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I'm getting ready to build an AR in 6.5G. and need some advice. I will be using it for multiple purposes ,hunting deer in eastern montana mostly (500to600yds) .How much will I gain with a 24in barrel over a 20in also I want to use a piston system like the AA from the start,any thoughts?
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Welcome. Hunting deer out to 500 & 600yds with a Grendel is asking a lot. Can it be done, out to 500yds yes...but there are certain bullets you would have to use, at certain velocities, namely the 129gr SST or 130gr Swift Scirocco pushed pretty fast.
You will also need to be a talented rifleman married to your blaster, and know how to call the wind for those shots. 300yds is more realistic for a max effective range with the 120gr class pills, while most people take their game within 200yds. There have been quite a few people who have killed medium and even heavy game at 300-400yds with the Grendel, but we're on the edge there.
The velocity increase from 20" to 24" isn't huge, but I have seen one forum member who had a custom lightweight 24" hunting Grendel built, and he has taken elk with it with pics posted. I would personally stick with a 20" gun, as it's a great balance point for a hunter. If you're exceptionally tall, you can go with a 22" lightweight barrel, but 20" is about ideal.
To get to 450yds and be around 1800fps/900ft-lbs of energy, you would need to push the 129gr SST at 2550fps, which is much faster than the factory loads with even mixed powders. I personally would want more velocity to take deer at 500yds, but if the wind was calm enough, and I had an exceptionally accurate blaster that I was confident in, I might take the shot after a register.
This is more of an ethics argument, but even the most ardent Grendel lover would have a hard time justifying 500yd shots on game in my opinion. You're talking about placing a shot with extreme resolve into a 10" circle at 500yds in field conditions with a projectile that's below the manufacturer's minimum recommended expansion threshold velocity, with about 2ft of wind drift with a 10mph full value wind. I would much rather make a shot like that with a .260 Rem pushing 2850fps with the 129gr SST, but the Grendel allows a lighter rifle.
Operating System
I like the DI system just fine the way it is, with the piston and bolt being one part in-line with the bore, contained in the upper receiver. The best op-rod driven AR's still don't have the smooth recoil impulse that the AR15 does, so I personally see the piston guns as an unnecessary fad that every manufacturer gets tired of telling customers "no" to, so they just break down and offer them, or buy into the reliability myth of an op-rod gun.
An op-rod will also add unnecessary weight to your blaster, which doesn't make a lot of sense for a hunting gun. If you do insist on an op-rod gun, they have matured a bit since the early 2000's when everyone was playing knee-jerk to the HK, which would have been a better gun if they had stuck with DI in my opinion. I've shot both the 10" HK416, and 16" guns, which handle a lot differently than each other, but I still prefer a DI AR15 that is well-built over an op-rod riven gun.
These are my basic thoughts to your intended use. Hope this helps.
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