True: polymers are extremely variable. You would have decent chances with a simple one like PETP. Though, many polymers, and more so for elastomers, are blends or copolymers
. The proportion of monomers changes the atomic composition of your elastomer.
That one seems to be nitrile rubber (is it?)https://en.wikipedia.org/wiki/Nitrile_rubber
See the picture on top right at Wiki? Depending on m/n, the proportion of nitrogen will change. also, elastomers can be loaded with many things like carbon black.
But if your aim is to evaluate a penetration depth for the protons (not a damage amount!) then you could live with an approximate composition. If some nitrogen replaces carbon, it will change the penetration depth little. So you could just count the C, N and H atoms from both the acrylonitrile and the butadiene on Wiki's page and deduce a mass proportion. With a little bit of luck, (C+N)/H is about the same for both monomers, and then m/n has little effect on proton stopping.
For more accurate values, you could let analyse a sample and get an atomic composition.
I doubt about your C and N amounts in 303 steel.