From ARP’s web page:

6.8 based wildcat- 16" barrel

If you are looking for the most reliable and best all out terminal performance on hogs and deer out to 300yds the TAC 30 is it. The rounds hit like Thor's hammer...DRT, a perfect match for Texas hog hunters. The Cavity Back bullets 125gr MKZ open to .800" diameter, imagine the wound channel! They are pushing the 125gr MKZ to 2700 out of an 18" barrel with 29.4gr of SW blackout. That is 150fps faster than a 20" Grendel and 450fps faster than a 300 Blackout. The Herrett using SW BO powder is as fast out of an 18" barrel as the 30 Remington AR was out of a 24" barrel. The TAC 30 is big brother to the TAC 6 cartridge. It is a wildcat that you must reload, form and trim cases but you do not need to fire-form.

4150 CMV 30 Herrett mid wt profile, Hybrid 5R rifling 11.25 twist . 2lb-2oz
M4 feed ramps, mid length gas system, .850 taper to.750 under the guard, .750 gas block dia. .700 dia from gas block to muzzle. 5/8-24 thread muzzle.

30 Herrett reloading dies are used with a 6.8 shell holder. 6.8 SPC brass is easiest to use, S&B, Hornady, Starline in that order. Trim to 1.570, 1.580" is max case length. It is important to size the brass close to the chamber size. Set the dies up where they push the shoulder back just enough to chamber the brass. Hornady 30 Herrett dies seem to work best but again be careful not to push the shoulder back too far. The 30 Herrett was a wildcat many years ago and the older dies vary in size. You do not need to fireform or turn the necks on the brass. Just trim, size, load and shoot.
The 30 Herrett in an AR can be loaded to higher pressures than the old TC pistols. With 125-130gr bullets Shooters World BO, 1680, 4198 and RE7 work well.