I use a Tasco 30mm tube-style RDS. Works great, on GBBRs, for over a year now.
50$ at Canadian Tire
http://www.canadiantire.ca/browse/pr...=1369164330432
It's heavier than most open-type sights, but a LOT more protected. It got hit so many times and still hold zero and works perfectly. 11 levels, I usualy put it on 4 for indoors and 8-11 outdoors/sunny.