SBR won’t chamber the next round

Discussion about rifles in 300 AAC BLACKOUT (7.62x35mm), hosted by the creator of the cartridge.

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Naturalgainsbro
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SBR won’t chamber the next round

Post by Naturalgainsbro » Thu Sep 08, 2022 5:30 pm

Built new SBR earlier this year (after a helluva long ATF limbo).

I’m running a maxim SCW gen 6 pistol system on it. 10.5 inch barrel and a fostech upper/lower.

I’m having some trouble with getting consecutive rounds to chamber. You’re already thinking to yourself “this guy needs to adjust his gas block”. . . Oh but I would argue you’re wrong.

I’m pretty certain it’s not a gas issue, because the rifle cycles 100% reliably when I hold my hand under the mag, I can dump an entire mag several times in a row (even with what I thought was my crap ammo stock) however as soon as I remove my hand from forcing the mag up inside the receiver another few millimeters, it jams the round without the nose of the bullet going into the port of the barrel and the bolt slamming against the side of the brass.

Any ideas? I have multiple mags and they’re all doing the same thing. I need the magazine to be seated higher inside the receiver, but not sure if this is fixable?

PS: why’s it always the rifles you spend the most on, perform the worst? Shoutout to the $600 budget build 16 inch AR sitting inside my safe that shoots like a gem.

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BJK
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Re: SBR won’t chamber the next round

Post by BJK » Sat Sep 10, 2022 11:09 am

Since no one has gotten back to you I'll do what I can by asking questions. Sorta like asking if a light that isn't working is plugged in.

Without manipulating the mag does the bolt lock back correctly? Manipulating the mag does it still lock back? More on mags in a bit.

You stated that you have other mags, same manufacturer and bought at the same time? Or wildly different manufacturers?

The other thing that I'm thinking is that it could be something incorrectly made in the firearm in the area of the magazine, but most feeding malfunctions are magazine related. Is there any way you can compare parts and measurements from the malfunctioning firearm to the one that works flawlessly? Even eyeballing where the rounds sit to get picked up might show an issue. I'm thinking Fostech made the upper and lower correctly and I'd be surprised if they didn't. But maybe the other parts manufacturers screwed up. Like the doohickey that locks the mag in place. I'm not going to check further by opening the safe but I couldn't remember significant movement on any of my AR mags when inserted. I just checked the 300BLK and sure enough other than minor movement the mag is in there correctly and to stay in place. I could try to manipulate the mag, but it wouldn't amount to anything major that would affect the function, it moves that little.

That's all I have. Good luck figuring it out. Please let us know what was causing it.

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dellet
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Re: SBR won’t chamber the next round

Post by dellet » Sun Sep 11, 2022 8:50 am

Have you tried screwing the mag release button in one or two more turns?

Also try inserting the magazine with the bolt locked back, it could the opposite of above and too tight, not allowing the magazine to up into the well fully.
300 Blackout, not just for sub-sonics.

Naturalgainsbro
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Re: SBR won’t chamber the next round

Post by Naturalgainsbro » Wed Sep 21, 2022 3:03 am

Current status is dropped off at a gunsmith who’s going to get it going and figure out all the timing.

Hope that in another week it’s ready to chop wood.

Interesting what you said about the mag release being too tight. I have thought it needed to be backed out a revolution or two, since there’s a lot of tension on that spring.

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