Been wanting one of these for quite a while but never pulled the trigger (no pun intended) on buying one. So....I'm doing some shopping around and see a "gen 3" description on one.....which I'm assuming means there is a gen 1 and gen 2. Can anyone tell me differences and options available or anything else to look out for while shopping for one? Looks like 18" and 20" barrels are the only 2 options? Anyone seen any inherent accuracy advantages between them?....I've got a ton of 155gr ammo to use if that makes a difference. Thanks for any guidance you can provide!