What makes a cartridge optimized for short barrels?


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Skribs
January 3, 2013, 05:44 PM
I've read that some cartridges were designed to work better in short barrels than the cartridges they are replacing. Usually this is regarding something like the 300 AAC Blackout or 6.8mm SPC compared with the 5.56mm round. Alternatively, on other rounds I've read you don't want to go short because they are not optimized.

What is it about a round that makes it work better in shorter barrels compared with another round?

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wanderinwalker
January 3, 2013, 06:05 PM
Just a SWAG, but I'm going to guess the reason the rounds are "optimized" for shorter barrels is due more to case capacity relative to bore diameter. In the cases you've listed the cartridges are meant to increase the effectiveness of a short barreled AR carbine. The effectiveness of the 6.8SPC and .300BLK depend less on raw bullet speed like the 5.56 by substituting bullet mass and diameter.

Now the other side is, picking on the .300BLK specifically, it will never be able to do with a 7.62x51/.308 cartridge firing platform will. No matter the barrel length, the 7.62 has more muscle and can generate more velocity. The .300BLK just lacks the case capacity.

My opinion when people say a round "isn't optimized" for a short barrel has more to do with the perceived "significant" velocity loss and increased blast and noise of firing, say, a .300 Win Mag from an 18" barrel versus a .308. This has nothing to do with a short-barreled .300 Mag being "no more powerful than a .308 with a lot more noise." (To grossly butcher the oft-cited claim about short-barreled .357 Magnums vs. 38 Specials.)

Skribs
January 3, 2013, 06:36 PM
Well it was meant to compete with x39 not x51, and the short barrel comments were regarding the idea of SBRing a 300 AAC or a 6.8 SPC vs. SBRing a 5.56.

KingTiger
January 3, 2013, 07:11 PM
The 300 BLK uses a faster burning powder so it's not spewing a fire ball out the end of a SBR like a 5.56 round would.

jmr40
January 3, 2013, 07:32 PM
Faster burning powder does not produce more velocity from shorter barrels, at least not any of legal length. The same slow burning powders that produce the best speeds from a 24" barrel will still produce the best speeds from a 16" barrel.

Some cartridge designs burn powder more efficiently. Short, fat cases allow the powder to burn more efficiently than long skinny ones. This produces more bullet speed, with less powder, less recoil and a little less barrel length.

For example a 300 WSM will develop about 98% of a 300 Win mags velocity with about 15% less powder and about 10% less recoil. You see similar numbers when you compare 308 to 30-06 for the same reasons.

ball3006
January 3, 2013, 07:33 PM
in the short barreled rifles. For example, My TC in 223 with a 10 inch barrel throws a muzzle flame like a short Mosin Nagant with off the shelf 223 ammo. I reload with H322 and now have very little muzzle flash and excellent accuracy.....chris3

helotaxi
January 3, 2013, 08:43 PM
Case capacity ratio to bore diameter combined with bullet weight dictates the correct powder burn rate. If that powder burn rate is sufficiently fast and the case capacity relatively small, the velocity difference between a short and a long barrel is minimal. In that sense, the cartridge isn't so much "optimized" for a short barrel but rather relatively insensitive to barrel length. In the case of the 300BLK shooting subsonic ammo, barrel length flat out doesn't matter because the goal is to keep velocity low in the first place. In that regard the ammo can be tailored to a short barrel as going to a longer barrel might put the bullet supersonic.

R.W.Dale
January 3, 2013, 08:52 PM
In real simplified terms the the more overbore a case is the more it suffers from short barrels.

This is a really really bad oversimplification but the gas produced from the burnt powder can only come out of the case and push on the bullet base so fast per bore diameter the more gas volume the more time that bullet needs to stay in the bbl for said gas to get to push on the bullet

Now this is the part that confuses folks.

This doesn't mean that a more overbore cartridge wont still be faster than a "optimized for short barrels" round. It just means that the bigger case looses much more velocity potential and probably has a very high muzzle pressure and therefore obnoxious blast.


All in all the buzz phrase "optimized for short barrels" just means intermediate round or glorified pistol round.




posted via that mobile app with the sig lines everyone complains about

jerkface11
January 3, 2013, 08:55 PM
It's called marketing. 300 ultra mag will outperform 300 blackout from any barrel length.

rsilvers
January 7, 2013, 03:16 PM
Just a SWAG, but I'm going to guess the reason the rounds are "optimized" for shorter barrels is due more to case capacity relative to bore diameter.

Yes, this is the answer. Something like a 220 Swift has a lot of capacity and a small bore, so it benefits more from a longer barrel.

http://bulletin.accurateshooter.com/2011/04/overbore-cartridges-a-working-definition/

taliv
January 7, 2013, 03:29 PM
also terminal ballistics. i.e. bullet designed to expand or fragment at reduced velocities

Revoliver
January 7, 2013, 05:21 PM
Woops, completely misread first post.

helotaxi
January 8, 2013, 03:38 PM
It's called marketing. 300 ultra mag will outperform 300 blackout from any barrel length.
Well, no kidding. The question becomes at that point is the additional powder, recoil and muzzle blast worth the marginal improvement if a short barrel is one of the objectives? There are so many cartridges between those two extremes that you would have to argue that it isn't worth the increased negatives when you can rapidly close the performance gap without taking as much in the way of penalties.

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