In the old days of IHMSA when the rams were still set full foot, many shooters used .308’s. Max barrel length was 15 inches. Plenty of horsepower, too much actually even for full footed rams. Shooting a 4.5 pound.308 handgun without a brake for 40 rounds in an hour can be a bit tough on a fellows wrist, however many did. One fellow was even shooting an XP in .458 American (a .458 Winchester magnum cut down to 2.5 inches) but I digress.
The .308 should be great at 18 inches, even 16 inches, but the 1-12 twist is going to limit you regardless of barrel length.