I think the rule of thumb is to reduce the charge weight by 10% & work up again. In cartridges with very small case capacities, like the 9x19mm & .380 ACP, the use of primers might give you unreliable ignition. The .38 Spl. case may be big enough that the magnum primers won't make a difference (as long as you reduce by 10% and work up again). MY recommendation would be to load up 10 or 12 in each caliber & see how they act.