Mossberg ATR100 & 4X4 Trigger Update
The following instructions cover how to modify the trigger in the Mossberg ATR 100 & 4X4 rifles 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 that happens to you, or to others, as a result of your attempts to modify your rifle. If you are unsure of your skills at performing a task of this nature, or lack the tools to properly perform these 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. Please READ THROUGH ALL OF THE INSTRUCTIONS BEFORE STARTING THIS PROJECT!
#1. VERIFY THAT THE GUN IS NOT LOADED. Physically inspect the magazine (box or removable) to verify that there are no bullets present. Remove the bolt and look into the chamber area to confirm that there is not a loaded round stuck in the chamber. Always keep the 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 isn’t.
#2. Clear off an area where you can work on the gun, and where small springs, pins, and tools won’t get lost easily. Use a solid light colored surface if possible. I used a bright white piece of paper on my work surface. If you have a magnetic tray, use that to hold the various pieces as you remove them so that they won’t roll away, or get brushed off of your work surface. I find magnetic trays keep things exactly where you stick them.
#3. Use the correct allen wrench, or socket headed allen wrench to remove the stock to action bolts-loosen them each a little at a time-don’t just take one out completely, then the other as this may cause a binding of the bolt.
#4. Once the screws are out, separate the stock from the action.
#5. Once the action/barrel is separated from the stock, remove the one screw holding the trigger assembly to the action. There is an alignment dowel on the front end of the trigger assembly (closest to the chamber). Make certain that this pin is not lost while you are working on the trigger assembly. Mine fell out at some point & I had to find a roll pin to work in it’s place. I recommend pulling that alignment dowel out at this point & storing it so that it does not fall out later and get lost. If you attempt to pull it out and it is in there firmly, you may choose to leave it alone. Of my 3 rifles 2 were in tight, but the 3rd fell out while I was working 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 how the trigger assembly works. If you look at this picture, you will note that there is a window on the 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 important later 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 is just a little smaller than the size of the pin, and supporting the trigger assembly on the other side (I use a socket for this as it has a recess for the pin to fall into, and also supports the trigger assy. at the same time.
You can see in the above picture that I am using my de-capper pin from my re-loading equipment. The decapper pin is the correct size, and is hardened steel so it really works well for this application. In that picture you can also see the socket on the underside supporting the 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” by the 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 be phosphate, or moly, or it might just be spray paint:
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 that spring now using your drift or punch implement:
Here you can see a comparison of the factory sear return spring, and the replacement spring that 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 a zip lock sandwich baggy to your local hobby store. Bring along your calipers as well so that you can match the OD of the new springs to those of the old springs. You want a new spring that is similar in diameter, similar in length, but with a lighter wire for lower pressure.
#11. Now that you have the sear & sear return spring out, let’s continue to take the assembly apart.
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. You’ll note that the end of the trigger 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 of the pin without causing much damage to the actual housing itself. You could use a Dremel tool, or even just the edge of a cut-off wheel. Even if you scratch the trigger housing itself, it won’t matter much as it won’t be seen once you get it all back together. It won’t rust either because it is ALUMINUM.
#13. Now that you have the end of the trigger pin ground off, you need to support the housing on the other side-use a socket here again-and drive the pin back through the housing. Don’t drive the pin out entirely. Just drive it back enough that you can remove the trigger while leaving the pin engaged in the off side of the housing. In this picture, you can see that the pin is 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 will NOT 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:
#16. Yes, that is correct, your new trigger return spring will come from a ball point pen. The only issue here is that the ball point pen spring has a larger OD than the spring supplied by Mossberg. How do we fix that?
You drill the original pilot hole for the trigger return spring, that’s what you do. I tried finding a replacement spring that was similar in OD to replace the factory one, but all of the springs that I found in that small OD were far too soft to work. After much head scratching, I decided to just open up the size of the hole, and then a whole new world of opportunities became available. Since Bic and I go way back-having used them several times for other trigger jobs, I gave them the new assignment on the Mossberg as well. I currently have Bic springs in my Rugers, Smith & Wessons, & Savages to name a few. Now Mossberg is proudly on that list.
