About 20 mo. ago I test-fired the BB 20As(?)--i.e., the 38+P / 158 LSWC / 1000 fps variety--in both my 640 and my M&P340 over my chrono. It clocked over 1000 fps from both revolvers--and it was stout enough I wouldn't want to carry it in the M&P340 and have to do a reload. At about $1.25-plus per round, I didn't shoot it any more--kept it for carry.
With conventional 38 Special recipes, one can get a 158LSWC / LSWC-HP (Speer or Hornady) running at up to 860+ fps at 38 CIP pressures, or <=21,750 PSI. As noted earlier / elsewhere, the Rem38S12s run maybe 825 fps--although the last chrono I did of them ran about 800- fps.
The reloads run maybe .12-.13 each; the Rem 38S12s were about .58 ea. ($28.00 / 50)
Jim H.