As usual, Uncle Nick did a fine job of explaining some important points.
There are, however some additional points I think need to be explained a bit particularly in light of comments from some of the newer reloaders.
SAAMI max pressure limits are NOT the blow up limits of any gun in proper working order. They are simply the industry agreed on upper limit for ammo that will work normally and properly in all common guns in the various calibers.
Industry standard. Works acceptably in EVERY GUN. Its NOT the point where your gun turns into a grenade. NOT EVEN CLOSE!!
Its just the place where the industry decided was a good place to stop. And, it is a good place to stop, for many cartridges and guns.
Consider this for a moment, proof loads. Proof loads are a SERIOUS percentage overpressure compared to standard ammo. They are called "proof loads" because they prove the gun will take a serious overload without failing.
(for a small number of shots, at the minimum)
For
some gun & ammo combinations, people can, and have been safely going well beyond SAAMI specs for a long time now. They do this SAFELY by restricting the higher pressure loads to guns that can take it. This is where the 3 tier load levels for different .45-70 rifles comes from. Its where "Ruger Only" .45 Colt loads come from.
Another important point is illustrated here..
I used to have 2 identical S&W model 28's. With a 158 gr jhp and a max charge of any magnum powder, one of them would run 200-300 FPS slower than the other.
Even apparently identical guns, firing the same ammunition can turn in widely different velocity readings. Most are fairly close to each other, but some vary a lot.
Trying to match the exact published load data velocity is, essentially pointless. And especially so when you are looking at small differences in velocity (under 100fps). The guns vary. Each individual round varies. This is why serious reloaders get into things like Standard deviation and Extreme Spread.
You can load the exact same components as the test data, and shoot them in exactly the same length barrel and get a higher, or a lower, or the same velocity they got, and its entirely normal. And there's no way to tell which you will get until you measure actual shots. NO computer program can accurately model what you will actually get, because there are variables in your gun and ammo the program cannot factor in. They get close, because most things fall within a given range.
Weather even plays a part. A few degrees warmer or cooler will make a measurable difference in velocity. Ammo in direct sunlight for a while, vs ammo in the shade, can make a large difference when its really hot.
Bottom line, you cannot use velocity to accurately determine pressure.
You CAN find data that gives speed and pressure with a given powder & amount, but it's only 100% accurate in the gun they tested it in.
Handloading data are guidelines, they apply in degree, and in different degree with different guns & ammo. They are not laws of physics that apply equally to all matter on earth.
