09-10-2018, 06:30 PM
LRRPF52 Wrote:What I notice in my large frame .260 Remington rifles is that recoil is very similar to a .308 Win., and both of mine were with heavy barrels.
The DPMS LR-260 has one of the heaviest barrels I've ever owned.
I had GA Precision build me one using a 22" Bartlein barrel after that experience, on DPMS receivers and BCG. That rifle is a laser, but it's a pig too. 140s will do 2660-2740 from it, but 2740 is pushing the gas system pretty hard using RL-17. I've literally bent at least 3 firing pin retaining pins due to the pressures generated in that thing, so I really had to be careful with what loads I run through it. I settled on 130gr VLDs going 2800fps, which is an awesome load for external and terminal performance. The rifle weighs 14lbs with optics and bipod, unloaded. If I load to anywhere near book max with it, brass life is lucky to make it past 3 loadings and then primer pockets are toast.
When I think about the recoil one would experience from the much lighter AR15 frame pushing a 140gr 6.5mm even at 2600fps, there is going to be a lot going on with the gas system, bolt carrier inertia, action spring, and buffer. It will really require a lot of gas system tuning and more forward port placement to behave well.
If I was doing it, I would use a +2" Gas System.
Another thing to look at is how .308 Win., 7mm-08, and .260 Rem are graduated downwards in SAAMI working pressure. My understanding is that this is due to bolt thrust, where the narrow flow path for the gas bottle-necks and creates more back pressure inside the actual case, shoving it as it expands against the bolt face harder.
So while the 30 RAR has a SAAMI MAP of 55ksi, a 7mm version would be less than that at ~54ksi, and a 6.5mm at ~53ksi.
.308 Winchester is 62ksi MAP, 7mm-08 is 61ksi, and .260 Remington 60ksi.
A 6.5CM has 52.5 gr H2O case capacity, whereas the 30 RAR has 44 gr H2O.
So we would end up somewhere between a 6.5 Grendel and a 6.5CM in terms of performance, leaning a little more towards the Grendel when you accept the case capacity and working pressure limitations.
If you wanted more working pressure, you could step up to the CMMG Anvil action and push 60ksi, but that Remington brass life is going to suffer.
You kinda start to see why I don't have one.
couldn't you just use the anvil action and push the grendel to 60k. That would get you half way to where you want to be but still allow you to use off the shelf ammo in a pinch.