Mossberg ATR100

8/7/2019 Mossberg ATR100

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Mossberg ATR100 & 4X4 Trigger Update

The following instructions cover how to modify the trigger in the Mossberg ATR 100 & 4X4rifles to achieve a lighter trigger pull, and to reduce pre-travel.

While I am NOT a gunsmith, nor do I claim to be-these steps have worked for me. I do not

recommend that you try this at home yourself. I cannot be responsible for anything thathappens to you, or to others, as a result of your attempts to modify your rifle. If you areunsure of your skills at performing a task of this nature, or lack the tools to properly performthese changes, take your rifle to a competent gunsmith to have the work performed.

If you have worked on triggers before, I think you will find this one is a piece of cake. PleaseREAD THROUGH ALL OF THE INSTRUCTIONS BEFORE STARTING THIS PROJECT!

#1. VERIFY THAT THE GUN IS NOT LOADED. Physically inspect the magazine (box orremovable) to verify that there are no bullets present. Remove the bolt and look into thechamber area to confirm that there is not a loaded round stuck in the chamber. Always keepthe muzzle pointed in a safe direction. Always treat the gun as if it is loaded, even if you

think you know for sure that it isnt.

#2. Clear off an area where you can work on the gun, and where small springs, pins, and toolswont get lost easily. Use a solid light colored surface if possible. I used a bright white pieceof paper on my work surface. If you have a magnetic tray, use that to hold the various piecesas you remove them so that they wont roll away, or get brushed off of your work surface. Ifind magnetic trays keep things exactly where you stick them.

#3. Use the correct allen wrench, or socket headed allen wrench to remove the stock toaction bolts-loosen them each a little at a time-dont just take one out completely, then theother as this may cause a binding of the bolt.

#4. Once the screws are out, separate the stock from the action.


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#5. Once the action/barrel is separated from the stock, remove the one screw holding thetrigger assembly to the action. There is an alignment dowel on the front end of the triggerassembly (closest to the chamber). Make certain that this pin is not lost while you are workingon the trigger assembly. Mine fell out at some point & I had to find a roll pin to work in itsplace. I recommend pulling that alignment dowel out at this point & storing it so that it doesnot fall out later and get lost. If you attempt to pull it out and it is in there firmly, you maychoose to leave it alone. Of my 3 rifles 2 were in tight, but the 3rd fell out while I wasworking on it.

You can see the pin I am referring to just to the left of the screwdriver in the above picture.And here is the separated trigger assembly:

#6. Now before we start taking the assy. apart-I want you to take a moment and study howthe trigger assembly works. If you look at this picture, you will note that there is a window onthe assy.. In the picture it has grease around it. Look now through that window so that you

can see how the sear rests on the trigger arm.Understanding how the sear rests on the portion of the trigger arm is going to be importantlater on when it comes time to reduce, or eliminate, the pre-travel.

#7. Now it is time to get to work. Using a small punch or drift with a head diameter that isjust a little smaller than the size of the pin, and supporting the trigger assembly on the other


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side (I use a socket for this as it has a recess for the pin to fall into, and also supports thetrigger assy. at the same time.

You can see in the above picture that I am using my de-capper pin from my re-loadingequipment. The decapper pin is the correct size, and is hardened steel so it really works wellfor this application. In that picture you can also see the socket on the underside supportingthe trigger assembly.

*Please note that the PIVOT pin for the sear is the first pin you remove:

#8. Once you have the rear (pivot) pin out you will note that the sear is still captured bythe retention pin that is just forward of the rear pin. Time to take that one out as well:

#9. After you get both pins out, you can remove the sear:

Please note proper orientation of the sear for re-installation.Here is the sear as it appears from the factory. It has some type of coating on it. Could bephosphate, or moly, or it might just be spray paint:


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Here is my sear after polishing & before re-installation:

#10. Now that the sear is out, you will note there is a spring under the sear-remove thatspring now using your drift or punch implement:

Here you can see a comparison of the factory sear return spring, and the replacement springthat I used. The factory spring is on the left, and the replacement on the right:

I recommend removing all of the springs first, then take the sear return spring with you in azip lock sandwich baggy to your local hobby store. Bring along your calipers as well so thatyou can match the OD of the new springs to those of the old springs. You want a new springthat is similar in diameter, similar in length, but with a lighter wire for lower pressure.


