I have an older 500A combo pak (18" barrel and 28" barrel, walnut stock and pistol grip, from early 80s. I also bought a new 500 and conversion kit to build what amounts to one of the new Shockwave 14" before they actually released the Shockwave already setup. I decided I would like to put the new 18" barrel, WITH HEAT SHIELD, and its magazine tube onto the older 500A. Got everything disassembled and to my distress the new 500 mag tube would not screw into the older 500A receiver. The threads on both the older and newer mag tubes looks exactly the same to me. I took a plastic pick and chased the threads on both mag tubes and the older receiver just to be sure no crud was holding up the works. The new mag tube will begin to thread but then stops for some reason. I don't want to force it lest I cross-thread something. Is it possible the new mag tube is bigger (thicker) than the older receiver? I've read that this is a compatible swap. The old mag tube will screw right onto the old receiver but the new tube just won't Any ideas would be much appreciated.