I have a 9mm PSA AR that I've got probably 1500-2000 rounds through. I had a doubling issue a while back that I resolved (I thought) by replacing the buffer spring. I shot 400-500 trouble-free rounds through it and figured it was good. However, this weekend it returned, with the added benefit of an out of battery discharge that sprayed my face with powder and shrapnel (and that, kids, is why I ALWAYS wear glasses when shooting!).

I assumed the problem was with the upper, but upon further testing, I determined that the fcg was not resetting. It wouldn't do it manually...it repeatedly passed all the function tests. But if I loaded and fired a single round, the trigger would not reset (hence, the hammer follow, doubling, and out of battery discharge). I replaced the fcg with another stock PSA unit that I had on hand, and it's back to working properly.

Now obviously I've been keeping a close eye on it, and after shooting it today I found this. The hammer is developing a notch in it where it appears to be contacting the top of the BCG above the firing pin. It's hard to tell from the photo, but this is not superficial wear...it's a groove that you can easily feel with a fingernail. I checked the old (defective) fcg that I took out, and it's hammer shows the same wear. None of my other ARs do, though.



So my question for those of you with 9mm ARs is this: is this normal? Should I be concerned? It seems to me that the hamner slamming into the BCG in this way is going to put undue stress on the hamner pin. Could this have been a contributor to the premature failure of my original fcg?

This message has been edited. Last edited by: 92fstech,

What bolt are you using? What spring and buffer?

quote:

Originally posted by RB211:
What bolt are you using? What spring and buffer?

Factory PSA 9mm Bolt. It is ramped. Also factory PSA 9mm buffer and spring.

I'm not completely familiar with with the PSA 9MM AR setup, so I have a couple of questions.

How much does the 9MM PSA Buffer weigh?

Are you using a 9MM Spacer to keep the Buffer from traveling excessively rearward / to keep the buffer speed down as it travels forward?

Have you had an bolt catches break?

I have a 9MM DDLES AR and have had great luck using HeavyBuffers.com 9mm-Q Buffer with a Wolff XP buffer spring . It's a heavier, extended buffer so you don't need a spacer.

My WAG (wild ass guess) is you may not have a spacer and the 9MM PSA buffer is too short and light. This might be leading to bolt bounce because the bolt is traveling too fast and the buffer and spring is unable to stop that.

If that is the case, maybe the bolt is contacting the hammer at some point in the process, causing that wear.

Again, lots of maybes / conjecture on my part - but after hearing how finicky 9MM ARs were, mine has been 100% reliable outside of 1 user error on my part (failed to seat the first magazine I ever fired with it). I don't have an eye watering number of rounds, only 1800 or so.

Good luck.

Is it possible your hammer spring is on backwards?
That can cause occasional doubling and is hard to diagnose.



Here is one explanation:
http://www.ktgunsmith.com/AR10_dt.htm

I know the above talks about an AR-10 but I have heard of it happening on AR-15's and AR9s.

I am inclined to think along the lines of Rhinowso - Possibly a bolt bounce issue due to your buffer/bolt combo being too light.

I don't have any PSA parts in mine (CMMG lower, Spinta bolt, Spikes ST-9X buffer (8oz) and a standard carbine spring). Probably the lightest buffer I would be comfortable with would be a 6.5oz, The standard PSA is 5.5oz IIRC. Many feel the magic weight is between 21 and 22oz, though, I have seen them work with less.

I would investigate the disconnector and the hammer, and where they interface.

You are seeing some wear on the face of the hammer, but that is not a crucial area. Unless it has worn enough that the bolt carrier doesn't push the hammer down far enough to engage the hook of the disconnector.


You may have a weak disconnector spring. Or you may have a broken hammer or disconnector.


It would be easier to diagnose if I had it in hand.

I strongly suggest posting this over on AR15.com, under the Pistol caliber AR section.

Rudy, ( owner of Macon Armory ) will see it and be able to expertly diagnose the issue here. Or, contact him. He's a great guy and really sharp on PCC ARs.

( You're a lefty, eh?)

This very issue came up on AR15.com a while back for myself and other PSA bolt owners. In fact I have 2 PSA bolts and this was occurring on both my hammers. It's a design issue with how the bolt is machined at the area where the hammer impacts. It's not a perfectly flat surface like most other 9mm bolts, and the hammer hits a semi-circular area on the bolt just above the firing pin and gets the indent marks.
The "fix" for me was to use a 1/4" spherical stone in a dremel to cut a relief into that area of the bolt so that the hammer has a "pocket" to drop into. Presto!
Sorry, I have no pics and I can't find the thread on arfcom, but look at the bolt where the hammer strikes, just above the firing pin, and you'll see what I'm talking about.
Hope this helps!

This message has been edited. Last edited by: Dakota Slim,

I’m guessing an out of spec soft hammer. The bolt drags across that area of the hammer during rearward travel to cock it. Some degree of wear is normal. I have a small amount in that area of my Colt M16 hammer. If it is peening that area, the hammer is likely soft.

