SpudFiles

It may just be a scratch with dirt in it. try to scrub it out. That is why metal barrels, and guns, for that matter, are nice.

It's not a crack or scratch - it's worse than that.

Some pipe has that - it's a line or seam that often comes in production of low pressure pipe. Your pipe is probably DWV. High pressure pipe does not have this because the production process is different (and more expensive)

DWV is not rated for pressure, and can be very dangerous.

You may need to re-build your entire cannon if you have used DWV.

that could be the case, but may not be as well. with the swat guns 2" barrel (the pvc one), originally, there were no black streaks, but after shooting spuds, and metal pipe, there were black streaks. although they were wider than this, there was no indentation at all, the pipe was just as structurally integral as before. I was wondering if this was the same thing.

It could be, but I've never seen it in non-DWV pipe.

I wouldn't trust it.

DWV pipe is fine to use. You just have to used dvw pressure rated pipe.

Yes, but that line won't be present in pressure pipe. That is symbolic of low quality DWV type stuff - it's a major weak point.