Now back to the drilling part, use a bit that is as close in diameter to the spring as possible. I got lucky and used a 3/16” bit, and the spring was actually a press fit. After inserting the new spring, it was in there tightly enough that it would not fall out on it’s own. Only open up the hole to the original depth of the existing hole! DO NOT DRILL THROUGH THE HOUSING! Stop, and check repeatedly to see when you are getting close to the bottom of the hole. Do not drill any further than necessary in depth.
Now I know what is on your mind-“how long should the spring be?”. And that is where some customization can come in on your part. You see it is this spring that really affects how much trigger pull you feel. It is a bit of work to put it all back together, for trial and error, but if you really want it “CUSTOMIZED” to your wants or needs, you are going to have to spend some time here putting it together to try a certain spring length, and then taking it back apart when you find you missed the mark. On your average ball point pen spring, I think I wound up using 1/3 of the original spring length. Again-this is what worked on my guns-but yours may require a different length! Start at just a little over half the original length of the spring, and see how it feels. If you want the pressure lower, cut it down some more a little at a time until you arrive at where you want to be.
In this picture you can see the new enlarged hole, and you can also see the pin for the trigger still sitting in one side of the trigger housing.
17. Once you get the trigger return spring sorted out, it is now time to work on reducing the pre-travel, or “creep” as some people call it. If you look at this picture you can see where I am measuring the ledge on the trigger arm where the sear rides. It is this ledge that the sear has to come across and then fall off of before the gun will fire. Think of it as a “Suicide Sear Assembly”. 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 what reduces pre-travel. The shorter the ledge the less travel of the trigger before it releases the sear, allowing the gun to fire.
Here you can see that I have started to remove material from the edge of the trigger arm to reduce travel
And here you can see how much I have reduced the ledge:
#18. After you grind the ledge down, you need to polish the entire ledge. Meaning, if the sear touches it, it must be MIRROR SMOOTH. This is how you get rid of the gritty feeling, and give it a nice smooth trigger pull, and allow it to break like glass. One other thing to note, when you reduce the ledge, try and keep the edge of the ledge as sharp as possible. DO NOT ROUND IT 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 to remove any paint. Maybe 4-5 strokes. You then move up to 600 grit and just a few more strokes-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 am sanding, 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 nice smooth 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 of the sear as well. The other side is what grabs a hold of the striker and holds it in the sprung position 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 move across each other, or touch each other when the trigger is pulled to be polished and slick as owl snot. Remember in the beginning when I told you to study the trigger assy. before you took it apart? If you did that, you will know what areas need to be polished. After reducing the trigger ledge to where you want it, you want to polish that surface as well, and even polish the area below the ledge where you were grinding. The reason you want to polish under the ledge is because the sear will travel across where you were grinding and if it is rough, you will gouge up the edge of the sear defeating the whole purpose of polishing it in the first place.
Okay, now that you have your new sear spring from the hobby store, your new trigger spring in place, all of your polishing done, it’s time to put the trigger assy. back together, and move on to the bolt. You have polished your sear, and the trigger arm on the ledge, and below the ledge. 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 those surfaces 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 trigger pivot pin driving it back into it’s original position while being certain that the pin will pass through the hole in the trigger arm. Once that is done flip the housing over again so that you can see the end of the pin you ground on to remove it. Now take a punch, or drift, with a sharpened point, and put it on the center of the pin where you were grinding before for removal. With the other side of the pin supported on a hard surface, put your drift on the center of the pin and strike the end sharply with a hammer to spread the end of the pin out again. If you don’t feel comfortable doing this, you can use Red Loc Tite on this end of the pin. Be very careful if you use the Loc-Tite because if it gets past that one side, it could freeze the trigger arm to the pin which would not be good. Alternatively you could mix up a little JB weld and put it on the end of the pin, and the housing to hold it together. I find that striking 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 pivot pin 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 help adhere 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 it through 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 the sear, 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 most of it.
#25. Now you are ready to put the trigger back onto the action. If you removed the alignment dowel from the housing when you initially took it off-put that back in now. If you did not remove your alignment dowel, then place the trigger assembly back onto the action insuring that 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 retaining screw. Now start the screw by hand insuring that it is not cross-threaded. Tighten with a Phillips head until it is snug. There is no need to really torque it as the Loc-Tite will insure that 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 back into 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. Don’t torque one bolt down, and then torque the other bolt down. I gradually tighten them both 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 can have an affect on accuracy. Tight, but not too tight.
