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Posted: 4/25/2014 5:06:11 PM EDT
Please one of you lovely ar gurus help me fix this problem. My AR works just fine all the time except when I am doing hammer drills. As soon as I start doing hammer drills I get a malfunction every so often. Not a single malfunction otherwise. This is a ground up build. The problem is when I start doing the hammer drills I will occasionally get a light primer strike. I talked to the gunsmith at my shooting range and he seems to think that I am pulling the trigger before the bolt is fully in battery causing a light hammer strike. He seems to think that my bolt carrier isn't moving fast enough. I thought that it might be a weak hammer spring but he said that if it works every time except when I do the hammer drills the spring isn't the issue. So what say the experts??

List of relevant parts
Trigger group from PSA lower parts kit
Daniel Defense 14.5" carbine gas length barrel
BCM BCG
Used both of these for buffers
BCM H2 buffer with a no name carbine spring
Just swapped to a Spikes ST2 buffer kit
Low pro gas block is a clamp on style Daniel defense

Is the gunsmith right in that between the full auto BCG and ST2 or BCM H2 buffers have too much mass causing the BCG to be moving to slow and causing light primer strikes while doing hammer drills? It is the only time I ever have any malfunctions. He recommended that I install a lighter buffer or use a higher strength wolf buffer spring. Thoughts??
Link Posted: 4/25/2014 5:19:49 PM EDT
Originally Posted By giabyul:
Please one of you lovely ar gurus help me fix this problem. My AR works just fine all the time except when I am doing hammer drills. As soon as I start doing hammer drills I get a malfunction every so often. Not a single malfunction otherwise. This is a ground up build. The problem is when I start doing the hammer drills I will occasionally get a light primer strike. I talked to the gunsmith at my shooting range and he seems to think that I am pulling the trigger before the bolt is fully in battery causing a light hammer strike. He seems to think that my bolt carrier isn't moving fast enough. I thought that it might be a weak hammer spring but he said that if it works every time except when I do the hammer drills the spring isn't the issue. So what say the experts??

List of relevant parts
Trigger group from PSA lower parts kit
Daniel Defense 14.5" carbine gas length barrel
BCM BCG
Used both of these for buffers
BCM H2 buffer with a no name carbine spring
Just swapped to a Spikes ST2 buffer kit
Low pro gas block is a clamp on style Daniel defense

Is the gunsmith right in that between the full auto BCG and ST2 or BCM H2 buffers have too much mass causing the BCG to be moving to slow and causing light primer strikes while doing hammer drills? It is the only time I ever have any malfunctions. He recommended that I install a lighter buffer or use a higher strength wolf buffer spring. Thoughts??
View Quote


Pretty sure you can't engage the trigger until the BCG is in battery. Have you tried an H buffer to eliminate BCG speed as a sanity check? Is hammer spring at the mil-spec weight?

Ammo? Firing pin?
Link Posted: 4/25/2014 5:26:32 PM EDT
Have not tried an H buffer yet. Firing pin is what came with the BCM BCG seems fine. It has done this with federal xm193 PMC xtac xm193 ppu 193 mk262 and tul ammo
Link Posted: 4/25/2014 5:27:41 PM EDT
Hammer is spring is what came from PSA and the claim mil spec.
Link Posted: 4/25/2014 5:39:25 PM EDT
You can't pull the trigger faster than the bolt cycles. Think of a full auto which fires 12 times a second or so. Unless there is some drag for another reason.
Link Posted: 4/25/2014 5:40:37 PM EDT
probably a FCG issue, not a buffer/spring issue. I'm no expert on carrier and buffer mass and spring strength but have read some technical threads on here where some fancy physics terms where being thrown around and basically stating that spring strength is no substitute for mass when setting up a properly cycling/functioning AR and his advise directly contradicts this. So my rule is = if you need a heavy spring you need a heavy buffer.

I run Wolff XP springs, one has a A5H4 buffer the other a H2, both function great.
Link Posted: 4/25/2014 5:54:30 PM EDT
Disconnector to hammer clearance marginal or excessive. Hammer is following BCG down during fast trigger operation. Re-time clearance between disco claw and hammer sear to 0.001~0.003" or simply replace parts and see what happens.
Link Posted: 4/26/2014 2:51:36 AM EDT
Okay I guess I will try and swap the lower with a new one I just finished that has a cmmg lower parts kit in it. If that doesn't work what else could be the problem???
Link Posted: 4/26/2014 5:22:03 AM EDT
I'm unfamiliar with the term "hammer drill" as applied to ARs. But from the other replies, it does sound like you're trying to fire faster than the rifle cycles.

I think it would be a bad idea to just blindly change out the FCG (or parts of the FCG) without knowing what exactly is causing your light strikes, especially when they're not very consistent or very common. Trying another lower is a good start; if you see the same sort of symptoms with another lower, it ain't a lower problem - unless the upper needs a different buffer. The first thing to try, whether it gives you the light strikes with another lower or not is to see how the upper reacts with different buffers. A heavier buffer will slow the cyclic rate, while a lighter buffer will speed it up. If the problem is that you're pulling the trigger too soon, before the bolt is fully closed, then a lighter buffer might reduce or eliminate that.

If changing buffers doesn't eliminate or substantially reduce the incidence of light strikes in these drills, then I'd suggest going with a completely new FCG, like an ALG ACT. I recommend this one in particular because there is almost no "break in" for the ACT, so you can discount any FCG function drag. For $65, it's an inexpensive upgrade over a stock trigger, and if you see fewer light strikes because of it, it should be worthwhile.
Link Posted: 4/26/2014 7:51:03 AM EDT
Alright I have narrowed it down to three things. One is the trigger group which I have a new lower to try and see if that is the problem. Next is the buffer. I went through my parts bin and found a standard carbine buffer. I will try that after the new lower. If those two things are not the problem then it has to be the gas tube. So off to the range I go to troubleshoot. I will give an update after the range.
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