Well Keith, we just returned from four days of glorious squirrel shooting her in Oregon, and my primary weapon of choice this trip was my Cooper M38 VR in 20VT also. Just installed a new Jard 10 oz. trigger in her, and "oh boy!", what a difference.
(I'll post the adventure when I get caught up with brass chores.)
In answer to your question, I cleaned every 100 rounds, and did not notice
ANY degradation in accuracy whatsoever. Last season I went 200 rounds, and the barrel still cleaned right up, and I could not see any accuracy loss even at that rate of fire. The barrel never got hot, just barely warm to the touch.....it could very well be
THE perfect rat round, at least it's becoming that to me. I was using the 32gr Nosler BT over RL-7.
One of the primary things I've come to love about this caliber, is the lack of barrel heat that provides much more shooting when targets are romping about everywhere. I can shoot four times longer with the 20VT than I can with any of my 204's due to barrel heating. I'll always have one of my 204's handy for when the wind becomes an issue or the range hovers around the constant 400 yard marker, but for ususal conditions out to 500 yards, it's the rat caliber of choice right now (to me).
Here's the reason the shooting was so hot.....have you ever seen so many rat mounds in any one area? This was the field we shot for three days, and you're only seeing a small slice of it in the pic......no problem burning up 300 CF rounds a day in this kind of target-rich environment:
You're gonna have a killer time out in SD with your Cooper 20VT, and I bet you shoot it more than your 204......we'll see, right?
Have a great shoot, Keith.