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#11. Now that you have the sear & sear return spring out, lets continue to take the assemblyapart.The sear spring sits on another pin, in this picture you can see the hole for that pin just above& to the right of the trigger pivot pin.

Drive that pin out. You should now have 3 pins out: 1. Sear pivot pin 2. Sear retention pin 3.Sear spring seat pin.

#12. It is now time to take the final pin out, the trigger pin. Youll note that the end of thetrigger pin has been expanded to retain the pin. You will have to CAREFULLY grind just that

portion of the pin that is sticking up above the top of the trigger housing:

You can see in this picture that I used a 6 bench grinder and carefully ground off the end ofthe pin without causing much damage to the actual housing itself. You could use a Dremeltool, or even just the edge of a cut-off wheel. Even if you scratch the trigger housing itself, it

wont matter much as it wont be seen once you get it all back together. It wont rust eitherbecause it is ALUMINUM.

#13. Now that you have the end of the trigger pin ground off, you need to support the housingon the other side-use a socket here again-and drive the pin back through the housing. Dontdrive the pin out entirely. Just drive it back enough that you can remove the trigger whileleaving the pin engaged in the off side of the housing. In this picture, you can see that the pinis still present, because it is holding the safety on:

#14. Once you get the trigger out you can remove the trigger return spring. This spring willNOT be re-used. It will be discarded, or saved for another project?

#15. Here is where you will be getting your new trigger return spring from:


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17. Once you get the trigger return spring sorted out, it is now time to work on reducing thepre-travel, or creep as some people call it. If you look at this picture you can see where Iam measuring the ledge on the trigger arm where the sear rides. It is this ledge that the searhas to come across and then fall off of before the gun will fire. Think of it as a Suicide SearAssembly. The sear has to walk the ledge and jump off before the gun will fire.

You can see that the factory ledge on the trigger arm is about .056

You want to reduce this number down to the .015-.025 range. Reducing this ledge is whatreduces pre-travel. The shorter the ledge the less travel of the trigger before it releases thesear, allowing the gun to fire.

Here you can see that I have started to remove material from the edge of the trigger arm toreduce travel

And here you can see how much I have reduced the ledge:


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#18. After you grind the ledge down, you need to polish the entire ledge. Meaning, if the seartouches it, it must be MIRROR SMOOTH. This is how you get rid of the gritty feeling, and giveit a nice smooth trigger pull, and allow it to break like glass. One other thing to note, whenyou reduce the ledge, try and keep the edge of the ledge as sharp as possible. DO NOT ROUNDIT OFF! The sharper the edge of the ledge, the more CRISP the release will be.

I use 3 grades of sand paper:

Starting with 400, I only run it across the surfaces enough to remove any large burs, and toremove any paint. Maybe 4-5 strokes. You then move up to 600 grit and just a few morestrokes-6-10. Then move up to 800 grit, and this is where you want to spend the most time.On a perfectly flat smooth surface, I lay the sand paper down. Then I hold the piece I amsanding, and run it back and forth across the paper. You want to get all of the scratches out,and get the surface to a point where it looks like a mirror. This is crucial to getting a nicesmooth trigger pull. In this picture you can see what I am talking about.

This is the side of the sear that rides on the trigger ledge. You want to polish the other side ofthe sear as well. The other side is what grabs a hold of the striker and holds it in the sprungposition until the sear releases it. So, polish the sear where it touches the trigger arm, and

where it touches the bolt portion of the striker. You want any and all surfaces that moveacross each other, or touch each other when the trigger is pulled to be polished and slick asowl snot. Remember in the beginning when I told you to study the trigger assy. before youtook it apart? If you did that, you will know what areas need to be polished. After reducingthe trigger ledge to where you want it, you want to polish that surface as well, and evenpolish the area below the ledge where you were grinding. The reason you want to polishunder the ledge is because the sear will travel across where you were grinding and if it isrough, you will gouge up the edge of the sear defeating the whole purpose of polishing it in


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the first place.

