Very true Gordon, however if I was shooting 40 cent rounds I wouldn't want the chance of one hitting the rubber of the apex ramp. The FSRs can be brittle and I'd rather not have a barrel break at the tip or the chance of one being thrown off course.
Another thing I forgot to mention is an underbore is pretty much required to get the rifling to work to its full extent. I know when I had my t9.1 I used a .684 Flasc unrifled barrel and it worked ok, but I believe UV Halo swears by his .683 and that is shown on the chart from Bryce.
So the rifled barrel wouldn't hurt normal paint, but would help FS rounds. That definitely helps the situation. The one downside to a rifled barrel that I have not forgotten from when I had that old Armson is that if you break a ball in the barrel/breach, just walk off the field. Because you need to fully rinse that paint out otherwise you'll never get 2 balls to hit within a foot of each other.
I do have to agree with Cheevo on this one, if there's a chance that the bumper will hit I wouldn't want him to risk it. FS rounds are too expensive for that risk. I've only seen a few Apex's where the bumper is fully out of the way to where I would feel comfortable.
Cheevo, are you talking about an underbored rifled barrel or just an underbored barrel with the rifled FS rounds?
What I've been considering telling him is to go with the backs in the link below. That way hes got some underbore and he can just install it inline with the stock barrel.
The .683 rifled barrel is underbored, and the FS rounds themselves have rifling on the skirt. But yes the barrel itself needs to be underbored AND rifled for full effect.
Actually no, I don't swear by the .683 rifled barrel. I used one until Bryce and Cockerpunk found that the Hammerhead performed nearly as well (at the very least to within statistical noise), and without the complications of the barrel fouling/jamming/inconsistency.