I have an A500G 12 gauge. It has been very reliable and I shoot mostly skeet with it. I performed a routine cleaning of the gas system, but when I went to reassemble it, I had a flat washer I couldn't place. It doesn't show up on the expanded view drawing in the manual and it's not mentioned at all in the reassembly directions. The washer fits over the magazine tube perfectly, but I don't know if it goes before or after the sleeve bar, gas piston, or gas cylinder. Any ideas?