I ran across a blurb somewhere lately that stated any good quality weapon chambered for the 38-special would also safely handle the +p. I think it makes sense, especially in guns like the 640. The construction and quality is the same in those early guns regardless of the serial number prefix. Many 38-specials we see on the market state they will handle +p loads, but are not stamped as such anywhere on the weapon. If you look at the load-tables, +p loads are not that much "hotter" than standard loads....maybe like 10% in the powder charge and generating an increase in pressure from around 15,000 in the special loads up to 17,000 in the +p loads.I doubt that Smith somehow purposely reduced the weapons strength in the models not designated as such. Probably ALL modern-day weapons chambered for the special will easily handle the +p loads. I can't see a manufacturer going to the trouble and expense to make a weapon that would not handle the +p load on purpose if it is designed to handle the special.

Powder manufacturers state there could be as much as a 10 to 15 percent variation in density, so the re-loader has a 10% error already built into the charge if he/she goes by the loading tables. I think it would take more than a +p load to blow the 640 apart. Some folks think the only reason they ever designated the 38-Special as +p was just to sell more expensive ammo and guns so marked anyway. Maybe some of the older weapons would have been pushing the envelope, but not modern-day guns.

Maybe a poor analogy....back-in-the-day you could order a new car with-or-without a heater in it and I know this was the case as recent as 1955. Nowadays....EVERY car comes with a heater.