Here's a quote from the manual:
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“Plus-P” (+P) ammunition generates pressures in excess of the
pressures associated with standard ammunition. Such pressures
may affect the wear characteristics or exceed the margin of safe-
ty. Use of “Plus-P” ammunition may result in the need for more
frequent service.
“Plus-P-Plus” (+P+) ammunition must not be used in Smith &
Wesson firearms. This marking on the ammunition designates that
it exceeds established industry standards, but the designation
does not represent defined pressure limits and therefore such
ammunition may vary significantly as to the pressures generated
and could be DANGEROUS.
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