FU-BUS are so cheap these days, you can have one on the gun and a back-up for your back-up in your pocket.
You wank the (presumed metal) one that's integral to the gas block... it's wanked, unless you have an FU-BUS on the rail too.
One of these days, someone is going to post a photo of a floating rail AR with about 15 sets of BUS installed... some on top, some offset, hell even a few on the bottom just for fun.
Cheers,
C