First build 9mm AR question

[Laidback]

Hey guys, I got done with my first build a few days ago and got to go to the range yesterday. I loved building it. So much fun to create something and then watch it work! I got through 92 rounds (almost 3 32 round mags) and then noticed that my bolt catch had broken off. I took it over the the "gunsmith" and he said that it looked like my hammer was hitting the bolt stop. The bolt stop sheared off inside of the upper/lower area and not at the pin location. It just snapped off inside. I had actually had two receivers and LPKs so I looked at the other one and the hammer did end up resting on/hitting on the top of the bolt catch. The LPKs came from CORE 15 here in FL and they have a pretty good reputation here. The gunsmith said that the small lip on the bolt catch may be what is causing the hammer to hit. I am not sure where the hammer is supposed to rest. Any help would be great.

I know there may be other forums to ask this question but I like this place better!
Thanks again!

 

[BigtimeAub]

This may be a dumb question, but are you using a 9mm trigger group or a standard 5.56 trigger group? I'm sure others with much more knowledge will chime in soon to be of more help.

 

[Laidback]

I asked to make sure that the hammer would work for a 9mm build before I bought the LPKs. Normally the only problem I saw as I was getting the information for the build was that if the bolt was not ramped for 9mm build. The whole upper is a CMMG 9mm upper so it is ramped. It wouldn't really matter as nothing in the lower would cause the hammer to hit something it is not supposed to due to the upper being a 9mm. I could put a 5.56 on there and the hammer is still where it is when I take the upper off.

 

[BigtimeAub]

Thanks for the explanation. I'm hoping to start a 9mm build for myself in the very near future and am just trying to learn what I can, however I can. Thanks again!

 

[ORD]

Let's break this down a bit more before blaming the hammer which isn't the culprit at all (the hammer isn't touching the bolt catch when the upper is installed and the bolt/carrier is in place...it only does that when the upper is not installed unless something is WAAY wrong with the lower/hammer/etc. Otherwise, when the rifle/carbine is fully assembled, the hammer contacts the firing pin/recess area on the bolt carrier.):

1) what mag block are you running?

2) what bolt catch are you running (the 9mm or 5.56 from the LPK...seemed like the 5.56 from the LPK, but just making sure)?

3) what buffer/spring/stock setup are you running?

4) what ammo were you running?

Breaking bolt catches on 9mm AR-15 type rifles is a well-known issue (and make no mistakes...it happens on the run-of-the-mill 5.56 rifles/carbines as well sometimes). When they break, it is usually in one of two places: a) most commonly in/around the hole where the bolt catch pin fastens the catch to the lower receiver (caused by there being too much play in the catch which when struck by the 9mm carrier, shears off at its weakest/thinnest point); and b) at the point where you are describing generally caused by waaaay too much travel (overtravel) in the shorter, heavier bolt assembly which builds up considerable momentum/force, slams into the catch and breaks it off at the point you described.

The problem can be found due to several different factors, including: a) poor quality parts to begin with (I don't know Core's reputation or from where they are sourcing their parts for the LPKs, but its possible it was a fluke due to quality issues); b) improperly configured 9mm AR, including using 5.56 and 9mm parts in an incorrect mix depending on how the system is setup with your other parts such as the carrier, mag block, hammer, etc.; and c) most commonly, using a buffer/spring/spacer/stock combination that is too weak/light/short/etc. which in turn permit the aforementioned damage.

The answers to the above questions should help narrow things down a bit.

 

[kaotic504]

Try a heavier buffer, or adding one of the Spikes 9mm spacers, and a stronger spring. It could have been a bad heat treatment on the bolt catch. But either way, either a 9mm buffer (longer and heavier than a standard carbine buffer), or a heavy buffer with the 9mm spacer (or quarters, whichever you prefer) are pretty standard updates when running a 9mm AR. You may chose to sway out your buffer spring as well, but I haven't had to do that.

Edit: it's not the hammer, if it was a hammer issue, you would be having a lot more issues getting it to fire consistently. But a tip to anyone that hasn't built a 9mm AR, never use a notched hammer.

 

[Laidback]

I made a few adjustments and just got back from the range. I got through the 3 mags with no problems.

