GASSING questions.

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

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Lead Head
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GASSING questions.

Post by Lead Head »

Hi all:
Am getting down to the wire with putting together my Blk build.
I get it that this round has basically pistol pressures and the gas system has to accommodate that.
To get to the point. The barrel I have has a gas port that measures .115 to be exact. This is larger than the ports on my AR .223/5.56 barrels which measure .065 and .070.
SO, the port on the Blk barrel is larger. Does the .115 hole seem adequate? That's one question.

Another is about the buffer spring. I've read where a lighter spring should be gotten as the pressure for the Blk is less.
OR is the lighter spring only for subsonic?
Do any of you use softer buffer springs for supersonic? I have no intention of ever shooting subsonic.
I'm trying to get all the details dealt with as I don't want to get to the range and have problems.

Thanks for replies
Freedom1973
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Re: GASSING questions.

Post by Freedom1973 »

Lead Head,
What are you going to be using the build for? What is the barrel length and the length of the gas system? I would not run lighter spring in a supersonic only gun. We need more information, what buffer weight and bolt carrier group do you have as well. Whatever else you could supply will help.
Freedom
Lead Head
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Re: GASSING questions.

Post by Lead Head »

Hi Freedom:
The build will be for pig hunting. And I don't anticipate anything really big.
The barrel length is 16" with a 1-8 twist.
As far as the "gas system", the length of the entire gas tube is like 9 1/2", which I suppose is considered a mid-length setup.
The buffer weights are 3 weights of 1 oz each; so 3 oz.
The BCG is an Abrams Arms.

All of what I'm using except for the barrel is what was on the AR I had that was rigged for a .223/5.56.
And that's all I can tell you. Got a Velocity trigger on it but that wouldn't have anything to do with going over to a Blk.

Thanks for any input
golfindia
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Re: GASSING questions.

Post by golfindia »

300blk SAAMI pressure is 55k psi.
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dellet
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Re: GASSING questions.

Post by dellet »

If the gas tube is 9 1/2” then it is carbine length, and the port is bigger than it needs to be.

Stay with a standard spring and if all you plan on shooting is supers, you might want an H2 or even H3 buffer.

You may already have an H2, total weight would be about 4.5 ounces, if your weights total 3 it’s possible. Standard carbine would be 3.8 total.

Other option would be an adjustable block if you want to tame the recoil a bit.

It will run they way it is, just not optimal.

As for pressures, most subs run relatively low pressure, most supers run the same as most 223 Ammo.
300 Blackout, not just for sub-sonics.
Lead Head
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Re: GASSING questions.

Post by Lead Head »

Thanks for the reply.
Regarding the buffer weights. Not long ago on one of my .223 ARs, I added an extra ounce to the buffer weights. Took out one of the 1 ouncers out and replaced it with a 2. I did so in an effort to soften the recoil a bit and figured that it would slow the action down on the BCG thereby making it a bit more reliable. That was my logic anyway. Didn't work. All it did was produce slam fires.
It's amazing what kind of looks you get at the range when that happens. Anyway, I put it back to the 3 one oz weights and it works fine.
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dellet
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Re: GASSING questions.

Post by dellet »

Lead Head wrote:Thanks for the reply.
Regarding the buffer weights. Not long ago on one of my .223 ARs, I added an extra ounce to the buffer weights. Took out one of the 1 ouncers out and replaced it with a 2. I did so in an effort to soften the recoil a bit and figured that it would slow the action down on the BCG thereby making it a bit more reliable. That was my logic anyway. Didn't work. All it did was produce slam fires.
It's amazing what kind of looks you get at the range when that happens. Anyway, I put it back to the 3 one oz weights and it works fine.
I’m skeptical that the change in weight is the actual reason for the slam fire.
300 Blackout, not just for sub-sonics.
hardcase
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Re: GASSING questions.

Post by hardcase »

Lead Head wrote:Thanks for the reply.
Regarding the buffer weights. Not long ago on one of my .223 ARs, I added an extra ounce to the buffer weights. Took out one of the 1 ouncers out and replaced it with a 2. I did so in an effort to soften the recoil a bit and figured that it would slow the action down on the BCG thereby making it a bit more reliable. That was my logic anyway. Didn't work. All it did was produce slam fires.
It's amazing what kind of looks you get at the range when that happens. Anyway, I put it back to the 3 one oz weights and it works fine.
I think the slam fire most likely would be due to the trigger or trigger finger.
Lead Head
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Re: GASSING questions.

Post by Lead Head »

The reason or reasons for slam firing are interesting.
At the time that I experienced it with my AR, another guy at the range had the same problem. A few of us were talking it over as to the cause and the other guys thought that the extra weight of the buffers put too much momentum on the BCG when it went forward to chamber another round.
Thing is, their theory ( and it does make sense ) is that the firing pin in the BCG is free floating ( at least to a point ) and the extra momentum caused the firing pin to gain extra force going forward and that was enough for it to set off the next round after it had been chambered.

Just saying
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dellet
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Re: GASSING questions.

Post by dellet »

Lead Head wrote:The reason or reasons for slam firing are interesting.
At the time that I experienced it with my AR, another guy at the range had the same problem. A few of us were talking it over as to the cause and the other guys thought that the extra weight of the buffers put too much momentum on the BCG when it went forward to chamber another round.
Thing is, their theory ( and it does make sense ) is that the firing pin in the BCG is free floating ( at least to a point ) and the extra momentum caused the firing pin to gain extra force going forward and that was enough for it to set off the next round after it had been chambered.

Just saying
There are a whole bunch of AR’s, M4’s and A2 stocks running around with much heavier buffers than 4oz that don’t slam fire. Of all the things that could cause one, buffer weight would be very low on the list, if it was on it at all.

Operator error, mechanical defect, faulty ammo would be the order of most likely I would normally suggest.

Since it happened to two different shooters, two different rifles, and likely two different ammo manufacturers, it’s clearly either the range or aliens causing the problem.

Just saying. :P
300 Blackout, not just for sub-sonics.
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