Okay, now that you have your new sear spring from the hobby store, your new trigger springin place, all of your polishing done, its time to put the trigger assy. back together, and moveon to the bolt. You have polished your sear, and the trigger arm on the ledge, and below theledge. If you are not at this point, do it now before we move on. Also WASH every part of the

trigger in solvent, and then blow it off with compressed air to get metal shavings off.

#19. After cleaning and drying, & applying a light coating of your favorite gun oil to all thosesurfaces you just polished, put the trigger return spring (Bic ball point pen spring) in place,and then put the trigger arm in place. Now flip the housing over, and gently tap on the triggerpivot pin driving it back into its original position while being certain that the pin will passthrough the hole in the trigger arm. Once that is done flip the housing over again so that youcan see the end of the pin you ground on to remove it. Now take a punch, or drift, with asharpened point, and put it on the center of the pin where you were grinding before forremoval. With the other side of the pin supported on a hard surface, put your drift on thecenter of the pin and strike the end sharply with a hammer to spread the end of the pin outagain. If you dont feel comfortable doing this, you can use Red LocTite on this end of the

pin. Be very careful if you use the Loc-Tite because if it gets past that one side, it couldfreeze the trigger arm to the pin which would not be good. Alternatively you could mix up alittle JB weld and put it on the end of the pin, and the housing to hold it together. I find thatstriking the end of the pin with a drift works just fine though.

***If you are assembling the trigger to test for pull-DO NOT spread the end of the trigger pivotpin yet as that will have to wait until you have the trigger return spring sorted to your liking.

#20. Next, drive the pin that the sear spring sits on back into the housing.

#21. Now, put your new sear spring in place. I find a little grease on the spring will helpadhere it to the housing until you get the sear in place.

#22. Next place the sear into the housing noting the proper orientation.

#23. With the sear now resting in the housing, put the sear retention pin in place and push itthrough the housing insuring that it seats in the aft side of the housing.

#24. Next put the sear pivot pin in place, and seat it while insuring that it passes through thesear, and then seats in the housing on the aft side.

Folks, unless you have some additional trigger return spring trimming to do-that was the mostof it.

#25. Now you are ready to put the trigger back onto the action. If you removed the alignmentdowel from the housing when you initially took it off-put that back in now. If you did notremove your alignment dowel, then place the trigger assembly back onto the action insuringthat the alignment dowel mates with the hole on the bottom of the action.

#26. Put a small amount of red Loc-Tite on the threads for the trigger housing retainingscrew. Now start the screw by hand insuring that it is not cross-threaded. Tighten with a


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Phillips head until it is snug. There is no need to really torque it as the Loc-Tite will insurethat the screw does not back out later. Tight, but not too tight.

#27. Now mate the action back to the stock insuring that you get the magazine spring backinto the stock the proper way. You can see how the spring goes in this picture:

#28. Once you get the action and stock together, you can start the 2 retaining bolts by hand.Be certain that you tighten them a little bit each at a time, until they both end up tight.Dont torque one bolt down, and then torque the other bolt down. I gradually tighten themboth up a little at a time until they are tight. You can use a little bit of BLUE Loc-Tite on

those bolts if you would like. I DO NOT recommend over-tightening these bolts as they canhave an affect on accuracy. Tight, but not too tight.

Now, lets move on to the bolt. There is a little bit of work to be done here, but if you havemade it this far, the rest will be a cake walk.

#29. Put your gun to the side-somewhere safe where it wont fall over, or be knocked off yourwork area. Secure it away from your immediate work space for right now.

#30. Before we begin working on the bolt, there is something I need you to do. Holding thebolt in your hand you will note a large pin that retains the bolt head to the bolt body. I needyou to clean your bolt well enough that masking tape will stick to it. Use Brake Cleaner, or

carb cleaner to get all oil off of your bolt. Now with a clean bolt in hand, take masking tapeand wrap it around the bolt where that black pin passes through it. You want to wrap itseveral times insuring that the tape holds that pin in place, and that the tape is actuallykeeping the black pin from spinning in the bolt. Once you remove the striker rod, if you donthave it taped, the pin can fall out, and the bolt head will fall off. Its not that big a deal toget it back together, but since there is no reason to take it apart in the first place, just usethe tape!

