6mm ARC

[Old Bob]

I suspect they are using slower burning powder in the 6mmARC which yields higher pressure at the Gasport and muzzle than the Grendel. It would not surprise me if powders as slow as 6.5STAball powder can get good velocity in the ARC

Progressive powders burn the most efficient under pressure and to an extent the smaller diameter bullet/bore diameter used the less room for hot gasses to expand in and the more time slower powders can do their thing effectively. For instant you can use slower burning powders in the 243 for great results than in the 7mm-08 or 308 and you can use even faster powders to better use in a 358 Winchester than the 308 or 7mm-08. '

Another way to think of it is we start with X case volume with the bullet seated, as the bullet moves under pressure of burning powder and resulting gas the further down the barrel the more volume the powder has, now with a larger bullet that volume of the chamber increases faster as th bullet moves down the barrel resulting in a larger Expansion Chamber to hold those expanding gasses resulting in pressure dropping faster than a smaller chamber due to the smaller bullet.

With a gas gun to deal with higher pressure at the gas port you can use a smaller gas port for less gas transfer but the pressure of that gas will be the same basically between gas port sized until it hits the larger diameter gas tube and the rest of the gas system. Ot using an adjustable gas block it reduce the volume of gas getting through, or simply move the gas port further down the barrel to where the gas pressure is optimal for the system with standard port size. It is a balancing engineering game getting the gas volume and pressure correct for timing with too little, just right or too much gas.

Hope that does well enough explaining this, sometimes us engineers have trouble explaining things.

I think I get what you're telling me. If I want to shoot the same ammo that works well in a 6mm ARC AR with a plus length gas system in another AR with a shorter length gas system (a 16" barrel with a mid-length system for instance), I oughta have an adjustable gas block to keep from over-gassing that rifle.

I've been wondering about which powders would be the most efficient with the 6mm ARC. There is zip/nada reloading data for the cartridge right now but I suspect that info will be forthcoming in the days ahead as reloaders start experimenting - after barrels become more plentiful.

Hmmm... I have a 22 Nosler I built 2 years ago but never fired 'cause my Grendel has been taking up all my time. That might be a good place to start working on a 6mm ARC sometime in the future.