Carbine buffer in a rifle buffer tube

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

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Jeepfe
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Carbine buffer in a rifle buffer tube

Post by Jeepfe »

I just put together a 300 blk AR and I am having what looks to be signs of over pressure with factory ammo, Remington UMC 120gr. No malfunctions other than the bolt won't lock back on the last round. I am using a magpul moe rifle stock and rifle length buffer tube. The gun has a 16" barrel with a carbine length gas system. I was told when I was putting it together that a rifle length buffer and spring should be fine so that's what I used (5.1 oz). About 3/4 of the fired cases show that the ejector is carving off some brass from the head as the bolt unlocks. I posted my issue on the ammo forum, subject title "over pressure or rough bolt" if you care to read it. It was suggested that I should be using a carbine buffer and spring because the heavier rifle buffer I'm using now is causing an over pressure because the heavier buffer isn't compatible with a carbine gas length. Does this sound right? I know heavier carbine buffers are common but lighter rifle buffers, not so much. I have a carbine buffer and spring in another rifle I can try but I need a spacer to use it in my rifle length tube. Does anyone know the exact length spacer I need? Spikes has one but I'm sure I can make one if I know the dimensions. Or ,any other options are welcome. Thanks
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GSO
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Re: Carbine buffer in a rifle buffer tube

Post by GSO »

Jeepfe wrote:I just put together a 300 blk AR and I am having what looks to be signs of over pressure with factory ammo, Remington UMC 120gr. No malfunctions other than the bolt won't lock back on the last round. I am using a magpul moe rifle stock and rifle length buffer tube. The gun has a 16" barrel with a carbine length gas system. I was told when I was putting it together that a rifle length buffer and spring should be fine so that's what I used (5.1 oz). About 3/4 of the fired cases show that the ejector is carving off some brass from the head as the bolt unlocks. I posted my issue on the ammo forum, subject title "over pressure or rough bolt" if you care to read it. It was suggested that I should be using a carbine buffer and spring because the heavier rifle buffer I'm using now is causing an over pressure because the heavier buffer isn't compatible with a carbine gas length. Does this sound right? I know heavier carbine buffers are common but lighter rifle buffers, not so much. I have a carbine buffer and spring in another rifle I can try but I need a spacer to use it in my rifle length tube. Does anyone know the exact length spacer I need? Spikes has one but I'm sure I can make one if I know the dimensions. Or ,any other options are welcome. Thanks
No, this does not look right. It's been a while, but I had a discussion with a Brownells gun tech who verified what I had read on the internet that a carbine upper will typically work just fine on a rifle lower (and vice versa). The design specs for the carbine were supposed to match M16/AR rifle specs in terms of spring force, deflection distance, etc.

That said, I would suggest that you may need more weight, not less. If you can get (or make) the nylon spacer, I'd suggest trying an H or H2 carbine. This would slow down the bolt carrier and result in a little more gas escaping (and lowering pressure) before the bolt unlocks. If you wanted to make one and can't find the dimensions, I have one at home and can measure it tonight.
I don't collect firearms; they just accumulate...

"Be Prepared" - My motto from an early age.

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Re: Carbine buffer in a rifle buffer tube

Post by Boolit_Head »

So this is a second thread about the same subject?

More weight may not be the best answer since I am running a carbine buffer on a pistol length gas system with no pressure signs. The 7.5 inch barrel length I have may play into that though.

The best way to solve this is to get to a known good situation and work from there. Carbine gas and Carbine buffer in a Carbine length tube is known to work. Putting a spacer in the rifle tube would be the same as a shorter tube so you could test and see what happened there. Then you could make a decision based on what you find. That should alleviate the bolt lock back issues.
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dellet
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Re: Carbine buffer in a rifle buffer tube

Post by dellet »

GSO wrote:
Jeepfe wrote:I just put together a 300 blk AR and I am having what looks to be signs of over pressure with factory ammo, Remington UMC 120gr. No malfunctions other than the bolt won't lock back on the last round. I am using a magpul moe rifle stock and rifle length buffer tube. The gun has a 16" barrel with a carbine length gas system. I was told when I was putting it together that a rifle length buffer and spring should be fine so that's what I used (5.1 oz). About 3/4 of the fired cases show that the ejector is carving off some brass from the head as the bolt unlocks. I posted my issue on the ammo forum, subject title "over pressure or rough bolt" if you care to read it. It was suggested that I should be using a carbine buffer and spring because the heavier rifle buffer I'm using now is causing an over pressure because the heavier buffer isn't compatible with a carbine gas length. Does this sound right? I know heavier carbine buffers are common but lighter rifle buffers, not so much. I have a carbine buffer and spring in another rifle I can try but I need a spacer to use it in my rifle length tube. Does anyone know the exact length spacer I need? Spikes has one but I'm sure I can make one if I know the dimensions. Or ,any other options are welcome. Thanks
No, this does not look right. It's been a while, but I had a discussion with a Brownells gun tech who verified what I had read on the internet that a carbine upper will typically work just fine on a rifle lower (and vice versa). The design specs for the carbine were supposed to match M16/AR rifle specs in terms of spring force, deflection distance, etc.

