One thing you need to check is the barrel chamber... Colt manufactured .38 Supers made from 1929 to 1990+- had the original headspace on the case rim chamber...gave very inconsistent accuracy... Guns made after that were headspaced on the case mouth just like a .45 ACP and give terrific accuracy...
To check the camber take a scribe or a sharp pencil and sloly go down the chamber wall. If the pencil tip stops dead then it is one of the later guns... If it goes right on up into the bore then for top accuracy you should have a new barrel fitted...
You can get tons of ammo off GunBroker....
