I think the anti-gunners would slap a 50% tax on all guns and ammo without even flinching. The sheeple would be powerless to do anything about it. If you doubt that, then simply take a look at how many anti-gun laws have been successfully overturned. You can count the successes on the fingers of one hand.
Why do you figure it would be any more easy for them to do it under a general sales tax, that it is now when there is no sales tax to put such a tax on top of.
A sales tax in place would act to impede such activity more than the current situation.