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Old February 4, 2019, 10:34 PM   #1
2ndchance
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5" 300BLK barrel with suppressor?

A friend of mine brought this to my attention and it was a bit of a head scratcher to me. He showed me a pic of a 300BLK AR platform using a 5" 1:7 barrel, running a SilencerCo Omega 9K suppressor. By design I'm guessing it's suppose to run subsonic only. Still, I was always under the impression that you need about 9" of barrel to properly cook off all the propellant. With a 5" barrel pushing a 208-220gr projectile at maybe 900fps, isn't that going to cause a bit of wobble? Baffle strike?

Plus, Pmags for 556 doesn't like feeding fat/subsonic 300BLK ammo, too.

What say you?

https://www.tactical-edgearms.com/pr.../rd300blkt.htm
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Old February 5, 2019, 07:29 AM   #2
Bartholomew Roberts
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Eh, if you are going to shoot subsonic only from a 5” barrel, might as well go 9mm or .45. I’ve never had any issues feeding 220gr (SMK or Rem FB) or 208gr (AMAX) in PMAGs, although thankfully, there are a lot more options than 220gr or 208gr for .300 subs now

I can’t imagine what accuracy looks like with a 1/7 twist on a 5” barrel for the longer rounds.
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Old February 5, 2019, 07:56 AM   #3
Mobuck
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"Plus, Pmags for 556 doesn't like feeding fat/subsonic 300BLK ammo, too."

In my experiences, this is not an issue. We use 10 round P-Mags for hunting and have not had any problems. Over length seating may cause hangups (with any magazine).
We have a "Harvester" can that is not rated below 10.5" w/subsonic and 16" super. I think they're be overly conservative but not going to risk damage just to find out. I now have a can in ATF jail that is rated for super sonic in a 10.5" as the performance of subsonic hunting ammo is unimpressive.
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Old February 5, 2019, 01:55 PM   #4
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I don't see the gas impulse being all that different with 200+ gr subs than I do with a 7.5" 5.56 barrel running standard ammo. Gas port size is a variable that can be toyed with to fine tune it, too.
It'll probably be loud without the can, though. And always messy.

Bullets being unstable? Nah. Barrel length doesn't matter there. If you can reach the velocity necessary to stabilize with any given twist rate, the bullets are stable. It's that simple.
The goal is rotational velocity. To achieve the desired rotational velocity, all you need to do is match twist rate to muzzle velocity. The higher the muzzle velocity, the slower the twist rate can be. The faster the twist rate, the lower muzzle velocity can be.
Barrel length matters not.

The 5" barrel doesn't matter much for propellant burn, either. My go-to subsonic load is estimated by QuickLoad to only burn about 91% of the powder by the time the bullet leaves the muzzle at 10.5". If I change barrel length to 5", propellant burn only drops to 82%. Yes, there's a difference. But it's not huge. .300 Blk subs are, essentially, handgun cartridges. And, handgun cartridges, in general, burn 70% of their powder in the first 3 inches of barrel. Nearly all handgun cartridges, including 'magnums' with slow powders, burn 70-80% in the first 4 inches.

---
9" inches is not enough for full burn. For 100% burn with most subs, you need a barrel in excess of 24". That's one of the 'unfriendly' byproducts of running low pressures with subs. Low pressure results in lower burn efficiency, which means even more barrel length is needed for full propellant burn than with full pressure [generally supersonic] loads.

If I take my go-to load again, as an example: It uses 10.5 gr H110 and only makes about 20k psi. 99% propellant burn takes 27 inches of barrel (at which point, the bullet is actually slowing down from bore friction!). But if I increase the powder charge to 15.3 gr to achieve maximum SAAMI pressure, the load, of course, goes supersonic. The bigger change, however, is efficiency. At max pressure, that load will have 98% propellant burn at 5", and almost 100% burn at 6.5".
Big difference.
---

PMags?
.300 Blk is one of the two-and-a-half cartridges that I actually use PMags for (the others being .223 Rem and 5.56x45mm). They don't play nicely with my fat cartridges like .475 Tremor, nor 6x45mm or .17-223, nor some round-nose style bullets in .300 Blk. But they generally work just fine with long-nosed spitzers, VLDs/ELDs, and bullets designed specifically for the cartridge.
Without .300 Blk coming into the mix, I would have given away all but the 20-rounders by now.
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Old February 8, 2019, 08:46 AM   #5
Bartholomew Roberts
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At the risk of being pedantic, you say the twist rate needs to match the muzzle velocity to achieve the proper rotational velocity and then say barrel length doesn’t matter. 1:7 twist is common in 8-9” .300 barrels. Going to a five inch barrel and not changing the twist is going to reduce muzzle velocity and not spin a long bullet any faster.
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Old February 8, 2019, 12:04 PM   #6
FrankenMauser
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Barrel length does not matter, so long as the muzzle velocity is adequate for the twist rate. (Or the twist rate is adequate for the muzzle velocity.)

A 1:7" twist will stabilize a 1.54" .30 caliber bullet at velocities lower than 250 fps. Even a 2" long bullet would be stable at 500 fps.

In this context, barrel length does not matter.

Many modern barrels have twist rates that are unnecessarily aggressive. This is one of those cases.
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Old February 8, 2019, 12:41 PM   #7
Bartholomew Roberts
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Interesting that AAC dropped its 1:8 twist for 9” barrels and went to a 1:7 twist for 9” barrels then. Based on what you are saying, a .30 bullet that fits in an AR15 magwell and doesn’t stabilize in a 1:8 9” barrel doesn’t exist. So why change?
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Old February 9, 2019, 07:22 PM   #8
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As to bullet stabilization, it will probably not be an issue. You are still getting roughly similar velocities, as .300blk subs usually use fairly quick-burning powder.
Sig already makes a 5.5in barreled .300blk. That said, it will likely run dirtier, as there will be more unburned powder.

PMags with heavy .300blk bullets can be problematic. I had some issues with the Remington UMC 220s. Magpul adjusted the internal dimensions slightly, and now sells a .300blk optimized version of the PMag, if that interests you.
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