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Posted: 2/8/2017 4:28:32 PM EDT
| I have a registered full auto lower. It is a conversion with standard M16 internals. It works fine with .223 and .22lr uppers but when I put on the 9mm upper it doesn't work. It works fine in the semi mode but not in the full position. It appears that the hammer is following the bolt. As I said it has standard M16 internals and I am using a CMMG ramped bolt. Any ideas? |
| As best I can tell, it is fine in the semi position but when in full auto I can pull the trigger and release the hammer but when I hand cycle it while holding the trigger the hammer does not lock back. It just rides the bolt forward. If you have any other questions please ask and I will do my best to answer. |
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When hand cycling in auto the hammer should catch on the sear while the bolt is held to the rear. While slowly closing the carrier you should hear the sear release right before the carrier finishes closing. That's when the hammer should drop.
You should be able to watch it work with the upper removed if you push the bolt/carrier back into the buffer tube and cycle it by hand. |
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Well not the same as mine is a bianary trigger I was having issues with the hammer following home as well.
What I did was change from the 9mm buffer to the light weight 3.5 oz buffer. This has solved all my issues so far and has been super fun. Use that info at your own discretion however as I don't know the difference between a fully auto and bianary modes. |
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I would also guess the ramp portion of the 9mm BCG's are long enough that the hammer is not being fully depressed..... or it is not being allowed to recoil far enough back.
I haven't tried it yet.... but I can't help but wonder if taking a little material off of the shoulder end of a buffer , would allow the BCG to move just that little bit more... and allow the trigger to reset properly. If you try this... I suggest you try a buffer you don't care about... since my thoughts are untested. If my thoughts are completely wrong.... Someone please chime in !! |
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An A2 rifle buffer & stock should have as much overtravel in a 9mm setup as a carbine buffer & M4 stock, assuming the OP doesn't have any spacer in there.
FWIW I did have a similar issue in my RRA 9mm AR, ran fine for years on a RRA lower/Hahn mag block w/ramped RRA bolt & single stage trigger. Tried to move it over to a NFA polymer lower (w/polymer FCG) and it would not reset when fired, but would reset when manually charged. Was running an 8 oz dedicated length buffer but also tried the factory RRA 5 oz carbine buffer with no spacers, no change. It just didn't like that FCG for some reason, but I had multiple 5.56 & 7.62x39 uppers that ran fine. |
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Quoted:
Yes, same hammer for all. Also, a friend let me borrow his ramped 9mm bolt and it didn't work in my gun either. Who was manufacturer of that bolt? Was it factory ramped or third party ramped? It's possible you could just have an unlucky tolerance stack within your 9mm upper. If your friends 9mm upper is not made from the same brands of parts as yours, you could try swapping their entire upper to see if that changes anything. |
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Can you post some pics of the FCG from the top and side.....I have seen several conversions where the auto sear pin hole were drilled with a slight cant.....and with the bolt velocity of a 9mm that can cause your problem...........
9mm bolts are also dimensionally different from the DI carrier.....same reason a DI will work fine with a Larue MBT and not with a 9mm bolt......in many instances the bolt isn't quite pushing the hammer down far enough to engage the disconnector or in this case the auto sear. If you can make it to the shop.....I can probably have it running in 15 or 20 minutes......but it is a damn long way form Kentucky to Macon GA 
I have some prototype bolts due in in a couple of weeks I hope that may solve the issue.....Lightening the buffet isn't the way to go...the trip wither catches the hammer nose on top or it doesn't you could also try a new auto sear.....it could be worn on the underside ...corner rounded.....and with the bolt pushing the hammer down not as far as the 5.56....it is slipping. |
| Worst case scenario, if you don't solve.....bring it to Knob Creek in April......not sure I'll be behind the table all weekend....but pretty sure I'll be there Saturday. I know several of the RO's on the subgun range.....so we should be able to test fire there... |
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Reread your post....99% sure it is bolt......it must depress the hammer further to engage the auto trip than the disconnector....that is why it'll run semi......pretty simple mod to the bolt if you have a tig welder...... |
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Old original "long slope ramp typically requires an OE COLT M16 hammer.... been gone around here and SUBGUNS I think in the past of similar concern...
Picture of offending ramp ?? Little more in "HERE" on down the page that re-inforce what I stated.. |
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Quoted:
Worst case scenario, if you don't solve.....bring it to Knob Creek in April......not sure I'll be behind the table all weekend....but pretty sure I'll be there Saturday. I know several of the RO's on the subgun range.....so we should be able to test fire there... Wow. That's a great offer. |
| Sorry I haven't posted for a few days but it looks like I have it fixed. It may have been a couple of problems or I probably did more than I needed to get it fixed. As I said it would only fire single shots with my upper and would stop with the hammer down on a live round. When I tried my friends bolt it did the same thing. When we tried his upper on my lower it would fire bursts and stop with the hammer down on a live round. Better but still not right. Luckily he had a spare stripped upper and we moved the parts from my upper over to the new one. It worked the same as his upper, firing bursts but stopping with the hammer down on a live round 4 or 5 times per mag. Also, when hand cycling we saw that the hammer was not catching on the sear and was following the bolt. Since it did work a bit better with the upper change I am going to say something was a bit out of spec with that upper. Next I decided to go ahead and install all new fire control parts in the lower. This appears to have fixed my problems. It worked properly when hand cycling and a quick test of a couple of mags in the back yard went perfectly. I will give it a better workout when I get to the range. What part in the fire control parts may have been worn enough to cause this? The hammer, at the point where the bolt makes contact seemed a bit worn and rounded but not that much. I am not sure how much use the parts have seen. I have had the gun for about 12 years (the 9mm is new) and put several thousand rounds through it. Is there a round count where parts should be replaced? Thanks to everybody for their help and suggestions. |
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Quoted:
Sorry I haven't posted for a few days but it looks like I have it fixed. It may have been a couple of problems or I probably did more than I needed to get it fixed. As I said it would only fire single shots with my upper and would stop with the hammer down on a live round. When I tried my friends bolt it did the same thing. When we tried his upper on my lower it would fire bursts and stop with the hammer down on a live round. Better but still not right. Luckily he had a spare stripped upper and we moved the parts from my upper over to the new one. It worked the same as his upper, firing bursts but stopping with the hammer down on a live round 4 or 5 times per mag. Also, when hand cycling we saw that the hammer was not catching on the sear and was following the bolt. Since it did work a bit better with the upper change I am going to say something was a bit out of spec with that upper. Next I decided to go ahead and install all new fire control parts in the lower This appears to have fixed my problems. It worked properly when hand cycling and a quick test of a couple of mags in the back yard went perfectly. I will give it a better workout when I get to the range. What part in the fire control parts may have been worn enough to cause this? The hammer, at the point where the bolt makes contact seemed a bit worn and rounded but not that much. I am not sure how much use the parts have seen. I have had the gun for about 12 years (the 9mm is new) and put several thousand rounds through it. Is there a round count where parts should be replaced? Thanks to everybody for their help and suggestions. Who parts did you buy ?? Colt factory parts ? Seems to me that the Colt M16 hammer remedied other person concern, as the off brand hammer was not tall enough or radius worn enough that geometry would not set hammer back far enough to catch auto sear paddle onto lug at hammer spur Do you have picture of bottom of your bolt to show ramp ? |
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