After a 50-week wait, I’m the proud and happy owner of a 12.5” SBR. It is a 12.5” BCM upper with Rainier Arms BCG on a Spikes lower with Spikes T2 buffet. I have not ran it with a suppressor and do not intend to. I’m having problems with the bolt locking back on the last round. I’ve been using Magpul magazines. So far, I’ve only had success with Magtech 62gr and my home load (75gr Hornady BTHP over 23.5gr H4895). M193, Privi Prartizan 62gr and Hornady Black 62gr 5.56 have not locked it back. I’m thinking it’s a buffer issue but I’m not sure. The Search button returned many results but nothing relevant.
The ammo you listed should definitely make it run. The first thing that I’d try is installing a lighter buffer.
Hornady black is actually pretty week. You may very well need a lighter buffer for that.
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Based on the build I think the easiest thing to try is a different buffer. Are you running a standard carbine buffer spring?
See if you can get a standard carbine buffer. 1 round per mag, and test for lock back. If it does, then you can try an H buffer. If it continues to function i’d probably stop there. A Spikes T2 buffer is 4oz from my googling, so it’s inbetween an H and an H2 buffer.
Past that, another BCG could be a good test too.
A BCM 12.5 should run PMC bronze with a H3 easily. It’s gassed pretty heavily for a 12.5 carbine. Seems something else is going on unless you have some crazy heavy spring.
Cant hurt to give it time to break in a bit.
Makes me wonder if you might actually have a gas leak. Either an improperly attached gas key or the gas block could be crooked, blocking some of the gas port. Or maybe even the gas block could be out of spec and allowing gas leakage.
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I have about 100-150 rounds through it, so maybe a few hundred more and a reassessment from there is needed. I’m running a standard carbine spring. I think the next step is a carbine buffer and see if that helps.
Do you have any other ARs to help diagnose with?
The first step is a static check of the bolt catch function.
Does it lock back on an empty mag when manually cycled?
Does the over travel distance look correct?
Thats odd. I have that same upper, and without the can I get bolt lock back with 55 grn ball. Using an H1 buffer and standard spring.
I think it’s the buffer. I have a standard spring, but I think the Spikes T2 buffer is just too heavy.
I can assure you it’s not. I have the same upper and it runs 100% reliably for THOUSANDS of rounds with an LMT E-Carrier and an A5. Before the A5 was used, it ran with an H3 and blue Sprinco spring.
You have something else happening. I would start by using a different buffer standard H2 or even H3 to start. After that I would be looking closely at that BCG.
I picked up a standard weight buffer today, I plan on hitting the range Sunday with some different ammunitions to test it out. What on the BCG would I need to be looking for?
Update: hit the range yesterday with standard carbine buffer installed. Brought five rounds each of Privi 62gr FMJ, Magtech 62gr FMJ, 75gr home load, and 80 assorted 55gr FMJ. All ran flawlessly and the bolt locked back after each 5-10 runs mag ran. Less than 100 rounds isn’t exactly a great sample size but it’s looking better than with the Spikes buffer.