Wabi is right on the money there. Pyrodex P is for pistol. Meaning it is a 3f grade of powder. RS is for rifle and shotgun. So it is a 2f grade powder. Select is nothing more then RS run through a screen a few more times to make it more uniform. I really used to shoot select until the store ran out once and I bought some RS. Never noticed the difference in the two powders.

Actually in my inline rifles I like the Pyrodex P for a lot of things. It seems to shoot very well and is a about 10-15% stronger in power then the RS they claim. I never ran it over a chronograph so I will not swear to it.