SpudFiles

In my area the 2.5" PVC isnt available neither is the reducers to fit it. I already have my cannon built but i wanted to buy a box of used tennis balls to shoot.

My chamber is 4" then i use a reducer down to 3" and my barrel is 3" around. The kids starter balls walmart sells fit like it is they are 75mm and roll right in and right out nice. Problem is they are a $1. for each ball starts to get pricy if you cant recover the ball.

Here is my idea, since the inside is 3" what if i buy another 3" pipe the same length as my barrel and cut a space out of it length wise, then squeeze it down and insert it into my barrel so the cut insert takes up the gap? It would be a sort of slip in adaptor.

I think i could use a cardboard tube also but it might not be thick enough to fill in the gap needed

Hmmm, I don't think using sliced PVC would solve the issue, it would just add extra cost and require quite a considerable amount of effort to cut, especially seeing as it's quite important to cut it straight etc. Also I've had problems with consistency when I've done this in the past as you often find that it warps from a perfect circle shape when inserted.

Although it's not the exact solution you were looking for, given that I presume you are muzzle loading this, why not just add a bit of lightweight wadding, if properly done it shouldn't effect performance that much.
Try to get the gap as as small as possible first using that card tube etc, then try using some wadding, should serve your purposes fine, and save you a bit of money in the process.

I think rolled up poster board will work, i didnt realize the normal size tennis balls were this big. Just a few sheets rolled up seems to fill the space fine.

Ill try it later today and see how it holds up.