Ok Dusty, I'll take a whack at your questions. First of all, since you now understand that it is bullet weight you were asking about and not powder weight, I have corrected your thread title.
Why not put velocity numbers on the box of ammo, say from a specific barrel length?
It would be impossible for an ammo manufacturer to know all the specifics of
your firearm so it would be impossible for them to predict the actual bullet velocity from their ammo. The same length barrel is only of many variables to be considered. Don't forget that lots of ammo can be fired from both a semiauto and a revolver, and the barrels of those two types are measured differently and even if they have the same length barrel will yield significant velocity differences.
Why not state type of powder and amount of powder?
They don't want people trying to duplicate their loads, not because theirs are so special, but from a safety standpoint. But, more importantly, they don't use powders that are readily available to the average shooter or reloader. They often use blends of powders that don't even have a name.
Does the type of projectile affect recoil? FMJ, JHP, etc. ( yes I know etc. is not a bullet type)
In general, no. As you now know, the weight of the bullet is more important than its shape with regards to recoil. That's not to say that bullets of the same weight won't have different recoil because they often will. For example, a lead bullet that weighs the same as a copper jacketed bullet might be loaded a little slower to prevent leading in the barrel. Therefore it will have a lower recoil value.
+P is achieved by using more powder, or different powder
what about low flash power versus non low flash? Shouldn't that be on the box as well. will that affect recoil and velocity?
The type of powder generally doesn't affect recoil as long as the bullet velocity is about the same. It would be nice if the manufacturers did state whether they used a low flash powder or not. Sometimes you can get that info on their website.
Often +P loads will yield more recoil than "standard" loads even if the bullet muzzle velocity is about the same. +P indicates a higher peak pressure and is not necessarily a higher velocity round. The bullet might be accelerated faster which would give you a higher felt recoil.
Why not put energy info on the box?
See the bit about bullet velocity.
shotgun shells don't refer to grains. They use shot size numbers.
Actually shotgun shells almost always state the "grains" of shot. Since you now know that grains is only a measure of weight, and shotgun shells almost always give their shot weight in ounces, you only have to convert to get the number of grains of shot. For example, for a shell with 1-1/8 ounces of shot, you would multiply 1-1/8 (1.125) times 437.5 (the number of grains in an ounce). So an average shotshell contains 492 grains of shot.
Shot size is a different aspect of shotshells from shot weight. It is selected by the shotgun shooter to match the intended use or target. (Same for gauge, shell length, shot velocity, powder weight (don't ask about that system), etc., etc.)
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