I carried the M2 Carbine in the M1A configuration as my secondary for 3 inserts and then a Car 15. I've carried the M16 and M14 as well.
All my own opinion!
As the range closes the 7.62 x 51 is detrimental to accurate follow up, as the range exceeds 500 yards it shows that it's long in the tooth. The heavy 6 mm and 6.5 mm projectiles while not a modern concept their respective efficiencies in there current configurations are modern.
With a 5 1/2 lb rifle 110 grain projectiles slows accurate follow up so while I love the M1 and M2 30 Carbine for close work (200 yards-) it's best when modernized with a new stock to weigh about 8 lbs.
The Car 15 of my day evolved into the M4 and for close up and personal work to 300 yards it's efficient, at 400+ yards it's dicey and as the wind kicks up it's a more of keep their head down proposition at that range.
At 52,000 PSI the 6MM ARC is the same pressure as the original 5.56 x 45, which gives me hope for durability. It remains to be seen if 1 base cartridge with a108 grain projectile can work in an M4 at 14 1/2", the M16 at 20" and a specialty rifle with a 20" or a 24" tube. The logistics people love that.
Ballistics are great for the 20" & 24" barrels I question the performance in 14 1/2" barrels and the sustained fire tests need to be perfect. Since a man's ass is on the line I'd like to see an M4 and M16 in 6MM ARC run uncleaned for 5,000 rounds and see what gives.
Re: 14.5" barrel and velocity: https://content.osgnetworks.tv/firearmsnews/content/photos/Hornady-6mmARC-8-770.jpg
You can go plug it into Hornady 4DOF, it's quite good at the distance it'd be used out of a 14.5" barrel. I was originally building an 18" but am considering a 16" now that I did the 4DOF crunching out to 1000. I'll probably build a 11.5" SBR, too.
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