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Lower clearance on the tube means less blow by, higher pressure in the tube and more stress to the shell. Assuming the tube can take the pressure and the shell is sturdy enough, you should get more...

Ops, sorry for the double post. Where is the delete or edit function on here?

The guys on Pyros also used some steel pipes for their big guns. There was an episode where they showed large a steel one that unzipped. I thought I saw some HDPE on there also, but not sure.
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The guys on Pyros also used some steel pipes for their big guns. There was also an episode where they showed large a steel one that unzipped. I thought I saw some HDPE on there also, but not sure....