i purchased a m1 garand used by a re-enactor
in .308 the bbl was threaded for a blank adapter
the threads go about 3/4 of an inch down the muzzle.
i have not shot it yet to see how the threading of the bbl
effects accuracy. has anyone ever seen this before?
i can purchase a new bbl and have it mounted,
should i ? please give advice or opinions
thanks bill  

Yes, you can easily have a garand rebarreled in .30-06 or .308.  

Whether or not you should rebarrel would depend on the condition of the bore, as well the accuracy due to the threads at the muzzle.  Some of the re-enactor rifles I've seen weren't cleaned well, or that often.

A new barrel would probably set you back about $250 (installed).

As a WW2 reenactor, I feel obligated to jump in.
A lot of blanks these days are pretty rough on the barrel, especially with the M1, since its BFA has to restrict the muzzle for backpressure to cycle...which means that all the crud in the blanks has a tendency to stay in the barrel, and more and more firings means the stuff keeps sandblasting its way down the barrel. If the user kept it nice and clean, that shouldn't be a huge problem.
The bigger issue is the tapping for the BFA. If the threads aren't centered on the bore, you can expect accuracy to go right out the window. (I have an adaptor that replaces the gas cylinder lock, works fine and still lets me live-fire it...and I clean regularly, even at events).
Take a look at the barrel, see if the rifling is any good. If so, try shooting it...if it still shoots well, probably no reason to worry about replacing the barrel just yet...though if you want to get it counterbored to clean out the BFA threads, that might reduce the chance of something marring your muzzle. Just my opnion on the threading though...more so than the rest of this, even.
In the end, though, it's up to you. If you think the wear and tear, or modification to the current barrel, makes it less suitable as a shooter, go ahead and replace it. If you're okay with it as-is, no point in spending the extra $$ until you actually do need a new barrel on it.

JMO, HTH

thank you for the help the rifling is sharp and the bore is clean
i hope to shoot it this sunday thanks again
bill