Hi folks, I need some opinions to help me make a decision. I'm building an AR15 for plinking and pest control, and I'm at an impasse deciding on a bolt and barrel. My two top choices are 300 blackout and 6.5 grendel. I intend to fire reduced loads for practice at my home range to avoid pissing off the neighbors. They work night shift, and don't appreciate excessively loud rifle shots when they're trying to sleep during the day. Standard velocity .22lr is no problem, as the range is 100 yds away from any buildings, but testing some full power smokeless cast boolit loads in my .45-70 definitely was a no-go.
I intend to load sub-sonic, and cycling the action is not particularly important to me. Bearing that in mind, I'm looking at a 16inch .300 blk barrel, and a 24" 6.5 grendel barrel...the longest commonly available in both calibers to help lower muzzle pressure. Both barrels i'm looking at feature a 1:8 twist, so I should be able to stabilize moderately heavy boolits...which to me means 140-160gn in the 6.5 and 200-240 in the .300 cost and availability are appx. the same either way, and I like that both can be reformed from common military brass. At this point, I do not intend to run a suppressor.
So what do y'all think will be less obnoxious? I'm leaning toward the 6.5 because of the longer barrel providing more expansion volume for the gas, and the projectile being lighter, allowing for smaller powder charges at the same velocity. However, if it's a poor choice for a cast boolit shooter, that opinion could change.