1. yes 2. yes 3. Gas block and gas tube: - How is this resolved with a DPMS barrel? use a dpms type "normal" AR15 length tube - Different length tubes would indicate differences in the gas key design - Use either carrier, although i'd suggest staying all dpms. there is no difference in gas key design, just gas port placement on the barrel - Might as well, its not much more $ 4. probably have to play with the buffer weight, but its mostly just important to match the buffer type to the tube type. Assuming you're using a rifle stock vs collapsible, it just matters whether you use the armalite or dpms length tube/stock