You say that, but in a regular gun the heat goes out into the atmosphere very quickly, and what feels a bit too hot to your hand can burn an internal membrane. In a buttplug the heat doesn't have anywhere else to go, and by the time you feel a burn in the sphincter the internal damage is done because the colon doesn't have receptors for that.

Once it started to get too hot the heat would still be travelling from the centre to the surface, and you'd need to pull out the hotter part past the sensitive sphincter, making it feel worse before it got better. That sounds like a special kind of horror to me.

I'd want a large metal flare for external heat dissipation and recoil absorption - because you really don't want the above situation added to the fact that it fired itself inside you - plus silicone insulation for the plug part, or extensive testing to confirm that the temperature stays very stable after firing.

Plus, training is important. If you wanted to practice to improve your aim you'd need a way to keep it cool. I think the insulation would help prevent training accidents, and in the field you really don't want to worry about damaging yourself if you somehow find yourself needing to reload for a quick follow up shot. I don't know what situation would necessitate that, but it would require supreme focus.