The most common one in the group I shoot with is Harrell's. I put an EC V2 on my VPT MTR because it simply screwed right on to the threads. I wasn't sure if it would improve groups with a 16" barrel but it did.
Most of the guys that use Harrell's shoot Anschutz and a few Rem 40X.
If you are trying to achieve the most accuracy/precision a piece of advice is find the ammo that shoots the best in your particular rifle and then tune for that. That's another difficult proposition now since really good match ammo is really hard to find. I'm thinking anybody who is even interested in a tuner is going that route. A tuner may not take but a tenth of an inch off your group size but in competition that can mean a lot.
There is some good information on Rimfire Central Firearm Forum if you haven't already found it.
