Problem is that barrels become leaded up to various degrees in and around the 250 round mark.
Hyper velocity just adds to the problem.
I got to work with a guy that built the Olympic rifles that stomped the Russains in the 1990 Olympics(?) We tried all kinds of ammo, and they developed a new bullet, lube, case, primer, which ended up similar to the Federal Gold Medal Ammo that is now available.
The same guy always wanted to go and Rim fire p. dogs and ground squirrels. We went on a couple of trips to N. Ca. on ground squirrels shooting std velocity CCI along with CCI Green Tag. With the lack of a sonic crack, we had one heck of a shooting trip to say the least. A lack of a sonic crack kept the squirrels from going down, same thing with p. dogs a few months later as we found a calm day in N. Wyoming up the the rolling hills. We walked the bullets in on the dogs from some really long distances.
If you have a super accurate 22, clean it regularly with top quality bronze bristle brushes intended for Rimfire available from
www.sinclairintl.com. As you push a brush through, examine the brush for lead, you will be very surprised. Depending on just how annal you are, you can clean after firing different types of ammo and see a remarkable difference in how much lead and powder fouling you pick up.
In my custom Ruger 10/22 that has a match chamber on a 40x barrel, it will put the CCI Standard Velocity in the same bullet hole at 75 yards. When the barrel gets a little leaded, the group will open up to about an 3/4". Even still, with the Leupold 6.5-20 with the target knobs taped for elevation, I can still adjust the shots in at 125 yards for one shot head shots in calm wind. After 200 yards the trajectory and the slightest wind becomes a Hail Mary on center of body shots.
I shot CCI Mini Mag HP on p. dogs to eliminate crawlers, which does a great job, but placing your shot is still key. In my rifle, the CCI mini mag HP held up the best when shooting 500+ rounds a day. Ammo is very different in the thickness of the copper coating on the bullet and bullet lube.
Search for a super accurate sub sonic round such as federal match or CCI standard velocity, you are a lot better off in the long haul, as 22 rimfires go from super sonic to sub sonic, they loose a lot of velocity.
It is against a lot of folks religion to clean their 22's, you need to re-think this mind set....use good bronze bristle brushes, throw them away after 100 strokes.