For now, i am using a 2'' barrel, with golf balls... as you can see in the photo below, they are a terrible fit. Soon i will be making some 2'' slugs, and trying sabots with the golf balls.
Also, if you notice, in one of the photos, there is a tennis ball in the barrel port... the port has 2'' female threads, and the ball fits right in.
![Smile :-)](./images/smilies/icon_e_smile.png)
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