A couple of years ago, there were two very long, ongoing threads about "hot" 38+P loads on the S&W forum / reloading. They started off a post by Erich (who also posts here), citing Speer #8 loads with 4756--with which he was running about 1000 fps, IIRC, from a 4" 38 Special barrel--with clean extraction, no pressure signs, etc., etc. The ensuing debate about loads being dumbed down in recent years, the reliability of current and prior testing methods, etc., etc., went on for 1000s of posts and spread over two threads. Unfortunately, I cannot find a link to those threads now--a forum migration on 1-11-2009 seems to prevent complete searching.
The debate attracted all manner of posters--one of who, writing under an assumed name, was apparently privy to the original testing and development of the "+P+" loads in the Seventies, IIRC. The bottom line was, these 'Treasury' / FBI loads run perhaps 875 fps from a 2.5" barrel on a 640.
Hoptob and I did some fairly comprehensive testing of the 4756 loads. The most (in)famous 38 Special version gets a 158-gr LSWC-(HP) up to a solid 900 fps from a 2" barrel. It also arguably has a pressure spike approaching 23,000. added on edit 2: or over 23,000--see hoptob's post here and the link he references.
I did some more anecdotal research about these hot 38 Special loads--they can best be seen in a historical context. At a time when LEOs still carried mostly 38 Special revolvers, overpressure rounds were developed to provide firepower to overcome criminals' superior (.357, .45ACP) firepower, or to overcome jurisdiction's penny-pinching to the relentless technological march towards semiautos.
There are several technical articles (my links to these are on another computer) that have examined these kind of high-end 38 Special loads as well--with conflicting interpretations. IMO, the varying interpretations can mostly be considered to be the result of imprecision in testing equipment (copper crushing was NOT that good, but it was what we had) and lack of standardization in the methods.
So, thirty-some years later, thanks to the (micro) computer revolution, piezoelectric (?) stress testing, and some standardization, we probably now have more accurate testing. Unfortunately, we also have a more litigatious business climate; the resulting application of of 'business judgement' to legalese CYA assessment has generally lead to lower pressure recipes.
FWIW, these +P+ loads can be replicated today fairly easily, if one is willing to load to those same pressures with the right powder. And, in fact, the Euro / CIP 38 Special standard seems to come very close to those 875 ballistics, on the right day with the right tweaks.
It's so much easier to do it in a 357 case, however. A 2" barrel, 900-fps 158 gr. load in a 357 case looks to be at a guesstimated 25,000 psi or so.
Jim H.