Will this fit gunstocks- 1 measures 1 5/8 inches X 5 inches. The other is 1 9/16 inches X 4 11/16 inches ?

One fellow says the small pad has these dimensions: 5 1/8"(5.125") x 1 3/4"(1.75") ... its the first "most helpful" 5 star comment. The medium size (which I have) has these dimensions: 5-7/16" x 1-7/8" (based on measurement with a ruler). It rather looks like the small might fit your first gun. Its hard to tell if the small pad would be too large for your second one or not. You might try buying a single small pad, as it likely will fit your first rifle. And when you get it, test it on the second rifle. If it fits, then use the one pad on either rifle. Or if you want each rifle to have it own then just buy another for your second rifle.

What size for Stevens 320 pump shotgun?

Medium fits perfectly. Small does fit and will work, but it looks like the picture of the sleeve on the wood stock. Just a touch small.

what size for remington 870 shotgun?

Medium size works fine with Rem 870 GA12

which size, if any, for a kel tec 2000 gen 2?

Fits well on gen 1 so it shouldn't be much different on gen 2 . Just to shoot it open it's fine but it will not lock closed unless you modify it. I only use it for long range sessions.It definitely stops the pain in my shoulder that I was getting from shooting lots of rounds. Worth the money.