That said, I would suggest that you may need more weight, not less. If you can get (or make) the nylon spacer, I'd suggest trying an H or H2 carbine. This would slow down the bolt carrier and result in a little more gas escaping (and lowering pressure) before the bolt unlocks. If you wanted to make one and can't find the dimensions, I have one at home and can measure it tonight.
This would not take into account wanting to run subs and the gas volume issue do to the different bore size of the 300.

I think he has two separate problems. One causing the pressure signs, the other keeping the bolt from locking back on empty.

A 5 OZ buffer instead of the 3 OZ it was designed to run with. Depending on port size it could work for supers, doubtful for subs. For the cycling issue.

The Half moon pressed into the brass he says he has, is a pressure problem.

This was not a clear explanation by me in the other thread.

The correct buffer should cure the not locking back.

There will still be a firing pressure issue to deal with.
300 Blackout, not just for sub-sonics.
Jeepfe
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Re: Carbine buffer in a rifle buffer tube

Post by Jeepfe »

Thanks everyone for the input. It was a tech from Brownells that led me to purchase the rifle length buffer and spring for this build. I've got a spacer on the way and I'm going to try a carbine buffer and spring from another one of my rifles, hope it works. I really don't plan on using subs unless the Hearing Conservation Act goes through and I don't have to go through the ass pain of the dreaded tax stamp! If I can walk in the store and buy a suppressor at a decent price and walk out then I'll get into the subs.
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GSO
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Re: Carbine buffer in a rifle buffer tube

Post by GSO »

Jeepfe wrote:Thanks everyone for the input. It was a tech from Brownells that led me to purchase the rifle length buffer and spring for this build. I've got a spacer on the way and I'm going to try a carbine buffer and spring from another one of my rifles, hope it works. I really don't plan on using subs unless the Hearing Conservation Act goes through and I don't have to go through the ass pain of the dreaded tax stamp! If I can walk in the store and buy a suppressor at a decent price and walk out then I'll get into the subs.
The one problem with this is that hundreds of thousands (a very rough SWAG) of people are thinking the same thing. If you are first in line at the store on the day it takes effect, then maybe. Or if you are a priority customer at a dealer somewhere. Currently there are about 8,000 SOT's handling all of the sales. If the HPA makes it out of committee (a critical event), it will most likely pass and then upwards of 450,000 FFL's will then be selling. I got an e-mail from a distributor that only does NFA products in December that estimated inventory would be sold out in 24-48 hours after it was effective. I think that's optimistic. Assuming websites don't crash and telephone lines explode, I'm predicting 4 hours tops. Manufacturing will catch up eventually, but I think it will be like AR15's post-Sandy Hook. And I believe those predicting huge price drops are oblivious to market forces. Sure, not all 55 million (guestimate) gun owners in the US will run out and buy a can right away, but supply will run dry quickly and so prices are likely to rise due to demand. Just my $0.02.
I don't collect firearms; they just accumulate...

"Be Prepared" - My motto from an early age.

Greg
Boolit_Head
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Re: Carbine buffer in a rifle buffer tube

Post by Boolit_Head »

Not only purchasing but making your own will be effected by the HPA. I spent about $150 building one that is a very good performer. If it passes there will undoubtedly be a rush on both prebuilt suppressors and parts to make them.
emmagee1917
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Re: Carbine buffer in a rifle buffer tube

Post by emmagee1917 »

I , too , vote for getting it now . First , there is no idea if or when this might happen . The current paperwork is no more a chore than the standard gun form you'd have to fill out anyway . AZ CCW holders have to be fingerprinted , so my LGS does that on site and for free . The $200 stamp will be small change compared to the higher shortage prices and the wait time may be as much or greater than the paper delay now . Also , there is a passage in the law that any tax paid from 2015 -ish will be refunded . If that stays in , you'll get your money back . If the law passes before you receive approval you just go in , cancel the paperwork , and pick up your suppressor . It looks like a win all the way around .
Chris
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GSO
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Re: Carbine buffer in a rifle buffer tube

Post by GSO »

Boolit_Head wrote:Not only purchasing but making your own will be effected by the HPA. I spent about $150 building one that is a very good performer. If it passes there will undoubtedly be a rush on both prebuilt suppressors and parts to make them.
This will be a good deal for those with the skills, tooling and machines to make them. But most folks won't be able to build one themselves. I saw a post somewhere (not here) where the author suggested every machine shop in the country will be making them...which absolutely will not happen. Even if removed from the NFA, suppressors will still be treated as firearms and those selling/manufacturing must get the FFL. It would be interesting, however, if machine shops were able to churn out parts kits that enabled the average "gun plumber" who can put together an AR to also be able to manufacture his own suppressor... Pistol and rimfire cans should be easy; sealed rifle cans which typically require welding might be in the too hard category.
I don't collect firearms; they just accumulate...

"Be Prepared" - My motto from an early age.

Greg
Boolit_Head
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Re: Carbine buffer in a rifle buffer tube

Post by Boolit_Head »

Rifle cans don't have to be sealed. Mine is not, it threads together. With the formed freeze plugs and cups clearly becoming legitimate after HPA it may make things much easier as it only leaves drilling the bore to make it functional.
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