I am in the process of building a AR pistol. However, my Rainier arms slab side upper will not fit on my Spikes Tactical lower. The upper looks like it lines up fine but I can NOT get the pins in. I have even tried taping it with plastic ended hammer. I have another Spikes rifle that I swapped the stripped upper on and it works fine. It is REALLY tight though. I believe its just a tolerance stacking issue. Does anyone know a solution to this problem? I would really like to move my uppers between all of my Pistols / Rifles / future SBR.