The AR15 FAQs list the muzzle velocities available from the 5.56mm cartridge with different barrel lengths, but they only go up to 20". Is this really the optimum? What velocities are achieved with longer barrels?

Tony Williams
Military gun and ammunition website: http://www.quarry.nildram.co.uk
Discussion forum at: http://forums.delphiforums.com/autogun/messages/

As I understand it, not a whole lot. I've only seen 24" barrels as a maximum length & those are for match use only.

Generaly speaking, due to powder differences, I'd say that 100fps would be the maximum difference between the 20" & 24" barrels.

Author Glen Zediker Chronographed his 26 inch barrel as 125 fps faster than the same load in his 20 inch gun.

The link has data on 22" all the way down to 10" barrels with different powders.

Very interesting info.

www.accuratereloading.com/223sb.html

Many thanks for the information, gentlemen.

I notice that the Steyr AUG/HBAR 5.56mm light support weapon has a 24.45" barrel, while the L86A1 (SA80 LSW) has a 25.4" one, but other MGs seem to settle for 20". The bullpup layouts of the Steyr and SA80 help to keep them compact, of course.

It does put an interesting slant on that table in the FAQs showing the minimum fragmentation distances of NATO ammo from different barrel lengths, which are frighteningly short for the short-barrelled carbines now being favoured. Presumably, these bullpup LSWs remain effective out to longer ranges than even the standard 20" barrel guns.

Tony Williams
Military gun and ammunition website: http://www.quarry.nildram.co.uk
Discussion forum at: http://forums.delphiforums.com/autogun/messages/

I think that 20" really is the optimum barrel length for the 5.56x45mm cartridge. Or at least very close to it. Both the original M193 55grn load and the SS109 NATO spec version were developed with and for 20" barrel rifles. Although slower burning powders are available now that weren't 20-25 years ago when SS109 was being developed, when you consider the available case capacity of the cartridge I am not sure that 24in and longer barrels improve performance enough to justify the weight even if you changed the powder.

Certainly if there was a clear performance advantage for a longer barrel on the M249 we would have them, and this may yet happen if heavy bullet ammo does come out. One of the advantages of having a quick change barrel system.