Originally Posted by drgonzo
The previous posts are untrue, you can underbore to where you have to practically hammer the ball into the barrel and if it makes it out, it won't break or curve wildly. The point at which a ball does not make it out of a long-back barrel is about .020+ underbored, with a shorter back you can even go tighter before it won't come out.
That said, actually feeling the paint drag happens at underbores like .012+ which is not ideal. The blow test finds a sizing for almost no underbore, which is also not ideal. It should be quite difficult to blow the ball through if not impossible with an underbore.
The biggest deal about it, to me, is you want the pump stroke to be smooth, because that affects your pumping reliability and timing (avoiding chops, for example), so you might as well bore up if you feel it.
As with anything paintball, your mileage will vary. I guess my previous post where toward a worse case/hammering paint into the barrel like the OP's post.
I personally underbore about a sizer down from whatever match would be. I'd use a .680 insert on paint that is roughly at the .682-.684. Most of the time, I would have no issues, but once in a blue moon you'll get that one paintball that is the bad apple of the batch and it gets hammered into the barrel and goes nowhere.... or if it makes it out of the barrel in my experiences, it hasn't been too straight of a flight path.
I also had a handful of instances in which brittle paint would swell a bit and it won't enter the sizer/back and goes kasploosh in the breech.