Here is a link to a very informative and interesting article on the RevolverGuy web site. I came across a reference to it today and read it. It is a good discussion of history/pressures/dates/etc. for the.38+P scope of ammunition. I learned that the FBI Load really started as the St. Louis Police load! Never heard that before.
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All About The .38 Special +P and .38 Special +P+ – RevolverGuy.Com
It really explains the answer in great detail to the frequently asked question of "Can I shoot +P in my......"