All this debate on muzzle velocity seems somewhat unnecessary. To me the real question is, did SAAMI change the chamber pressure rating of 38 special after 1972. I have read it was 20,000 psi and after 1972 is now 16,000 PSI with +P being around 20,000.
Seems to me, it that is true, then all the velocity testing is a mute point. Manufactures used to load 38 special above current +p and now they load it light.
If these numbers are true, then any older gun rated for 38 Special would have no problem shooting +P because it was designed for the higher chamber pressures at the time.