I am running a Spikes Magwell Block.
Standard bolt catch
Spikes 9mm buffer, Wolfe extra power buffer spring, pistol buffer tube (until I get my paperwork back) and $2.75 in quarters for spacer.
Ammo was PMC FMJ.

After looking things over I may have had the magwell block too high...not too sure really. The bolt catch broke not at the end of the magazine when you would think it would break (although I did change the spacers number to be just past when the bolt would catch, added 4 quarters to make this happen) but it broke when I had 4 or 5 rounds left in the mag.

 

[kaotic504]

You don't need the spacers/quarters if you have their ST9x buffer, it's longer already.

 

[ORD]

You don't need the spacers/quarters if you have their ST9x buffer, it's longer already.

+1 on this. The Spike's dedicated 9mm buffer (the ST-9X model) is essentially the length of a standard AR-15 carbine buffer, PLUS the length of the spacer added to it, and designed specifically to eliminate the need for the spacer(s). Adding spacers to a lengthened buffer can cause problems...not likely to fix them.

I made a few adjustments and just got back from the range. I got through the 3 mags with no problems.

I am running a Spikes Magwell Block.
Standard bolt catch
Spikes 9mm buffer, Wolfe extra power buffer spring, pistol buffer tube (until I get my paperwork back) and $2.75 in quarters for spacer.
Ammo was PMC FMJ.

After looking things over I may have had the magwell block too high...not too sure really. The bolt catch broke not at the end of the magazine when you would think it would break (although I did change the spacers number to be just past when the bolt would catch, added 4 quarters to make this happen) but it broke when I had 4 or 5 rounds left in the mag.

The Spike's magwell block (along with allowing you to run more standard parts) is a good product all around any my personal preference aside from the Hahn dedicated blocks, but as with all "adjustable" magwell blocks for 9mm conversions...it does need to be "fine-tuned" or otherwise adjusted properly to ensure proper reliability/function (and prevent broken components like bolt catches). That very well may be at least part of the issue.

As for your buffer/spring/stock setup...again...see above, but I'd keep the change in your vehicle's cupholder for tolls and OUT of your receiver extension if you are indeed running the dedicated 9mm buffer from Spike's. The Wolff XP spring is the same I run in ALL of my 9mm carbines, so I doubt that's an issue (unless its an issue in conjunction with the spacers).

With the catch breaking with rounds still left in the mag, I return to my "poor part" comment previously made. While the Core stuff may be ok...it could be a simple "bad apple" in the bunch that failed at a weird time.

As for the ammo...I'd skip the PMC in the future not because it is bad or too hot, etc., but because it is just filty @#$%ing dirty stuff. If you are going to run your SBR 9mm suppressed, it'll need cleaned pretty quickly because of all the damn fouling it will create.

 

[Laidback]

I am using the Spike's Heavy tactical buffer. I now remember that I went with this and the newly learned "quarter trick" because I could not find ANYONE who had the Spike's spacer. Not even at a jacked up Gun Broker price. It just wasn't out there.
It will be suppressed eventually. What are you using on your set up, ORD, if you are running suppressed? I am still looking around. Leaning towards SWR Octane 9 so far.
I really wish I knew what caused the bolt catch to break off. Is it normally under any pressure or stress during use until the magazine follower pushes it up (or my magwell block follower extension...thingy. Not sure of its name)? If not, if the magwell was properly positioned then it would have no reason to break when it did. So if that is the case good ol' logic would tell me that I have the magwell block too high in the receiver and it ended up pushing the bolt catch up when it wasn't supposed to be up...then even at that...shouldn't it have just stopped it like it normally would if the mag was empty...I dunno. I have this pistol set up for now until the SBR paperwork comes back for my other receiver then I will just switch the upper over and go from there. So this is kind of a test run.
Any other things that you guys found helpful when putting stuff together with the 9mm platform? Starting to get info on triggers and optics. I think an entry level EOTech should be good. This isn't a long range weapon. I will start getting stuff together for a .308 build when things get back closer to normal or get a 300 Win Mag to go even further later.
Thanks for all your help and insight!