at my club we have a large drainage plastic ribbed piece of 'pipe' its about 36" in dia & 5 ' long. interior has 4" insulation held in place by chicken wire. raised up to above bench height in a cradle of 2x3's on wheels. with your muzzle about 1' in, fov at 100 yards is about 20'. a 50 bmg sounds less than a 30-30. i'll try to post a picture in a day or 2. for mostly hand gun the tube length could be reduced & therby opening-up your field of fire a whole lot. as an aside: some1 correct me if i am wrong, but since JFK it is a federal offense to shoot out of a window.
Make a fire for a man and you warm him for the night
Light him on fire and you warm him for the rest of his life