OK, I shot one or two more matches with the 20 in gunner, using some PMC X-TAC ammo because that load seemed to work well enough for sighting in the gun originally. With this load, I was able to engage targets out to 300 yards with no problems. I haven't had a chance to shoot it much further beyond 300 yet. None of this is a surprise since I never had a real problem with the X-TAC. My beef is with the XM193.
After having run a few hundred more rounds through the barrel, I went back out to a regular range and tried shooting the XM193 again at 100 yards. This time, instead of not being able to hit an IPSC size target, I was able to hit my target. Group size sucked big time, like 4+ inches for 5 shots, but it was a lot better than the last time I tried this ammo. I do acknowledge that some barrels just don't shoot XM193 well, but I had higher hopes for this barrel from Faxon. Given that I saw a great improvement after shooting several hundred rounds, I'm hoping that I just need to break this in further. Maybe I'll see even greater improvement in the future. I'll try to update this thread when I have more data using XM193.
I just don't know why the heck the gunner barrel performs so poorly with XM193 while shooting other stuff relatively well. My handloads are nothing special, but I was able to get groups just over an inch at 100 yards.
One last thing on the 20 in gunner, I tried shooting a few rounds of the 75 grain gold dot LE ammo at 100 yards. Group size was no good, like probably 3-4 inches. This is a very consistent round, should be able to shoot a lot better than that. I hope it tightens up after further break in.
As a side note, I had a great time blasting my 11.5in faxon gunner with an SLR adjustable gas block. That might be the softest shooting gun I own. The brake on it helps, but even for a braked gun, this thing is buttery smooth. The dot doesn't move at all when I pull the trigger.