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Old December 3rd, 2018, 01:12 AM #1
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300 BLK barrel length(super sonic)

I plan on building a 300 blk upper for deer hunting as it's more cost effective for me than buying a new bolt gun. I know people build 300blk guns because they are very quiet when shot with a suppressor. That's really not my thing and I'm only going to use super sonic rounds since it will be my deer upper. Since I'm putting the upper together my self I figured I'd build it to harness the maximum energy potential of the 300 blk round.

Increasing barrel length has diminishing returns, and the information I'm finding online says the 300blk round burns all of it's powder 8-9" into the barrel. I'm guessing this is for subsonic loads, or does this hold true for super sonic rounds as well? Is it worth going longer than a 16" barrel for super sonic rounds?

Given my usage would the carbine gas length suit me better than the pistol or will the difference in dwell time not make a difference in bullet velocity? As I understand manufacturers are switching to the pistol length system so people can shoot subsonics more reliably but like I mentioned before I wont be shooting subsonics.

I'm pretty much doing the opposite of what everyone else is doing with the 300 blk platform which is making it hard to find the correct information. All my shots will be within 150 yards which is well within the capability of the round, but knowing I set it up just right for that extra 2% difference will give me that good feeling.
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Old December 3rd, 2018, 03:09 AM #2
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Go with a 16" barrel.

They are for the most part correct that most of the powder is burned within the first 8-10" of the barrel for supers or subs.

Don't bother with a carbine gas system, the cartridge isn't designed to operate on carbine gas. Get the barrel with a pistol length gas system, due to that is what the cartridge was designed for.

I built a 300BLK upper for the exact same reason you did. I took my time with it and did some accuracy enhancing things, during the build that paid off in spades. You can see the 300BLK in the thread I linked to.
https://www.mdshooters.com/showthread.php?t=213710

I'd be willing to offer advice an input, if you would want it.
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Old December 3rd, 2018, 06:58 AM #3
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If you're only going to shoot supers and cost is a concern, why not use 7.62x39, which is ballistically the same as .300AAC and WAY cheaper? The magazine situation is basically solved at this point, certainly for the small-cap mags you'd be using to hunt deer.
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Old December 3rd, 2018, 08:12 AM #4
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For deer, 6.8 spc or 6.5 grendel would both be much better options(assuming you're sticking with an ar15 platform and not just jumping up to an ar10). 300blk is really best used as a way to get good ballistics out of a short barrel and/or suppressed. If you're sticking to 16" and not suppressing it, it's not really using it for what it's good at.
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Old December 3rd, 2018, 03:28 PM #5
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Quote:
Originally Posted by smokey0118 View Post
For deer, 6.8 spc or 6.5 grendel would both be much better options(assuming you're sticking with an ar15 platform and not just jumping up to an ar10). 300blk is really best used as a way to get good ballistics out of a short barrel and/or suppressed. If you're sticking to 16" and not suppressing it, it's not really using it for what it's good at.
Agreed.
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Old December 5th, 2018, 05:31 PM #6
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.300 blkout works great for hogs
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Old December 5th, 2018, 05:46 PM #7
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Unless your 7.62x39 has a .308 barrel, there are more bullet choices for the .300 BO, assuming you reload. I agree with the 16 inch barrel and, since I have never seen any problems using supers in a carbine gassed upper, I would say it is also good to go.

IMHO, the 6.8 SPC doesn't have the drive it once had. Again, if you reload, the 6.5 Grendel will give you more versatility when it comes to hunting game.
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Old December 5th, 2018, 06:45 PM #8
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Quote:
Originally Posted by John from MD View Post
Unless your 7.62x39 has a .308 barrel, there are more bullet choices for the .300 BO, assuming you reload. I agree with the 16 inch barrel and, since I have never seen any problems using supers in a carbine gassed upper, I would say it is also good to go.

IMHO, the 6.8 SPC doesn't have the drive it once had. Again, if you reload, the 6.5 Grendel will give you more versatility when it comes to hunting game.
He doesn't reload. AFAIK
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Old December 5th, 2018, 08:58 PM #9
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It's a versatile cartridge, runs well in ARs, and plenty have built hunting rigs around it. While it was built to be efficient in short barrels, loads vary, and there is still enough pressure to accelerate a bullet past the point it is all ignited. Most charts I have seen show around 100FPS increase per inch out to around 8 or 9, then only about 50 gained for the next few inches, and as little as 10-25FPS gained as the barrel length goes beyond 12" or so. Might not be much, but they do increase velocity, and longer barrels will reduce muzzle blast and noise even without a suppessor, and of course no need for an NFA stamp or brace. The round doesn't have the pressure curve of 5.56 or most other rifle calibers, it's more like a magnum pistol, small case capacity, and faster burning powders. The short gas system helps ensure enough port pressure and dwell, even with longer barrels, you rarely need to choke down pressure as dramatically with an AGB, or increase buffer weight as often as with 5.56. Pistol gas works with all bullet weights, and even on longer barrels with supers it isn't overgassed to the degree 5.56 often is, while carbine gas can cause problems with subs, there really isn't a benefit in recoil, velocity, or reliability to justify it.
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Old December 6th, 2018, 12:50 AM #10
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Quote:
Originally Posted by John from MD View Post
Unless your 7.62x39 has a .308 barrel, there are more bullet choices for the .300 BO, assuming you reload. I agree with the 16 inch barrel and, since I have never seen any problems using supers in a carbine gassed upper, I would say it is also good to go.

IMHO, the 6.8 SPC doesn't have the drive it once had. Again, if you reload, the 6.5 Grendel will give you more versatility when it comes to hunting game.
I can find factory ammo that ranges from 110gr all the way to 220gr. Reloading allows you to utilize projectiles like the 165 SGK, 168 SMK, 175 SMK, 230 SMK, and 240 SMK. Even so, the most widely found loads for range use are 147gr or 150gr. You can find 120gr OTM at WalMart in the ammo section, too.
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