6.5CM both SA/bolt questions

General Firearms Precision Rifle - Semi-Auto
1

Hi. I am trying to decide on what I want for my AR10 lower to be built at. I would like to make a 6.5CM AR10 for ammo commonality with my 6.5CM bolt gun that I want to build, but I am starting to second guess that strategy.

When I am going to load for my bolt gun, I am guessing that my COAL will be longer for the bolt gun just from the nature of having more space in the magazine, and if those don’t fit in a Magpul AR10 mag, then I would have to load two different cartridges anyways.

Is there someone out there that loads for a bolt gun that can also use the same OAL so it fits in an AR10 magazine, but doesn’t see that much reduction in their bolt gun usage? Is what I am a valid reason to want two different cartridges for my AR10 and bolt gun, or am I making a mountain out of a mole hill?

2

COAL/mag length is going to be the same with most all common, popular bolt guns (Tikka/Ruger/etc) as the AR platform gas guns.
From memory 2.180” is about max for both gas guns and bolt guns.

3

You arent going to like the loads from the bolt gun in the gas gun anyway.

Gas guns dont seem to like the 140-143 stuff like the bolt guns do. They do better with the 120-123 stuff.

BUT if you want a longer magazine the KAC and Larue SR25 pattern mags are longer than the Magpul stuff.

4

Same load (41.5 gr R16/140 gr. Hornady ELD-M, CCI BR2, Hornady brass) from 22" Aero 6.5 CM, M-5E1 “AR10” and 24" Howa chassis bolt gun. COAL 2.810" (I had COAL number transposed above- going from memory).
Velocity is 2740 for 24" bolt gun, 2725 from gas gun- losing 15 FPS for the 2" shorter barrel of gas gun.
(Black squares are 3/4" X 3/4")

Aero gas gun:

Howa bolt gun:

5

Just as a point of reference to what the lighter bullets do in gas guns.

Im pushing 123g AMAX with Varget(39.9g if I am not mistaken) to 2900fps out of my 22" JP barrel.

6

My AR10 tends to like the heavier projectiles.

7

OP, you may be able to get one load working well for both guns, but it’ll depend on a few factors. You will need to load around your gas gun first, as that will have more demanding requirements for the load to work well in the gun (bullet weight, powder, headspace/shoulder bump, neck tension, case mouth crimp, and COAL/mag length). 140+ grain bullets tend to not work well in gas gun loads, and it is difficult to get a good load with the shorter COAL and a 140 bullet, as the bullet starts taking up too much case volume. I’d start with a 130 (Berger Hybrid, or Nosler RDF) load for the gas gun, and then see if the same load works in your bolt gun. The 30s were developed for mag length match loads in gas guns. You probably wont be able to get the bullet ogive close to the lands on the bolt gun barrel, but the 130 loads might jump to the lands just fine. You’re gonna have to try it to find out.