Originally Posted by Steve C
Alk8944 is right on the money. The image below was taken from the 1972 Guns Annual. Note the test barrel lengths and remember they where likely unvented pressure test barrels and not real revolvers or semi auto's. No real difference to today's ammo when one subtracts the expected velocity gain from longer barrel and no cylinder gap except the term +P wasn't in use. Higher performance ammo was called Police loads or Hi-speed which was the equivalent to today's +P.
Nope: .38 Hi-Speed used to mean the hotter .38-44 ammo. A 150 grain bullet at some 1150 FPS. It was meant only for heavy-frame .38's and use in .357's. The then-Sales Manager at S&W told me that it wouldn't blow up a current Model 10, but that if much was shot, the gun would loosen up far sooner than with standard ammo.
We didn't discuss using it in older M&P revolvers, but I wouldn't do that, except in an emergency where it was the only ammo there. It might well damage those older guns. Before the 1930's, the ammo wasn't known and the guns weren't heat-treated for it.
However, modern ammo is checked from a four-inch barrel with a gap like that on a revolver. Older pressure test barrels were longer and non-vented, so they gave higher velocities.
S&W has stated that ONLY steel frame guns new enough (1957-) with the model number stamped in the frame are regarded as suitable for Plus P use. Modern ones with alloys ike titanium are another matter.
Of the M-60, the factory advised me that the M-60-4 and later are warrantied for Plus P. But all M-60's were made after the mid 1960's of modern steels and some have fired quite a bit of Plus P in them.
Saxon Pig and gun scribe Mike Venturino have shot a lot of ammo in M&P's made in the 1940's with no problems. But I think it's wise to follow S&W's advice.
All Ruger .38's and.357's are safe with Plus P ammo. They didn't arrive until modern heat-treatnig and steels were in use.
Colt advises having small frame guns checked by the factory after 1,000 rounds of Plus P if alloy-framed, and after 3,000 rounds if steel. These mean like the Cobra and the Det. Spcl. and Police Positive Special. The Official Police and the SAA are presumably so large that it isn't an issue.
I don't know about other brands. Almost certain that Manhurhin is safe with Plus P. Most are sold in .357, anyway.