I'm keeping away from aluminum. I had an aluminum piston in there before, and it shreded all around the gear box. I only have a pdi120 in there, so not a problem. The thing is I'm not aiming for either precision nor maxing FPSs, but focussing on RoF and loudness.
So, A steel cylinder with an aluminum head (proposed by illusion)
or a polycarbonate piston and non aluminium vented piston head (proposed by Vondnik)
or a deepfire silicon piston head (proposed by Spitfire)
I will put a lot of BBs throught this gun, I did not buy a drum barrel JUST for the look (ok, it did play a good part, but still, I am trigger happy

)
I've checked Arnie's and other sites, but can't find any real comparison on pistons/cylinder/heads... (there are 348927 articles on springs do...) it seems all people want if high FPSs.
Mike!