The M-640 when introduced with serial number CENXXXX in the frame cylinder window were marked “TESTED FOR +P+”. In later production that wording was removed with the explanation that there was not a industry standard for +P+.
Factory +P is loaded 3,000 PSI below maximum allowable chamber pressure. It is not a hot load. It is not even a warm load. It's really a good target/practice load. The notion that +P is hot is a fallacy created by brilliant marketing to make buyers think they are getting high performance ammo. A 125 at 925 FPS? Give me a break. Very mild. Standard ammo from the 1960s used a 158 at 800 and this generated the same chamber pressure.
If it says 38 Special on the barrel it's "rated" for +P since +P is NOT loaded to high pressure.
Pictured below is a 1942 M&P that had 500 rounds of Remington +P and another 600 rounds of my own +P+ (125@1150) put through it with zero effect. A member of the S&W Forum fired 1,000 +Ps through an alloy K frame with no effect (what a surprise that ammo loaded well below max pressure didn't blow up the gun!).
The CENxxxx prefix was used for the first (1989) run. Your BNNxxxx serial number dates your gun to 1993. If the early guns were +P rated yours will be as well.
that S&W 640 is probably an old NYPD off-duty revolver. The market got flooded w/those guns when NYPD switched to 9mm pistols. With that being said the all steel 640 guns are rated for +p ammo. The NYPD issues Speer GDHP 135gr. +P for any current officer still carrying the .38 special to include the S&W 640.
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