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Interesting testing of 5.56 velocity and exit pressure vs. barrel length
#1
This has been out a while, but for those considering the tradeoffs of various AR-15 barrel lengths it should be interesting ...

Small Arms Defense Journal: Barrel Length Studies in 5.56mm NATO Weapons
Quote:There has been a cultural shift from the 20-inch barrel length in the AR-15/M16 weapon systems chambered for the 5.56×45 NATO cartridge to progressively shorter barrels for the purpose of producing an increasingly more compact assault/entry weapon without resorting to a bull-pup design. Simple usage of these short-barreled weapons has shown the necessity for both sound and flash suppression, the intensity of which (in exceptionally short barrel lengths) approached the intensity of a flash-bang diversion device. This shift toward shorter barrels has resulted in the U.S. Army and Marine Corps adopting the 14.5-inch barreled M4 carbine with a re-design of the 5.56×45 from the 55 grain SS-109 to the 63 grain M855 ammunition to optimize this barrel length. The differing bullet design also necessitated a change in the rifling twist rate from the original 1:12 inches to 1:7 inches.

Law enforcement and some special operation units have continued this trend by using weapons fitted with 10.5-inch barrels, and there is some misguided law enforcement interest (in these author’s opinions) in the M16 type weapons using 7-inch barrels. Besides the horrendous flash and sound levels, these ultra short barreled weapons introduce significant ancillary issues, including weapon functioning and reliability as well as projectile stability and cartridge lethality.
-JD-, proud to be a member of pa2a.org since Sep 2012.
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#2
Interesting. So it would seem that for a PDW, you would be better off with a pistol cartridge caliber than an AR pistol, as the greater mass makes up for the lower velocity.

This certainly helps with my shopping decision.
tolerance for failure meter... LOW
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#3
Good read. I think it clearly points out some of the problems our military has been finding out about first hand with the M855 round in shorter barrels. I really would like to see the same tests done with a 55 gr bullet just to see what sort of numbers they come up with.
The law? The law is a human institution...
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#4
This is why I recently disassembled and sold off my SBR, and am eagerly awaiting the release of the CZ Scorpion EVo 3 S1.


Justin
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#5
Just my opinion but 14.5" would be the limit I would go. 20" is too long for a weapon one is carrying around in and outdoors on a regular basis. 16" seems to be a good compromise all around rifle.
sgtsandman, proud to be a member of pa2a.org since Nov 2014.
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#6
14.5" seems to be a good threshold for a military carbine. It doesn't seem that noticeable, but I can feel a small difference in the handling between my issued M4 and my AR at home. Every little bit helps when you're already loaded down and bulked up from a flak and kevlar, and mag pouches, and other PPE.

It's interesting to see that the next major drop off in velocity occurs at 10" in length, so an 11.5" barrel is also a viable option for an AR pistol or SBR build for closer engagements. 10.5" barrels seem to be pushing it, and probably won't carry enough velocity to be useful after a couple hundred yards.

What's amusing for me is that shorter barrel lengths start to, for all intents and purposes, require suppressors to be useable without destroying hearing and/or becoming disorienting for the shooter. The added length from a suppressor negates whatever length reductions are gained until you get to the really short barrels. At that point you've given up a lot of velocity, and may as well have switched to a pistol round, or 300 BLK, to get more weight behind the slower round.

If the shooter was already factoring in mounting a suppressor, however, it would make some sense to go for something like an 11.5" barrel. You can still retain a lot of the velocity of the round, and keep the rifle fairly compact for something with a suppressor on it. It'll still be loud since .223 and 5.56 is a pain in the ass to keep quiet, but it won't be a mini flashbang every time you pull the trigger then.

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#7
ShaulWolf;157921 Wrote:14.5" seems to be a good threshold for a military carbine. It doesn't seem that noticeable, but I can feel a small difference in the handling between my issued M4 and my AR at home. Every little bit helps when you're already loaded down and bulked up from a flak and kevlar, and mag pouches, and other PPE.

This is where the fundamental design change of the bullpups comes in to play. You can have a platform that just squeaks over the 26" min. O.A.L. US-civie requirement and has a 16"+ barrel. The additional benefit is that the C.O.G, is moved way back toward the body, resulting in faster swing and realistic one-hand firing if necessary.

Currently a bit pricey in the Tavor incarnation at about $2K street (with an aftermarket trigger - factory is good but extremely heavy at 11lbs.+) - you get a 5.56 piston-driven platform with a trigger approaching that of best of combat style ARs.

Desert Tactical's upcoming MDR promises to be better yet with more AR-like controls, forward-side ejection, easy caliber convertibility (5.56x45 and 7.62x51 to start with standard AR/SR25 pattern mags), 16" and 19" barrels, and hopefully a great trigger (given DT's pedigree of bullpup precision rifles.) It ain't gonna be any cheaper, though.

A few more years of innovation and we should see the prices coming down.
-JD-, proud to be a member of pa2a.org since Sep 2012.
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#8
-JD-;158022 Wrote:This is where the fundamental design change of the bullpups comes in to play. You can have a platform that just squeaks over the 26" min. O.A.L. US-civie requirement and has a 16"+ barrel. The additional benefit is that the C.O.G, is moved way back toward the body, resulting in faster swing and realistic one-hand firing if necessary.

Currently a bit pricey in the Tavor incarnation at about $2K street (with an aftermarket trigger - factory is good but extremely heavy at 11lbs.+) - you get a 5.56 piston-driven platform with a trigger approaching that of best of combat style ARs.

Desert Tactical's upcoming MDR promises to be better yet with more AR-like controls, forward-side ejection, easy caliber convertibility (5.56x45 and 7.62x51 to start with standard AR/SR25 pattern mags), 16" and 19" barrels, and hopefully a great trigger (given DT's pedigree of bullpup precision rifles.) It ain't gonna be any cheaper, though.

A few more years of innovation and we should see the prices coming down.

The price for a bullpup is prohibitive for a lot of folks, and it becomes a no-brainer when I can build a stock AR for a quarter of the cost of a Tavor. The inability to adjust the length of pull leaves something to be desired too. However, the tradeoff for having a more compact rifle while still maintaining barrel length is pretty awesome.

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