Update!
A couple of days ago, I mounted my Leupold Rifleman 3-9x40. I was almost sick with the results. The scope came off my modified 10/22 that is capable of shooting of shooting 1/4" groups at 25 yards. On the 15-22, I could only get about 1-1/2" at 25 yards. I've never ever had a 22 rifle that couldn't hold under an inch at 25 yards with a scope and good ammo.
I had trouble believing the scope is the problem, so just a half hour ago I decided to take a closer look at the 15-22. I squirted a little rem oil on the outside of the bbl extension where it mates up with the receiver on the inside and gave the bbl a little wiggle. Sure enough, I could see some oil being displaced as I wiggle the muzzle of the bbl. I found a bent up aluminum handle from a broken snow shovel and made a makeshift bbl nut tool. Damn bbl nut offered no resistance whatsoever when I spun it off. So I'm thinking it was not torqued down to spec, or didn't stay torqued to spec. After taking it apart, I checked the play between the bbl and the steel sleeve it sits in. I must say that I'm a little surprised just how much play there is between the bbl and the steel sleeve in the receiver. I can see that if the bbl isn't torqued down properly, and if the bbl nut does not make physical contact with the steel sleeve when torqued down, it could have some accuracy problems. Honestly, I was getting better groups with iron sights when it was brand new.
While I have it apart, I'll get some pictures posted of the parts. I'll also see if I can sketch up a quick diagram on how the bbl, handguard, and BBL nut interface so you can see what I'm talking about.
I think my next two steps are:
1- put the scope back on the 10/22 and verify that there isn't a problem with the scope. It's been a few months since I've shot the 10/22, so I want to eliminate the scope as a problem.
2- Once I've verified there's not a problem with the scope, I'm going to put it back together without the handguard. That way I can make sure that the bbl nut is contacting the sleeve and try to shoot for groups. Not sure how stable I can get it on the bench without an handguard and not resting the BBL on anything, but it's worth a shot.