pellets out of smooth bore barrel

[inonickname]

That does not mean that they are always right

You seek to belittle me, sir! You leave me no choice but to challenge you to a duel!

Please scream "it's ovaaar nine-toousanddd!". I've always wanted to hear you (type) that.

Try chamfering the ends of the barrel with a drill or countersink, then fine them up with sandpaper. If that does nothing get back to us.

[mark.f]

To add to what Gipetto and qwerty have said, the end of your barrel has not necessarily been cut by machinery, but most likely a tubing cutter, if it is air, water, oil. etc. tubing. This will leave a small reduction in inside diameter at the end of the tube.

[Ragnarok]

Please scream "it's ovaaar nine-toousanddd!". I've always wanted to hear you (type) that.

To what aim? I see nothing of the prerequisite magnitude to illicit such a reaction. At least, nothing to which I have not already alluded.

However, I am sure you will hear me use the given words at some point in time.

[sniper hero]

try inonickname's idea
I think there is too much air coming out of the barrel behind the pellet giving it an unstable flight