98 flatline problems

Too high a velocity is the first likely culprit. Don't assume that Marbs are "good" paint. Over the last year I have found them somewhat fragile and I have found other brands that are consistently more accurate. Lack of accuracy at distances as close as 30 ft indicates a problem that needs to be figured out, not a characteristic of the barrel.

My Flatline has been very accurate at distances where others are, but it took some tuning and effort to keep it clean (hot water and dish detergent...like Dawn). Even when you don't break any paint in the barrel, residue from the ball shells builds up on the matte finished internal surface. The bead blasted inside of the barrel has a texture like very fine sand paper which grinds away at the ball shell leaving deposits that impact accuracy.

Flatlines are somewhat finicky, but if care is taken to keep it clean and fit to the marker properly you do not give up accuracy for distance.

If you are getting shots that curve in all directions or fly in a spiral path sometimes, it is not the barrel itself. It is out of round paint, ball shell residue or contamination in the barrel from broken paint or even rainwater. Don't rule out the occasional opponent shot that breaks on the muzzle of your Flatline contaminating the bore.