Breech Slider

[Panzerfaust]

I just discovered something usfull in lowes today, go there and look for a "compression repair fitting" in your pipe size. they are small pieces of pipe with adjustabe piecies of rubber on either end contained inside screw caps. Standard pipe fits through the fitting making an airtight seal with the rubber ends. Leave it not tightened fully and u can slide it back and forth to close over your breach hole. If you absolutly need airtight then tighten it after you slide it into place, but i think it would be airtight even when it is loose enough to move.

[SpoonZ]

I found myself a perfect seal by using electrical insulation. Called heat shrink. Comes in different sizes. You apply heat and it shrinks. so i slipped some over my pipe and it seals it 100%.