Hi everyone, I bought a M&P12 a few weeks ago, I did the initial cleaning prior to the first range trip including field strip etc. Took it out to zero the red dot, fired maybe 200 rounds through it and broke it down again to clean, went back together no problem but the pump would not fully cycle so as I tried to disassemble it again the upper will not disengage from the lower, seems like the forearm rails are hooked to something in the receiver, any thoughts on what I may have done to cause this? Thanks for any ideas, I’m concerned about causing a bigger problem if I try to muscle it apart.