Re: Remington 51 .380
The M51 is a rather interesting pistol. For example, I have read that Gen. George S. Patton carried one as a backup gun. Springfield Research Service has noted that in WWII the OSS purchased a small number of M51 pistols from commercial sources for issue to field agents.
If you have access to the 1979 Gun Digest 33rd Edition there is a great article about the Model 51.
While I have no problem shooting standard velocity ammo in the M51, I have seen several broken breach blocks caused by shooting high velocity ammo through these pistols. Be sure you are using standard velocity ammo, no extra hot self defence ammo, "+P" or whatever.