I'd like to get a good dedicated .22 upper for my AR-15. Preferably something that has a flat top, flash hider, and can take a standard quad rail (or possibly free float) so that I can set it up identically to my real AR-15. I also need it to work with the black dog .22 magazines. So who's making a good dedicated .22 upper these days? I've found a few makers online, but I don't know who's good. Can you switch out the gas block on these the same way as a regular AR-15?