Those that can afford the weight (and price!) use a Nightforce, typically the *Benchrest* model, and typically the 12-42 variable.
There are a fair number of other variable power scopes on the line. I would guess that a 24 power is the most common fixed-power scope. I would have no problem using a good 20 power scope. A scope that doesn't change point-of-impact is far more important than a bit of magnification. You still see old Unertl's winning matches, but again, they wouldn't be my first choice.
You can go to the IBS website & look at match reports. Every once in a while, an equipment list is posted.
You can also go to the individual clubs websites -- North Carolina, Ohio, Virginia (alphabetically) for sure have relay results that list equipment. Usually by brand only, though.
Remember that Weaver & Sightron, as companies, have been bought & sold in recent years. So my 24-X Weaver from 1995 might be real different from a currently manufactured Weaver. Certainly the repair service has changed. Just one more reason why Leupold is highly though of. For a new Leupold, I believe you would have to go with a variable on a 30mm tube to get a 20 to 30 power scope now, but don't know for sure.
Edit -- hmmm. The past year's relay results seem gone from the NC website. Virgina still lists the nationals results try
http://www.virginia1000.com/07.Nats.LG_1.relay.full.htm
etc.