Now, let’s move on to the bolt. There is a little bit of work to be done here, but if you have made it this far, the rest will be a cake walk.
#29. Put your gun to the side-somewhere safe where it won’t fall over, or be knocked off your work 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 the bolt in your hand you will note a large pin that retains the bolt head to the bolt body. I need you 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 tape and wrap it around the bolt where that black pin passes through it. You want to wrap it several times insuring that the tape holds that pin in place, and that the tape is actually keeping the black pin from spinning in the bolt. Once you remove the striker rod, if you don’t have it taped, the pin can fall out, and the bolt head will fall off. It’s not that big a deal to get it back together, but since there is no reason to take it apart in the first place, just use the tape!
#31. Now with the bolt on your work space in front of you, you’ll note that the you can release the spring pressure on the bolt by holding the bolt body, and turning the piece just behind the bolt handle. If you look closely you will see that the piece just behind the bolt handle 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.
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 you’ll note that I have the striker rod secured in a vise. You don’t have 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 the spring 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 which will allow you to push your drift through hole in the striker rod, and then you can pull the bolt actuator 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 bolt actuator on the back of the striker. With that pin now exposed, take another drift and drive 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 end of the striker rod so that it will catch the spring. Now-you guessed it-pull the drift that is holding 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 my guns.
#37. What you want to do is to cut that spring down to about 4.5”. You should start by cleaning the spring with Brake Clean, or carb clean, then masking it with tape. Like this:
#38. Then you want to cut it like this:
I used a cut off wheel on a pneumatic ¼” grinder to cut my spring. It leaves a fairly flat face on the spring when done.
Now with the spring cut, remove the remaining masking tape and let’s move to some polishing.
#39. With the bolt actuator piece off the bolt body, you can see that the sharp pointed edge carries 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 paper with more strokes, then finish with the high grade paper with lots of strokes for polishing. If your lady friend has an Emory board around, grab it as it will make getting into the small areas easier. You need to polish on BOTH sides of the actual point. Hard to see in the this picture, but here it is nice & polished:
#41. After polishing on both sides of the point, work on that part of the bolt that the point will be making contact with. Polish it just like you have everything else.
#42 Now, this guy here:
has more than 1 job. In addition to cocking the gun when you actuate the bolt, the square part (on top in the picture and directly behind the point) is the piece that rides against the sear in the action. You need to polish that square piece where it contacts the sear. If you hold that piece against the bolt body, and simulate how it would fit into the action, you’ll see where the contact points are. Polish every surface of 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 carb cleaner 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 the black 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 in the cocked position. Likewise, hold the trigger while closing the bolt to make sure that the bolt is releasing as a result of the trigger being held. If it passes those tests, try dry-firing the weapon. I know some people think that this will hurt the gun, but you just had it apart-did you see anything that would be damaged by dry-firing it? I didn’t! Dry-fire away! You want to make sure that the gun is dropping the hammer EVERY TIME you pull the trigger. Now that you are done admiring the feel and instant response of the new trigger-we need to do some testing for SAFETY.
#46. Test 1: With the hammer cocked, and the safety OFF, take the gun and slam the butt of the weapon on the ground. You are testing for a “Slam Fire” condition. If the bolt remains cocked, move on to the next test.
#47. Test 2: With the striker cocked, put the safety ON and with the barrel pointed in a safe direction, 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 safety off. 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 brass loaded with ONLY THE PRIMER. Put this brass into the gun keeping the gun pointed in a safe direction, with the safety off, now pull the trigger to make sure that the striker will crush the primer. 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 rounds down range.
If it fails ANY of the tests above-STOP!-Do not attempt to fire the weapon! Something is wrong 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 won’t have problems with yours!
If you experience any type of problem-take the gun immediately to a gunsmith and let them know exactly what you did to the gun so that they can fix the problem! This is not worth your life, or anyone else’s-DON’T TAKE UNNECESSARY CHANCES!
If anyone has any questions about this process, or has anything to add-or if you just want to point out omissions or mistakes-please feel free to contact me and I will make adjustments as necessary, when necessary. I can be reached at poolstuff @ Juno DOT Com (silly spammers). If you 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 without opening 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:
ATR 100 http://www.mossberg.com/manuals/100ATR.pdf