The 590 and 835s have open-ended mag tubes and are also available in 9-shot versions. They can be swapped onto your 500 but you will also have to change the barrel, spring and mag tube cap. The mag tube, spring and cap can be had from Brownells for cheap, but the 590 barrel is hard to find. (I did see one on gunbroker.com for $64.)
If you go that route, I think the shortest 590 barrel is 24 inches, which is substantially longer than what you probably have, the 18.5 inch plain barrel.