#31. Now with the bolt on your work space in front of you, youll note that the you canrelease the spring pressure on the bolt by holding the bolt body, and turning the piece justbehind the bolt handle. If you look closely you will see that the piece just behind the bolthandle has a pointed end that is designed to fit into a recess in the bolt when it rotates. To

disassemble the bolt simple continue twisting the cocking assembly past the cocked position,and eventually it will release from the bolt body.


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Once you find the sweet spot, and the striker assembly and spring come out of the bolt body.

It will look like this:

In the following pictures youll note that I have the striker rod secured in a vise. You donthave to have a vise to hold it but it really makes it a lot easier!

#32. Now, if you look closer at the striker rod itself, you will see that there is a hole that has

been drilled through the shaft about 1.5 in front of where the bolt hand rides. Compress thespring and then slip a small punch, or drift through the hole like this:

You will note that there is a slot cut in the metal piece that rides against the spring whichwill allow you to push your drift through hole in the striker rod, and then you can pull the boltactuator up & away leaving the spring compressed and pinned with the drift.

#33. Once you have the spring pinned, you will now see a pin exposed that retains the boltactuator on the back of the striker. With that pin now exposed, take another drift and drive


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that pin out. It will look like this:

#34. Then with that pin removed, you can slide the bolt actuator off like this:

#35. Now with the bolt actuator off, and out of the way, take a rag and place it over the endof the striker rod so that it will catch the spring. Now-you guessed it-pull the drift that isholding that spring pinned to the striker rod.

#36. Now you should have a spring that looks like, and measures like this

As you can see the factory spring measures about 5.25. It was this same length in all 3 of myguns.

#37. What you want to do is to cut that spring down to about 4.5. You should start bycleaning the spring with Brake Clean, or carb clean, then masking it with tape. Like this:


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#38. Then you want to cut it like this:

I used a cut off wheel on a pneumatic grinder to cut my spring. It leaves a fairly flat faceon the spring when done.

Now with the spring cut, remove the remaining masking tape and lets move to somepolishing.

#39. With the bolt actuator piece off the bolt body, you can see that the sharp pointed edgecarries the full weight of the spring when you are actuating the bolt. You can make that

process smoother by polishing this guy:

#40. Just like before, start with your heavy grit to remove burs, then your mid grade paperwith more strokes, then finish with the high grade paper with lots of strokes for polishing. Ifyour lady friend has an Emory board around, grab it as it will make getting into the smallareas easier. You need to polish on BOTH sides of the actual point. Hard to see in the thispicture, but here it is nice & polished:

#41. After polishing on both sides of the point, work on that part of the bolt that the pointwill be making contact with. Polish it just like you have everything else.


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#42 Now, this guy here: has more than 1 job. In addition to cocking thegun when you actuate the bolt, the square part (on top in the picture and directly behind thepoint) is the piece that rides against the sear in the action. You need to polish that squarepiece where it contacts the sear. If you hold that piece against the bolt body, and simulatehow it would fit into the action, youll see where the contact points are. Polish every surfaceof this piece that touches any part of the sear!

#43. All polishing should be done at this point. Clean all bolt parts with brake clean, or carbcleaner and then blow them dry. Once dry, apply your favorite gun oil, or grease in this case,and put it all back together!

#44. Once you get the bolt back together, remove the masking tape you applied before to theblack pin, and then put the bolt into the action.

#45. Cycle the bolt several times to insure that the bolt is cocking and holding the striker inthe cocked position. Likewise, hold the trigger while closing the bolt to make sure that thebolt is releasing as a result of the trigger being held. If it passes those tests, try dry-firing theweapon. I know some people think that this will hurt the gun, but you just had it apart-didyou see anything that would be damaged by dry-firing it? I didnt! Dry-fire away! You want tomake sure that the gun is dropping the hammer EVERY TIME you pull the trigger. Now that youare done admiring the feel and instant response of the new trigger-we need to do sometesting for SAFETY.

#46. Test 1: With the hammer cocked, and the safety OFF, take the gun and slam the butt ofthe weapon on the ground. You are testing for a Slam Fire condition. If the bolt remainscocked, move on to the next test.

