I will be changing my 6 pos M4 stock for one of the ACE Skeleton stocks that uses an A2 buffer tube. Question is do I need to get an A2 buffer? or can I use my current buffer that came with the 6 pos stock? I assume I will need to change the spring because of the longer buffer tube length. I have a 16'' carbine length gas system if that means anything. Thanks for your help.