SAAMI stands for Small Arms Ammunition Manufacturer's Institute. It is an organization in which member companies establish standards for the production of all types of commercial ammunition. This includes the actual dimensions of a given round—width, length, case wall thickness, etc—as well as the pressure level to which it is loaded. When the SAAMI member companies agree to these yardsticks, gunmakers may proceed with building guns that can comfortably accept SAAMI-standard ammunition. Although handloading is an active and widely practiced adjunct to the shooting sports, no gunmaker will ever warrantee his gun when it is used with handloads. That's because they cannot exert any control over the way the ammo was loaded or what components were used. With virtually all commercial ammunition sold in America loaded by SAAMI members, the shooting public can proceed with their sport in relative safety. It is an excellent example of an industry regulating itself.
In the case of a few cartridges, the need for greater performance has driven SAAMI to create a so-called “+P” level of pressure. This simply means that the ammunition is loaded to a slightly higher pressure and is safe to use in guns rated by their makers to be strong enough to handle it. Some shooters are insistent on squeezing every last bit of velocity out of their hardware and this +P business is a controlled and safe way to get it done. But only a few cartridges have a +P standard and printing that mark on a box of hot-loaded cartridges of another type just won't do.
I have heard that .45 Colt ammunition loaded to high velocity and marked +P is available from several makers. The .45 Colt is an elderly cartridge, probably our longest-serving handgun round. Originally designed for use with black powder, the cavernous .45 Colt case will accept large charges of smokeless, which can destroy older revolvers. As loaded by SAMMI standards, .45 Colt ammo is on the mild side. Yes, I am aware that there are much stronger .45 Colt guns on the market, and some will take a great deal more pressure. But if higher pressure ammo finds its way into older guns, you have a recipe for accelerated wear and possiblydisaster.That's why there is no SAAMI standard for .45 Colt +P.