Sorry, I can't help with your main issue, hell i don't even have a lower for mine yet, but with regard to this:

quote:

Originally posted by 92fstech:
However, this weekend it returned, with the added benefit of an out of battery discharge that sprayed my face with powder and shrapnel (and that, kids, is why I ALWAYS wear glasses when shooting!).

If your upper has a dust cover (many of the PSA 9mm uppers unfortunately do not), then installing one of these might help with the blast:

https://www.cmmginc.com/produc...-with-gas-deflector/

I realize it was a huge issue related to the out of battery discharge, but as a lefty (long guns only), I noticed there is a lot of blast from the 9mm ARs I've shot recently, so I'm considering getting tbis set up for mine.

Thanks for the responses everybody...it's been helpful. Rhino...you make some good points, and while I have not sheared any bolt catches, I think that's probably more because of the fact that my gun does not have LRBHO capability (hence the bolt catch never engages) than the fact that it's tuned right. The buffer does not have a spacer, and I'm not sure of the weight. I don't have it on hand right now to check. I've considered getting a blitzkreig hydraulic buffer, but I wanted to make sure all the function issues are ironed out before I drop that kind of money into this thing.

ARMT Guy: no, I'm not a lefty, but having a case explode in the upper receiver sure threw plenty of crap into my face anyway! If I was a lefty it likely would have been way worse !

I think Dakota Slim's response here addresses the exact issue I am having with the hammer:

quote:

This very issue came up on AR15.com a while back for myself and other PSA bolt owners. In fact I have 2 PSA bolts and this was occurring on both my hammers. It's a design issue with how the bolt is machined at the area where the hammer impacts. It's not a perfectly flat surface like most other 9mm bolts, and the hammer hits a semi-circular area on the bolt just above the firing pin and gets the indent marks.
The "fix" for me was to use a 1/4" spherical stone in a dremel to cut a relief into that area of the bolt so that the hammer has a "pocket" to drop into. Presto!
Sorry, I have no pics and I can't find the thread on arfcom, but look at the bolt where the hammer strikes, just above the firing pin, and you'll see what I'm talking about.
Hope this helps!

Dakota, any chance you could tell me how much material you had to take off of the bolt to stop the hammer from impacting? Did you make the relief cut all the way across, or only on the corners where the hammer appears to be impacting? Any reliability or function issues after relieving that area?

My concern is that even if the hammer pin doesn't snap, over time this groove at the top of the hammer is going to wear down far enough that the hammer will fail to engage the disconnector, and I'll start getting hammer follow again. This may very well be the reason that my first FCG failed.

quote:

Originally posted by 92fstech:
Dakota, any chance you could tell me how much material you had to take off of the bolt to stop the hammer from impacting? Did you make the relief cut all the way across, or only on the corners where the hammer appears to be impacting? Any reliability or function issues after relieving that area?

My concern is that even if the hammer pin doesn't snap, over time this groove at the top of the hammer is going to wear down far enough that the hammer will fail to engage the disconnector, and I'll start getting hammer follow again. This may very well be the reason that my first FCG failed.

I made just enough of a relief cut so that the hammer had a pocket to fit into and no longer made contact with that ledge on the bolt. It actually required very little material removal in that area; much less than I thought it would, so go slowly. Both my guns ran like sewing machines from the get-go and continue to after the bolt mods.
Hammer follow can be difficult to diagnose in these guns (especially over the internet), but the likelyhood of wearing that area of the hammer down far enough to inhibit disconnector engagement is about nil. That level of wear is something that I've never even heard of. You have to remember that that area with the damage is only the beginning of the area that the bolt ramp pushes on. I'd say no worries there.
I decided to mitigate the possibility of broken hammer pins with a set of KNS pins. One extra layer of protection worth looking into, IMO.

Well, I went and took my dremel to it. It felt like a bad thing to do, but it needed done. I need to go shoot it now and make sure it still runs, but I don't see why it wouldn't. The area I relieved shouldn't have any impact on anything, apart from eliminating the hammer impact. The real trick is going to be determining if I actually went far enough to resolve the hammer impacts, since my trigger already shows wear. When my Larue eventually gets here for my Grendel build, the ALG QMS trigger from that is going in the 9mm, so it should be readily apparent once that happens.

quote:

Originally posted by 92fstech:
The real trick is going to be determining if I actually went far enough to resolve the hammer impacts, since my trigger already shows wear.

Just take a sharpie and fill in the area previously damaged on the hammer, take a few (10) shots and see where the ink is removed. It's a quick and dirty method, but will give you an idea of the kind of contact you're now getting. Good luck!

good call...ill give it a try

Get a heavier and longer buffer. I use a KAW Valley 7.5 ounce buffer in my PSA AR9. Lower recoil, and hasn't broken an extractor like it did when using the buffer that came installed on the gun from PSA.

May not be the source of your problem, but your AR9 will last longer with a lower bolt speed and shorter bolt travel.

I got mine at AIM Surplus:

KAW Valley buffer at AIM