#47. Test 2: With the striker cocked, put the safety ON and with the barrel pointed in a safedirection, attempt to pull the trigger. The gun should not fire.

#48. Test 3: Now release the safety and listen for the gun to fire when you take the safetyoff. The gun should still be cocked after releasing the safety.

#49. Now pull the trigger, the gun should dry-fire.

#50. Test 4: Now, if you are a reloader like me, the next test is to take a piece of brassloaded with ONLY THE PRIMER. Put this brass into the gun keeping the gun pointed in a safedirection, with the safety off, now pull the trigger to make sure that the striker will crush theprimer. You should hear a POP like a cap gun when the primer ignites.

If it passes this test, then you are ready to head to your local range and put some roundsdown range.


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If it fails ANY of the tests above-STOP!-Do not attempt to fire the weapon! Something iswrong somewhere and needs to be addressed before you go forward!

I have had ZERO problems with mine but that is not to say that you wont have problems withyours!

If you experience any type of problem-take the gun immediately to a gunsmith and let themknow exactly what you did to the gun so that they can fix the problem! This is not worth yourlife, or anyone elses-DONT TAKE UNNECESSARY CHANCES!

If anyone has any questions about this process, or has anything to add-or if you just want topoint out omissions or mistakes-please feel free to contact me and I will make adjustments asnecessary, when necessary. I can be reached at poolstuff @ Juno DOT Com (silly spammers). Ifyou send me an e-mail you MUST indicate something in the SUBJECT line that I will recognize,and also you MUST include your name. If these items are left out, I will delete them withoutopening them. Too many viruses for me to chance opening non-recognized e-mails.

It probably took you longer to read this, than it will to actually perform the changes.

And in case you have not seen them yet, owners manuals:4X4 http://www.mossberg.com/manuals/4x4_revA.pdfATR 100 http://www.mossberg.com/manuals/100ATR.pdf

Thanks guys for the kind words.. Let me make some additional comments about doing theupdates.

First, let's address the issues with the bolt.. I am recommending that the striker spring (firingpin spring) not be cut shorter as I mention in the original post. This change in thinking hasnothing to do with the problems that Mossberg has supposedly had with the bolt body/bolthandle separations. This change in thinking came about because men smarter than myself

explained to me what "lock-time" is, and what can happen from a weak (shortened in thiscase) firing pin spring in regards to inconsistent primer strikes. In short-don't shorten thefiring pin spring! *If you have already done this, and are not having any problems (like me!)then don't worry about it!*

Let's now touch on the problems that I have become aware of with the Mossberg rifleswherein people are getting injured. For the record I don't know exactly what is happening. Idon't think anyone that is not directly involved in one of these injuries will ever know thetruth about what happened in those cases. I "read" peoples thoughts on why these failureshave occurred, and find those thoughts plausible, so I'll discuss them. I have read that thebolt handle is simply pressed into the bolt body on these guns. It is up for debate whether ornot this junction is silver soldered for extra strength. If you can imagine that the bolt head

must be "locked up" with the locking lugs in order for the bolt to sustain the load of the riflefiring. Now, if the bolt handle were not silver-soldered, and the press fit of the bolt handleinto the bolt body were loose enough to allow the handle to rotate independent of the boltbody.. You'd possibly have big problems. The rifle could fire with the bolt handle in the downposition, however, the bolt head would not be locked up in the lugs. Upon firing, this couldcause the bolt to be ejected rear-ward from the gun into the shooter. What do you do nowthat you have a Mossberg rifle and you are aware of this potential problem? Well, if you foundthis link and you are into modifying rifles, why stop with trigger work? Here is how I further


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modified my rifles to give me peace of mind..

How To Modify Bolt:Remove bolt from gun, disassemble bolt (remove firing pin, firing pin spring, bolt headretention pin, and bolt head. Now that you have the bolt body with affixed bolt handle inhand. Degrease the entire assembly.

