They likely meant Fudd, a term for gun owners who are interested only in hunting or shotgun sports, rather than other shooting related hobbies. They tend to prefer guns with wood furniture, usually bolt action rifles or double barrel shotguns. Think Elmer Fudd. Autocorrect probably got the best of them and corrected it to FUD.

FMJs are generally useful for feeding through the firearm more smoothly and are able to be fired at higher muzzle velocities. They're nice because you need to clean your gun less frequently, in theory. They're also cheaper than hollowpoint or softpoint rounds. There are about 4 reasons an indoor range might ban them, they are more likely to damage the backstop or berm because they don't spread out, like an HP or SP round would; they're more likely to ricochet; they may have a steel, bismuth, or tungsten core which increases the damage to the backstop and the likelihood of ricochet, and because they want to sell you more expensive hollowpoints. The main reasons are for reducing their costs and increasing their profits, ironically the exact type of capitalist shit Texan gunbros say they love.