So out of curiousity how do you know what type of buffer you have/need. I know M4 buffers are shorter correct? But beyond that I read on the army review of the m4 that certain buffer swap made it much more reliable. So umm if I am getting a used ar15 how do I know the type of buffer should be in there. On top of that how do I know what type of buffer is ideal for 6.5 grendel? I have no idea.