No, it isn't; dependent on context obviously.
In your example, if the counter argument's assumption is true, and disabled people are at risk of violence they could not defend themselves from without gun control, then that's relevant to gun control.
It's not an ad hominem unless you infer something from the conclusion of discrimination, besides the need for gun control.
I would just reply that very few disabled people would be at much risk of violence without guns, point to how there's no disabled rights activists arguing for less gun control, etc..
But it does depend on context.
If someone claims e.g. that intellectually disabled people do not often make for good surgeons, then the counter-argument of prejudice doesn't make sense, even-though it isn't an ad hominem and we may be sympathetic to its motivation.