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Posted: 9/21/2014 3:28:43 AM EDT
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How do you approach a trigger job on a mil spec trigger hammer?
I did the following to a P-Tac LPK. The trigger was definitely rough to begin with. Used 600grit 50% diamond lapping compound on the sear with the hammer/trigger installed in the lower. Pulled the trigger catching the hammer so it didn't hit the frame about 80+ times and reapplied lap compound 4 times during the process. Went to 1500 grit 10% diamond lapping compound on the sear and did another 50+ trigger pulls while reapplying the compound. Used a dremel tool with a buffing wheel and buffed both trigger and hammer sear engagement perpendicular to the grain of the lapping. Using a 1/4-28 tap, ran it into the fire control pocket and ran a 1/4" set screw up to adjust over travel. Installed JP yellow springs. After this the trigger is still gritty. I'm not sure how far I can go before removing too much material and the hardened surface on the sear is compromised. Any thoughts or recommendations? |
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I have done dozens the same way; I stone them and sand (400- 600 grit on a piece of Glass on the bench) until no tool marks are visible under a magnifying glass, the buff them to a mirror shine with a dremel.
Grease it up, good to go. 
Tool marks are what give you that dragging the trigger down a gravel road feel. My take on the case hardening; Gunsmiths have been stoning triggers since there were triggers. Triggers are not such a high stress part that they are going to wear out from use by an average shooter, especially if you keep it lubed well. As long as you aren't changing the sear angles or removing metal, I would do what is needed to slick it up. Soft Babbitt bearings suffer much more stress and last for thousands of hours in engines if they are well oiled. I think there's way too much fuss about it. If you are really anal about it, get some Case Hardening Compound from Brownells or Cherry Red from Midway and re-harden after you are done. People don't seem to mind that their $400 match barrel is going to wear out after 5000 rounds, but have conniptions over the idea that a $50 FCG could do the same because you tuned it. I'll do what I can to make my trigger as good as I can get it, and if it wears out, replace it. Haven't had one malfunction yet..... Tacked thread about trigger work in " Build it yourself". if you want to read more about it. |
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Thank you for the info.
I was having interference with the safety after adjusting the set screw. The fix seems to be to remove metal from the tail of the trigger to allow clearance. Hadn't come across this information before. I'll carve a little from the trigger tail so the slack can be taken up more. I'll also light stone the trigger (I'll have to check if I can find a piece of glass to work with instead) and check the hammer for tooling marks and light file as needed. I have some hardener if needed. Just didn't want to get to that point if I could help it. I've done pistol triggers with success, but also went too far with an HK91 and had doubles/triples resulting. I only did the hammer so I replaced it and sent to Bill S and he did a nice job on that one. I have several AR's to do and want to optimize the results. Some are 80% builds and doing the trigger job as well gives some additional satisfaction. |
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Be careful with the set screw; my rule of thumb is to adjust it until the hammer falls, then back up one full turn. I have never taken metal off anywhere, if I have interference, I back off until I don't.
One of the engineered safety features of milspec AR triggers is that the sear surfaces are not 90 degrees to each other when cocked. The hammer actually cams backwards to align the sears when you first apply trigger pressure, then the trigger sear surface begins the downward slide. You can watch this while adjusting the set screw. You do not want to completely remove this safety zone and set the sears on the edge of breaking. The set screw is essentially "pre-pulling" the trigger for you, moving it off the sear. It's important to retain enough contact to remain safe. I always "drop test" any rifle that I do trigger work on. 6-10"drop on the butt over a padded surface until you are completely satisfied that it cannot drop the hammer from impact with the safety off. Too far with the set screw and you won't have enough sear contact, and it will be dangerous, and capable of bump firing. Better to have a little creep and be safe than the other way around. Oh, and if you have a collapsible stock, I highly recommend collapsing it prior to bump testing.....  
Unsubstantiated internet advice; YMMV |
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Thanks.
The problem I was having was after adjusting the set screw, I couldn't turn the safety. Even with quite a bit of creep still left in the trigger. I took a bit off the tail of the trigger and went further than expected. With the safety on and pulling the trigger the hammer doesn't drop. Banged it around some as well. Pulling the trigger with the safety on and then moving off safety it is close to being a set trigger. Light for what I was expecting. I'll spot weld it and try again and take less material off next time. Better safe than sorry. Worked the hammer/trigger more and it still needs more work. All part of my learning curve...
Edit: It's a 300blk pistol BTW. Did the drop bouncing the weapon 4X a rubber mat on a concrete floor and had the buffer clacking pretty good. This was after I put the safety on and took up the slack in the travel. Did the test 3X and found that the travel on the trigger increased after drop (bounce) test. I believe this is what you were referring to as inherent in the sear engagement design. |
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I know ya'll are talking about mil-spec single-stage triggers, but I just wanted to drop in and say... If you exert the same level of "trigger job effort" on a RRA 2-stage, you'll end up with a much nicer trigger. It will then rival way more expensive match units. That's been my experience anyway. |
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Budget Gun with Standard FCG ; cost to improve with JP spring kit, set screw: $12, 30minutes labor
Budget Gun with Standard FCG; cost to improve with RRA trigger, $80-100, + labor. Every AR I own and most of my friends and family have had the trigger massaged to a nice clean 5lb trigger for a little over $10 and some grunt work. We could never afford that with aftermarket triggers. 
Then there's the whole two stage-single stage discussion....... |
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