16” 6.5 Creedmoor barrel worth it?

[markm]

If I remember to do so, I'll pull our come ups for 600 and 1000 off of the dope scriptures in the Ammo Box tonight.

[Todd.K]

Varget and 95gr vmax is a great shooting combo in my .260 bolt gun. My boys shot quite a bit of it when they were younger because the recoil is very light.

[constructor]

I’m toying around with the idea of a battle rifle or CSASS built around a 16” barrel chambered in 6.5 CM. I’ve built three AR10s in the past, all with 18” - 20” barrels, and I’d like to have something a bit more compact. I’d also like to try 6.5 CM as it seems to be all the rage these days. Realistically speaking, how does 6.5 CM compare to .308 when fired from a 16” barrel? I’m particularly interested in impact energy generated.

I would say no, you lose too much velocity. I picked up a 16" 243, 260 and 308 on a whim to test. In the end I have a TAC30 (6.8 case necked up to 30 cal) that will shoot the 130gr MK319 bullets 100fps slower with 3/5ths the amount of powder from 16" barrels using the best performing powder in both cases. The TAC30 with an 18" will shoot faster than a 308 if I use an 18" with much less recoil. The .243 is worse, the case capacity to bore area ratio just doesn't work, you need longer barrels for the slower powders to burn. I'll never use less than a 18" barrel for a 308 and 20" for the 260 or .243 again.

[constructor]

This is the enemy of every new cartridge.

People were expecting scaled down 6.5CM ballistics down to a 16-18" small-frame AR15 with the 6ARC. Meanwhile you see hype of the 6.5CM being hyped as having a ballistic arc/trajectory to .300WM. We've already seen in this thread the inevitable 6.5G to 6.5CM comparison cause "It's all 6.5mm"...about like a Frenchie and a pitbull are comparable as "terriers".

People always gettin' set up for disappointment making comparisons outside of a straight "apples-to-apples":
A) Cartridge type (magnum vs. short action rifle, intermediate vs. short action rifle, etc.)
B) Platform type (large frame vs small frame vs bolt)
C) Barrel length inconsistency

Ya know what ya don't see much of:
- Here's what a 6ARC is doing out of an 18" AR compared to 77smk out of an 18" AR
- Here's what a 6.5CM is doing out of a 16" SR compared to a 16" 308 SR


For the record, I think there's a better mousetrap to be had in a 6mm AR15...

Agree, Hornady chose the wrong case for an AR15 and they had both examples at the same time when they were making my custom dies a year before they decided to make the 6mm ARC. The 6.8 case would have been better, we have 30 round pmags that function perfectly, bolts don't break as easy and we can run 58,000 psi with no issues.

[constructor]

10-12 years ago on the military arms and ammo forum we were talking about a true mid sized cartridge and what would be most efficient for military use with shorter barrels. Cris Murray was working on a new machine gun cartridge that would reduce the size and weight of the 240. He had it right, a cartridge of 42-45gr H2o capacity is about perfect for barrels 14-18" long. His 7mmUIAC cartridge would have worked but to realize the benefits they would have needed to make a new rifle platform between the AR15 and AR10. That would handle cartridges 2.6" long and a bolt and BCG not as large as the AR10 but larger than the AR15. .050" larger in dia than the AR15 would have been enough. I machined 10 bolts so he could test his 7mm UIAC in a MG42 he converted. The case is the diameter of a Carcano, just a little larger dia than the Grendel but longer.
Cases larger than that are blowing powder out the end of a 16" barrel, cartridges smaller just can't push the bullets fast enough without excessive pressure.

[markm]

to realize the benefits they would have needed to make a new rifle platform between the AR15 and AR10. That would handle cartridges 2.6" long and a bolt and BCG not as large as the AR10 but larger than the AR15. .050" larger in dia than the AR15 would have been enough. I machined 10 bolts so he could test his 7mm UIAC in a MG42 he converted. The case is the diameter of a Carcano, just a little larger dia than the Grendel but longer.
Cases larger than that are blowing powder out the end of a 16" barrel, cartridges smaller just can't push the bullets fast enough without excessive pressure.

Yeah. I remember an instructor talking about this a long time ago. They need to make an "AR-12" was sort of his point that goes along with your point.

[Hammer_Man]

Yeah. I remember an instructor talking about this a long time ago. They need to make an "AR-12" was sort of his point that goes along with your point.

I think a platform between the AR15 and AR10 would be perfect. You could scale up everything just enough to run a 6mm or 6.5mm based cartridge without any of the drawbacks associated with 6mm ARC due to the Russian based casing.

[markm]

I think a platform between the AR15 and AR10 would be perfect. You could scale up everything just enough to run a 6mm or 6.5mm based cartridge without any of the drawbacks associated with 6mm ARC due to the Russian based casing.

There'll be a lot of wasted raw materials and machine time building nonsense before someone gets it right. I'd bet a 6mm bullet would be the solution in the correct cartridge/platform. Pick a bullet weight range and velocity, and work backwards. (easier said than done, I'm sure)

[Hammer_Man]

There'll be a lot of wasted raw materials and machine time building nonsense before someone gets it right. I'd bet a 6mm bullet would be the solution in the correct cartridge/platform. Pick a bullet weight range and velocity, and work backwards. (easier said than done, I'm sure)

Agreed, but in the long run it would be better to start with a clean sheet of paper. I think a smallish semi auto 6mm/6.5mm/6.8mm cartridge/weapon is an excellent concept on paper. However trying to retrofit said cartridge into a platform designed for 5.56 NATO, using a Russian case isn’t the best way of going about it. You could design an all new casing, and scale up the dimensions of the AR15 where it needs it to avoid the magazine/feeding/bolt breakage issues of 6mm ARC.

[HKGuns]

Agreed, but in the long run it would be better to start with a clean sheet of paper. I think a smallish semi auto 6mm/6.5mm/6.8mm cartridge/weapon is an excellent concept on paper. However trying to retrofit said cartridge into a platform designed for 5.56 NATO, using a Russian case isn’t the best way of going about it. You could design an all new casing, and scale up the dimensions of the AR15 where it needs it to avoid the magazine/feeding/bolt breakage issues of 6mm ARC.

Standardization would be key as AR10 is already a bit confusing to build. I can see that being a viable approach and may resolve some of the magazine issues with cartridges like 6.5G that I have experienced. Small (AR15) - Medium (AR13) - Large (AR10)