Re: springfield 03A3 308
The only reason might be standardization. I briefly considered using a Navy Sleeve in mine since everything else I have including a Garand is .308. But the downside is that you would probably need to shorten the magazine follower and use a custom spring with a spacer block to prevent the shorter rounds from unstacking under recoil unless firing single rounds.
Too many cons, not to mention lowering the collector value of these fine rifles. They are already getting hard to find. Won't be long before they are as rare as Civil War Springfields. A lot of them were produced too, relative to the population level of the period.
Regards, Sherlok