Do not down load mag. brass! If you want to shoot 38 sp or +P then load that brass up right.
If you don't like the results of say .38 sp or +P shot out of your mag. rated pistol buy a .38 tack driver.
Down loading mags. can be done, and is done by folks. BUT you can get squibs, hang fires or the can't hit anything double pop's. Which are like squib's that clear themselves.
When I was at my range last weekend there was a group of guy's having the above issues. Down loading .44, .357 and .50 BMG........