//How far round the twist will he be allowed to go and remain in office?//
He is not round the twist though is he? He is delivering to his voters. That may not please the left wing in the US or here in Europe but non of them would vote for him if he was the best person in the world.
I think he is being pragmatic. There is no way that 400 million guns can be removed from the country so the only way, in the immediate future, is to protect. Not a situation I would relish but I have, as yet, not seen anyone come up with a better idea.
Alongside this The Donald is starting to put some additional checks in place. Of course this is not going to change everything but if small steps are made they will get there.
The US still need to get to the bottom of why their citizens do this. Other countries are armed and don't do it so why so many in the USA? If they don't address this then the perpetrators of such atrocities will simply use bombs or trucks.