As said above, the difference between a .38 Special and .38 Special +P is pressure. That said most (but not all) .38 Special +P factory ammo is barely at the standard .38 Special pressure limits and for the most part is an excuse to charge you more.

Ad for the M66 and Magnum ammo, you can shoot all the 158gr full power .357 Magnum ammo you wish to in your M66 as long as you stay away from ammo loaded with a 125gr bullet. The additional velocity generated with a 125gr bullet can cause forcing cone damage but the 158gr bullet won't.

__________________
Freedom is never free!!
SWCA #3437