300 BLK barrel length compared to 6.8 and 5.56mm

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

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este
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Re: 300 BLK barrel length compared to 6.8 and 5.56mm

Post by este »

tank wrote: With an 8" or 9" AR upper in 300 BLK, you can now have a silenced weapon that's as short as an unsilenced M4, yet it can hit just as hard, or harder.
And less gas in the face, and less noise, and more ammo selection, and less cyclic rate change, and easier on silencers.... (... and?)

It's definitely the one most worth SBRing.
dtwowhtwow
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Re: 300 BLK barrel length compared to 6.8 and 5.56mm

Post by dtwowhtwow »

Ok, so here’s maybe a dumb question from a newbie.
Is there much gain going from a 16” to 18-20” barrel in the 300Blk? I don’t yet have one but am very interested because of the AR15 form factor (same lower, mags, BCG, brass, etc). In trying to get long range reach out and knock em down, the only other option is the .308 AR10 but now you’re talking another form factor that’s not interchangeable with anything else. I know one of the primary reasons for the 300Blk is in suppressed mode but can you also get long distance knock down with a longer barrel and how well does that function?
I’ve not seen any 300Blk uppers longer than 16” and was wondering the reason.
Thanks.
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300Blk
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Re: 300 BLK barrel length compared to 6.8 and 5.56mm

Post by 300Blk »

If you wanted a barrel longer than 16 inch in 300BLK, that is like wanting a barrel longer than 24 inch in 223. Those are the sweet spots for each. The fact that people use 223 and 5.56mm in 16 inch and shorter barrels is a massive compromise. Not so with 300 BLK.
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talonxracer
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Re: 300 BLK barrel length compared to 6.8 and 5.56mm

Post by talonxracer »

I swear my Encores barrel became more accurate when I went from 16" to 13.5"
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300Blk
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Re: 300 BLK barrel length compared to 6.8 and 5.56mm

Post by 300Blk »

More likely the crown than being shorter.
badkarmaiii
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Re: 300 BLK barrel length compared to 6.8 and 5.56mm

Post by badkarmaiii »

Shorter barrels are stiffer for the same diameter.
Could also be a harmonic effect.

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Re: 300 BLK barrel length compared to 6.8 and 5.56mm

Post by Trinidad Bill »

Thanks for sharing the data. The information is great.
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Re: 300 BLK barrel length compared to 6.8 and 5.56mm

Post by nolwark76 »

Caution- I'm going to beat a horse that's been dead awhile.

Looking at the OP that Robert posted, I am curious about something. Yeah, yeah, at some point you have to end the barrel and use the thing. Not going to contest the 16" as a great length. It looks like going down to a 14" loses 60 fps and going up to 18" gains 50 fps.

If the goal is maximizing the supersonic range of the 300 BLK, and I'm only concerned with sub sonic ammo cycling (barrel length might push it above sound barrier?), is 18" really a waste?

Shooting a bullet faster doesn't automatically mean better performance, my .308 bolt gun does better with my handloads loaded slower than the max safe charge, much better than the hotter superformance Hornady ammo.

I'm just trying to learn. I've got a BLK SBR, 16", and would like one for mid range 500-600+, and am leaning towards trying an 18". Because I've got the other two rifles, the 18" doesn't need to keep sub sonic rounds below the sound barrier, just go bang. If I could get another 50 fps and only add 2" of barrel, I think I'm good with that. Especially since losing 2" drops me 60 fps for that round.

I would probably opt for the 125 smk so it will be slower than the speeds of the 110 obviously.

It's just kind of fun to do things outside of the primary goal of a caliber, just for fun. I'm load testing 195 gr bthp over 47.5 grains of Reloader-17 in my 20" barreled .308, because it can. So far I'm getting just over 2500 fps without pressure signs. These are loaded for single feed and are showing promise.
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Jim Timber
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Re: 300 BLK barrel length compared to 6.8 and 5.56mm

Post by Jim Timber »

I'm getting much better accuracy out of 150gr bullets than many folks are seeing out of 125's. Length of bullet has a factor there - the longer (heavier) bullet has less distance to jump into the lands. It's also possible that the harmonics are such that a heavier/slower bullet is a better match for consistency. Then you get into over rotation, and things get even worse for the little speed demons.

Accuracy is the product of consistency. The barrel is going to "wiggle" as the pressure and shock wave travel through it. Pushing a load to just short of case failure is not likely to be the most consistent (often, it's detrimental to accuracy). If you need ultimate ft/lbs for hunting ethics, you might have to give up some accuracy.

The 300BLK's throat is a compromise for being able to eat big heavy bullets for subsonics, and still being able to run the lighter supers. It's not a tight benchrest chamber. Accuracy will take more trial and error than a less versatile cartridge which operates in a smaller window. There's no free lunch.
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Unobtanium
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Re: 300 BLK barrel length compared to 6.8 and 5.56mm

Post by Unobtanium »

300Blk wrote:I am thinking the sweet spot is 16 inches - equal to the efficiency of a 20 inch 5.56mm and yet has 100 more ft-lbs of energy than a 20 inch 5.56mm.

I would say most people should want a 16 inch if they think 5.56mm should be 20 inches.

A 9 inch 300 BLK matches an M4 (14.5 inch).
I prefer at LEAST a 10" barrel because I prefer AT LEAST a 9" rail surface for weapon handling purposes. I also like running things suppressed, and with a full-size can, that makes it equal a 16" carbine with FH.
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