The .22 long rifle to .22 magnum conversion is fine. My personal .22 magnum began life as a .22 long rifle "match" stainless 10" Contender barrel and was rechambered by my local gunsmith. It shoots very well and hasn't given me any problems. I suspect much of the accuracy comes from that tighter bore.
The convertible Single Sixes deal with it by using a .224 bore which is why a lot of them don't shoot for [bleep]. My first one was pretty good. I've had about a dozen since and not a one of them shot .22 long rifle ammo well enough to keep .. which is why I didn't. One among them shot .22 magnum ammo well. Given that track record I am truly regretful of selling that first one.
Tom
