Internal porting?

[SlartyBartFast]

There is no vacuum behind the ball unless your barrel is too long. If the barrel makes even the slightest 'pop' noise when a ball is fired it means there was pressure behind the ball.

Once the ball is free of the barrel, vacuum behind the ball is normal and unavoidable. Simple flow dynamics/air resistance.

[Kip Walker]

The only way this would help besides silencing the gun would be if you hoocked microline to the end of the horizontal ports out of the tip of the barrel and attached them so they would semi fill the air chamber before firing the next ball. ///
///
/-----------------\///
| o oooooooooooo o///
| o o//
| o o/
| o o|
| o oooooooooooo o|
\-----------------/

/= microline
o= horizontal ports