FCG problems on Milled 80%-100%

[RangerAJ]

I bought a billet 80% receiver and finished it right using a local shop with professional tools and expertise.

I have been in no hurry to complete it and have been watching for sales for components for my 1st build. I recently picked up a LPK and FCG combo from MidwestPX.com that came with a CMMG LPK and the ALG ACT FCG I wanted to try.

During installation, I found the hammer pivot pin hole to be very tight. I found a couple little burrs (aluminum shavings) in there, so I blew it out with some compressed air. After cleaning it, I still had some difficulty getting the pin in.

I ended up taking a 5/32 carbide bit and turned the outside edge by hand to try to ease things.

The pin and FCG is in now, but I have noticed that the bottom end of the hammer rubs against the front of the fire control pocket.

I have also found that when I pull the trigger (not letting it hit the receiver) and hand cycle it back to the reset position, the hammer drops during a slow release of the trigger (reset). If I let it out quickly, the trigger sear catches it.

I have checked the obvious recommended issues, (disconnector spring in upside down) and have not found it's cause or remedy.

All of this is being done without an upper receiver installed. As I mentioned above, this is my first build and I am a novice when it comes to the AR platform. My reason for building it myself is to better learn every part and how they all work together.

Prob # 1 Hammer pin hole
Prob # 2 Base of hammer rubbing inside fire control pocket
Prob # 3 Slow reset causes hammer to fall

These issues may be related, any and all advice will be appreciated.

[vw69turbobug]

post some pic of your fcg. but sounds like you need to cut more material off the front (front being toward the magwell area) to clear the hammer raduis at the bottom of the hammer.

[RangerAJ]

Here's a couple pics of the FCG pins. If the hammer pin is out of spec, it can't be by much.

[IMG][/IMG]

A little closer

[IMG][/IMG]

Good quality FCG by ALG:

[IMG][/IMG]

Spot where base of hammer rubs against front of FCG pocket in lower receiver.

[IMG][/IMG]

Would the pressure of the friction on the base of the hammer be enough to slow the rotation of the hammer and prevent the sear surfaces from catching?

Should I grind & polish just the spot where the hammer is rubbing or put it back on a mill and expand the whole pocket?

I saw the nightmare drill press pics from another post & don't want to cut into the mag release pocket.

[RangerAJ]

double post

[RangerAJ]

Holy giant pictures!

This is my first time posting pics, I hope that is adequate without being too much.

[RangerAJ]

double post

[vw69turbobug]

^^ what he said if i remember correctly it should be around .1050-1100 in thick

[croc4]

looks like the right wall is thicker than the left.

Take dremel with a sanding drum on it and go over the problem/ contact areas, should solve the issue. I had a similar issue on one of my very first lowers I did.

croc4

[Mr.Hipp]

One of my walls is thicker than the other too. I used KNS anti rotation pins and had to try several selectors to get it to work proper.

[RangerAJ]

Thanks all for the comments & advice. I'm working on it & will let you know how it turns out.

Any info on the reset (possible double fire) issue?

[sengk]

I have the same problem with the ALG FCG. I can't duplicated it dry firing but it does happen during live fire. The FCG was fine for about 60 rounds then I started getting double fires on trigger reset. I have since removed the FCG from my lower.

**edit: I contacted ALG and they are sending me a replacement FCG. ****

[ar15barrels]

I have noticed that the bottom end of the hammer rubs against the front of the fire control pocket.

I have also found that when I pull the trigger (not letting it hit the receiver) and hand cycle it back to the reset position, the hammer drops during a slow release of the trigger (reset). If I let it out quickly, the trigger sear catches it.

The lower was machined out-of-spec.
You can simply clearance the lower some so that the hammer does not rub.
Then you can re-time your disconnector so that it does NOT release the hammer until your trigger is in place to catch it.

Fix the lower BEFORE you work on the disconnector as the disconnector timing could even be related to the hammer being pushed against the front of the FCG pocket.

[RangerAJ]

*Problem solved*

I beat up the hole for the hammer pin test fitting my LPK.

This caused the hammer to wiggle in the now out-of-round hole. Spring tension caused the base of the hammer to rub against the front of the fire control pocket. I could feel this friction. While working the hammer. While troubleshooting the problem, I was inadvertently causing the hammer to lever against the pin and lower, making the problem worse.

I wound up with an out of spec lower due to my own ham handedness.

Anti-walk pins (with the external arms) could have fixed the problem, but I opted to start over due to the safety issue and for some peace of mind.

MidwestPX, ALG and Dr-Death (& Rick) all helped diagnose and fix the issue. I can't recommend them enough.

[Gunsmith Dan]

Yea I was gonna say your hammer hole looked to big giving the hammer "wiggle" room.

FYI for you and the others when dealing with burrs and holes that are slightly small take the pin and rub a light coating of lapping compound (600 grit or 800 grit). Then slowly hand rotate and push the pin back and forth till it moves smoothly, completely clean off all the lapping compound everywhere. Then repeat for the other side and that should solve the issue without messing anything up, takes some work but the results are worth it.

[VytamenC Tactical]

The lower was machined out-of-spec.
You can simply clearance the lower some so that the hammer does not rub.
Then you can re-time your disconnector so that it does NOT release the hammer until your trigger is in place to catch it.

Fix the lower BEFORE you work on the disconnector as the disconnector timing could even be related to the hammer being pushed against the front of the FCG pocket.

+1 machined out of spec.

i see what your talking about all the time. like i tell every body before you leave the "build party" throw your fire control group it and run your function test on it. seem like everybody with a cnc is now a ar specialist