I have a 20" fluted sporter barrel in my old DSP, and it cut the group sizes down by half of the original barrel, which was never a winner in anyone's book. That was with most any ammo. I tend to shoot cheap ammo whenever I can, but it likes CenterX and Pistol King a lot (under a half-inch).
Normally, though, I shoot Aguila SE-HV or Blazer in it, and it will shoot that stuff into 3/4" for 10 shots at 50 yards. Plenty good enough for my usual plinking/squirrel hunting stuff.
I had to sand a little on the barrel channel, not enough to work up a sweat, though.