Now if you study the junction where the bolt handle is pressed into the bolt body, in yourmind envision that there is a sleeve that is part of the bolt handle where it is pressed insideof the body. What I did was drill a hole through the bolt body that intersected with the sleevethat is part of the bolt handle. Where to drill the hole you ask? Well, I did not want thisplugged hole visible once the rifle was assembled, and in battery, so I drilled the hole on theunderside of the bolt (when assembled). After drilling, I degreased the assembly again toinsure no oil, or grease was in the hole. Once you get the hole drilled, you'll need access to awelder. Either a MIG, or TIG, welder. Now that the hole is drilled, and the piece degreased,you want to plug weld the hole. Once finished, you can polish the plug weld so that it doesnot stick up above the parent metal. This will permanently bind the two pieces together andinsure that the reported idea above never happens to you. Alternatively, you could simply

tack-weld the bold handle @ bolt body junction in a few places around the outside whichwould accomplish the same thing, but may be visible when the gun is in battery. Or, if youdon't have access to a welder, you could drill holes through the bolt handle/bolt bodyjunction and then press roll pins into the holes. If you go the pin route, just be sure that youare drilling deep enough that the pin will engage the bolt handle sleeve under the bold body.

Lastly, it has been mentioned that possibly the bolt head retention pin might be shearing, andallowing the bolt body to rotate to the down position, but still leave the bolt head in an un-locked position as the lugs will not be engaged. For this potential issue, I don't have ananswer as of yet. I'm checking on the viability of using a Savage Magnum bolt head pin, andpossibly modifying it to work in the Mossberg bolt head. I've heard that the Savage Magnumpins have a smaller firing pin hole drilled through them which leaves more metal for a

stronger pin. These pins can be purchased separately and I will post a source if I find thatthese pins can be made to work. I have a Savage 7MM Rem. Mag. at home I will tear down andmeasure the pin. Then, I'll see what the chances are that it will work in the Mossberg bolt,and if so what modifications (if any?) are necessary. I'll post back on that later.

With all of that said, and typed, I think it should be said that none of this should benecessary. Buy a Marlin XS7, or XL7 and shoot to your hearts content. They come withadjustable triggers, they don't have any bolt issues that I am aware of, they are as accurateas all get out, and cost LESS THAN the Mossbergs. I can't see a reason why anyone wouldpurchase a Mossberg now that the Marlins are available.

If you have a Mossberg(s) like me, and don't feel comfortable selling a potential problem to

someone else, then by all means modify them until you feel they are safe, and keep themforever.Here's another update... I have been corresponding with another gentleman from Australiaand he has sent me pictures of his Mossbergs bolt. From the pictures, it looks like Mossberghas changed/updated the bolt design! I can't positively confirm this as I have not held therifle.. Mossberg has NOT mentioned any bolt updates or changes in their advertising becauseto do so would be admitting that the old bolt design was flawed. But, maybe they did?


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Anyway, if your Mossberg has a seam within 1" of where the bolt body meets the bolt handle,you have the *potentially* updated bolt. Is this newer bolt stronger than the old one? I don'tknow! I have not seen one in person yet. Is it possible that this bolt is only available on riflesdestined to leave the US? I don't know! I'll have to stop in my local Academy and handle anewer rifle to see if it has the updated bolt. I just hope they have a newer one, and not justold stock.

Also, if your rifle has the newer bolt design, you may find that it has a newer style trigger aswell. On the gun that I received pictures of, his trigger came from the factory with an over-travel set screw, and also a sear adjustment screw. He tells me that the screws are set at thefactory, and that the heads of the screws are ground off to prevent the end user fromadjusting them more. Since the trigger housing is already tapped for the screws, it would beeasy to remove the existing screws and replace them with new screws so that you can furtheradjust them (if necessary?).

Even with the *possible* updates, I still would steer people to purchase the Marlin over theMossberg.

I'll post back later with any other updates as they become available. Thank you to thosepeople who have contacted me with information on these rifles.

Okay,I thought I would post the pictures as seeing is believing. Here is a picture of the updatedbolt. You'll note that this guy has TIG welded the bolt at the seem for extra assurance thatthe bolt handle won't separate from the bolt:

And here is a shot of the trigger on his gun:

In the picture of the trigger you can see there is a screw with lock-nut on both sides of thetrigger housing.

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