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View Full Version : Why 12" barrels for 5.56mm?


Bartholomew Roberts
May 29, 2007, 06:59 PM
OK, lately I've noticed a couple of trends in 5.56mm rifles. Short barrel for the SCAR - 12", Masada - 12", XCR - 12", XM8 - 12". On top of that, you have the Novseke Afghan barrels (12") and other 12" barrels offered for the AR15.

Clearly, someone has a requirement that says 12" is as short as they want to go for 5.56mm. My first thought was that perhaps mechanical function was the issue. After all, direct-impingement rifles like the AR can have function issues as barrels get shorter and 12" is a nice compromise; but four of the rifles mentioned above are gas piston and don't have that same problem.

So what is the driving factor behind the 12" minimum barrel requirement? My second guess would be terminal ballistics; but looking at the numbers, I don't see a huge difference between 10.5" and 12" in performance, so why the 12" number?

RockyMtnTactical
May 29, 2007, 08:00 PM
With what specific load?

Bartholomew Roberts
May 29, 2007, 09:03 PM
With what specific load?

Any load really... I don't have a lot of muzzle velocity data on 12" barrels; but looking at the few numbers I can find reported, the difference between 10.5" and 12" for 55gr M193 is about 80fps.

This means the 12" barrel hits 2700fps at 12yds and 2500fps at 65yds. The 10.5" barrel never hits 2700fps and hits 2500fps at 44yds. With 75/77gr the ranges extend a bit; but not too much due to the slower initial velocity.

Even if you consider that 21yds significant, why 12" instead of 11.5", 12.5", 13" etc.?

Zak Smith
May 29, 2007, 09:19 PM
My SBR is 12". Regular Noveske lengths are 7.5, 10.5, 12.5, 13.7, 14.5, etc.

I think somewhere around 12" is the shortest you can go and still use a carbine gas system. Someone please correct me if this is incorrect.

I chose 12" for a compact package that would still produce decent velocity.

Bartholomew Roberts
May 29, 2007, 09:23 PM
I think somewhere around 12" is the shortest you can go and still use a carbine gas system.

Well, there are the 10.3", 10.5" and 11.5" barrels that still use the carbine gas system; but I think 12" is around optimum for function and compact in the AR. Still, why a 12" barrel for all the rifles that aren't direct impingement?

Something out there is driving that requirement but I can't figure out what.

Limeyfellow
May 29, 2007, 11:39 PM
I guess its a compromise with the amount of speed loss with even shorter barrels and that stopping the ammo fragmenting properly.

HorseSoldier
May 30, 2007, 12:30 AM
I think Limeyfellow is correct. The short barrels are really nice for hopping in and out of vehicles, CQC, etc., but they're a trade off at longer range. 12" seems to be the current compromise people are going with.

Of course part of it may be that since they gave every Schmo in the 82nd Airborne an M4 with a 14.5" barrel, you obviously have to go shorter than that to still be one of the cool kids. ;)