Just target shooting out to 400 yards max and the benefit of having mini AR10 performance. Was a curiosity of mine since I enjoy reloading and would like a new project. I also plan to test it in some ballistics gel.
Seems from everything I can find 30HRT or 30 Harrett is about the most you can get out of the AR15.
Oh and I love 30 cal over anything 6.whatever
30HRT....30 Ham'r....300 Blackout
If you're interested in downrange performance rather than just muzzle numbers, the bullet profiles in this pic hold a pretty big clue. One of those has a much higher b.c. than the other two.
I've done a few different variations of the 30 HRT / 30 Herrett AR / 30 American or whatever you want to call it. I'm calling my latest iteration the 30 SPC Improved. Ballistics from this 20" barrel are a 125gr NBT @ 2,725 or a 150gr FN or RN at 2,450. But, the case is too long for anything with a decent ballistic coefficient, so velocity drops off pretty quickly. If your goal is 30/30 lever action ballistics in an AR, some variation of the 30 HRT is an interesting way to go. FYI though, I ran into a bunch of 6.8 brass that really sucked for wildcatting, seeming to have pretty soft case heads. But dies are easy for most of these variants - just a 30 Herrett die set.
IMO a better version of this round is the same thing but in 35 caliber - based on the 357 Herrett dies (shortened in the lathe). My 16" (I call it the 358 Herrett, due to use of a .358" 1:14 twist barrel blank) in an AR produces similar muzzle energy to my 20" 30 cal, and for the woods hunting that both rounds are suited to, the 35 cal version is superior to the 30 cal in my experience.
If I were approaching this again and determined to stick with 30 cal, I'd go with some variation of a 30 Grendel, like that 30 ARX above. You mentioned 7.62x39 - the Grendel case is essentially an improved (blown out body with sharper shoulders) version of the x39 case for added capacity. So, ignoring some minor quibbles about shoulder position, the 30 Grendel wildcats are basically 7.62x39 Improved, but with better (small primer) brass and better .308" bullet choices. The downside is dies - there's nothing out there to modify dies from, so you need a special purpose die set.
Or as mentioned above, just consider the 6.5 Grendel ballistics in comparison to the 7.62x39. Both shoot a 123gr bullet, with the Grendel at slightly higher velocity for similar barrel length, but a big improvement in ballistic coefficient and down range trajectory. On the other hand, wildcatting is fun, and any of the rounds mentioned above are interesting projects. Feel free to holler if you jump into one of them seriously, I can offer some comments on what powders worked best, gas system details, etc.
