I've got a 6.5 Grendel build that I really like but I've had some growing pains with it that I haven't dealt with in other AR builds.

It shoots excellently, with 120 grain Federal Fusion. Best group to date:




Without going to deep into the background, the rifle was overpassed, especially when suppressed, so it now has a Superlative Arms Bleed Off Gas Block and functions well.



So, with both steel case and brass cased ammo (Federal Fusion), the bolt catch will occasionally catch the front of the bolt carrier and not the face of the bolt lugs.




Details:
--I have a standard carbine spring and a regular H buffer in the gun now.
--Magazine in photo is an Alexander Arms 24 round. It has also happened with a 17rd AA mag as well as an ASC 24rd magazine.


What's the fix? Do I need to swap in a T2 buffer? Heavier?

Other thoughts?

Sounds like now you're slightly under-gassed.

Adding weight will probably just make it worse.


That's not the worst thing in the world. Heck, just more lubrication and/or break in may fix it. If it were mine.. unless I planned on taking the rifle into battle or using it in "dynamic" competition (reloading on the clock), I wouldn't worry about it.

quote:

Originally posted by cas:
Sounds like now you're slightly under-gassed.

Adding weight will probably just make it worse.

I agree.

Sorry, forgot to mention that this was after the rifle was tuned for unsuppressed ammo with the adj gas block AND continues to happen even when suppressed.

It also occurred before adding the adj has block which makes me think the bolt is moving too fast.

The adj gas block was primarily needed because extraction was happening so fast that the case was striking the rear of the ejection port, occasionally causing the spent case to bounce in front of the bolt. After cleaning up the extractor and adding the gas block, brass is reliably landing at 3 o’clock suppressed.

I even took slo-mo video of extraction to see what was happening!

If your brass is going forward it might be going too fast.


While it's only a sample of two, both my "race guns" with lightened setups eject forward and always lock back proper.
I wouldn't call 3 o'clock horrible.

Too much spring on the bolt catch or bolt catch too tall? I have seen different catches.

Thanks beltfed.

I measured the height of the bolt catch when activated by an empty magazine with the upper off and it was within a few hundredths of another AR. I did swap it for another bolt catch to see if that helped...no joy.

Another data point: it has happened with two different lowers...a new build and a well used SBR lower, same buffer weights.

That leaves buffer weight or magazine as the cause...unless I’m missing something.

Try a regular carbine buffer.

quote:

Originally posted by Chowser:
Try a regular carbine buffer.

Easy enough. Will try one this next range trip.

quote:

Originally posted by cas:
Sounds like now you're slightly under-gassed.

Adding weight will probably just make it worse.


That's not the worst thing in the world. Heck, just more lubrication and/or break in may fix it. If it were mine.. unless I planned on taking the rifle into battle or using it in "dynamic" competition (reloading on the clock), I wouldn't worry about it.

That's the thing. While the primary use right now is hunting, I hope to shift to using it as my primary HD SBR (it has a 12.5inch barrel) after I get all the bugs worked out and figure out the most reliable mags.

quote:

Originally posted by beltfed21:
Too much spring on the bolt catch or bolt catch too tall? I have seen different catches.

I wasted hours one day trouble shooting a friends gun that wouldn't lock back. With two uppers it was fine. With a 3rd it was not. So we SWORE it was that one upper. After much time, parts swapping and cursing, out of final desperation we determined that for seemingly no reason what so ever, his bolt catch just didn't like that upper!?! We swapped in a new one and all three uppers worked fine.

I have a 12” Grendel running with a carbine buffer and superlative arms bleeder gas block. No issues suppressed or unsuppressed. Only issues are mags.
The Elander mag from Alexander Arms is the only magazine I have no issues with so I will buy more of those.

quote:

Originally posted by cas:

quote:

Originally posted by beltfed21:
Too much spring on the bolt catch or bolt catch too tall? I have seen different catches.

I wasted hours one day trouble shooting a friends gun that wouldn't lock back. With two uppers it was fine. With a 3rd it was not. So we SWORE it was that one upper. After much time, parts swapping and cursing, out of final desperation we determined that for seemingly no reason what so ever, his bolt catch just didn't like that upper!?! We swapped in a new one and all three uppers worked fine.

Mine has the same symptoms of the OP, and after replacing a couple LPK parts, determined I somehow ended up with a fixed stock buffer spring (too long) so the bolt couldn't retract far enough to engage the catch. Fed & functioned fine, but had to manually charge the first round of a magazine until I replaced it with a carbine spring.