gfl216 226 Posted October 11, 2016 I have an older Bushmaster M4 A2 that I bought probably 15 years ago .I was wondering if I was to buy a complete gas piston upper would I have to replace the buffer in the lower or any other parts to make it function or would the existing parts work with the new upper? Quote Share this post Link to post Share on other sites
Vlad G 345 Posted October 11, 2016 Ask the people selling the upper? Most of the time it should be fine but there are no "standard" gas piston uppers so you are best checking with the maker Quote Share this post Link to post Share on other sites
MartyZ 692 Posted October 11, 2016 The only thing you would need to change is the BCG, that is if you don't buy it with the upper. The BCG for piston does not use a gas key but instead has a flat portion for the piston to push against. Quote Share this post Link to post Share on other sites
Screwball 483 Posted October 11, 2016 I can take my Spikes upper with AA piston off my one lower, and swap with my D/I FDE rifle (one has a carbine buffer, other a rifle buffer) with no issue. Can even mount both on my .45 D/I lower, as it runs a standard carbine buffer. Cannot really swap BGCs or specifically bolts, being my piston doesn't have gas rings on the bolt. Quote Share this post Link to post Share on other sites
MartyZ 692 Posted October 11, 2016 If you have a complete piston upper, with BCG, then you can put it on any lower. Quote Share this post Link to post Share on other sites
gfl216 226 Posted October 12, 2016 What dose BCG stand for? Quote Share this post Link to post Share on other sites
MidwestPX 172 Posted October 12, 2016 What dose BCG stand for? Bolt Carrier Group Quote Share this post Link to post Share on other sites