Yep, I got it, after I reread your post(read twice, post once....I know). My brain apparently stopped part way through, then picked up again on the how to make the barrel. Sorry about that, if you read my edit on the post I think it'll be pretty hunky-dory.
By the way, we are currently building our own t-shirst shooter!Yayyy!!

And heres an idea we came up with: Our school uses really cheap, lightweight garbage bags in the small cans, and if the shirt(or other projectile of choice) is placed in the bag first, it slides much nicer in the pvc, as well as acting as a kind of sabot on launch. As far as the bags go, think of one of the plastic produce bags that are on the rolls and that you put your fruit in at the grocery store.
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-Sir Francis Bacon
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-Albert Einstein