I love the idea. But if your can says only 14.5"+ from 308, I would pin and weld a muzzle device to get you to 16" and build it as a rifle with whatever stock/chassis you wanted.
Or if you want to get a new can, get one with a shorter or no barrel length restrictions and go wild. 10" - 12" 308 would be awesome, I have no idea what velocity numbers you will get, but they will be much higher than anything that can fit in an ar15 size platform, especially reloading playing with different powders. The YHM resonator is a really great can if you want a cheap beater to throw on there. Built like a tank, full auto rated (not that it matters for a bolt gun), not too heavy, 10.5" 308 barrel length restriction, and can get them for around $400.
Or if you want to get a new can, get one with a shorter or no barrel length restrictions and go wild. 10" - 12" 308 would be awesome, I have no idea what velocity numbers you will get, but they will be much higher than anything that can fit in an ar15 size platform, especially reloading playing with different powders. The YHM resonator is a really great can if you want a cheap beater to throw on there. Built like a tank, full auto rated (not that it matters for a bolt gun), not too heavy, 10.5" 308 barrel length restriction, and can get them for around $400.