because of the .22lr conversion unit, and because an 11" barreled, 6 lb CAR-15and 10"(12 ozs) of can equal a quiet, still handy pc. Put the required 14", 24ozs of can, on the end of a 22" barreled, 9 lb M1A, and the pc is bipod-only. The .308's lack .22 conversion units, which makes them pretty worthless for foraging. Calling in every enemy within 2 miles, every time you pop a skunk or raptor, just means you have to do that much more fighting. The odds will get bad because of such stupidity, real fast. You belong in either darkness or thick cover,(if shtf) and a 308 has ZERO advantage in either sort of situation,over the 223. In fact, if the 223 is canned and the 308 is not, the 308 is a HUGE handicap, by comparison. A good can means ZERO muzzleflash at night, or in the darkness of thick cover, ya know. That's one of the ways we evaluate just HOW effective a can is. If it's still got muzzleflash, it aint very good at suppression of the hot powder gases.
The 308 burns nearly twice as much powder as does the 223, so your 308 can, in order to be as quiet as is the 223 can, has to have twice as much internal volume. The bigger the hole in the baffle, the more powder gas can escape, so that makes the 308 need more volume, too, if you want as high a level of suppression.