In the US, it's common for gun supporters to say they don't support any sort of gun regulation because of the 2nd Amendment. I suspect many of these people only support the 2nd Amendment because it supports what they already want to be true. Would these people suddenly get rid of their guns tomorrow if it was discovered that without a doubt the 2nd Amendment was faked and the real text meant to drastically restrict gun ownership?
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(*) I like shooting guns. I think they're neat pieces of machinery. I like action movies where people shoot guns (and this means by near-necessity I like action movies where people get shot by guns, though it's a little uncomfortable to admit that).
I'm also fully supportive of effective gun control measures. I'd gladly give up rights to gun ownership because it seems to me the evidence is clear enough that it would save a lot of innocent lives if done correctly. 
Or to slightly modify your interesting hypothetical, how would the same folks react if the 2nd Amendment were shown to be applicable to only a "well regulated militia being necessary to the security of a Free State… " ?
The place where they get really uncomfortable is when you ask if they are in the "NO INFRINGEMENT" camp, and if they are, you ask if they are in favor of letting current prison inmates and psych ward residents having guns.
Because let's face it, that is an infringement of their rights. The fact that a prisoner has had their gun rights restricted by legal action doesn't change the fact that their gun rights have been restricted. So the absolutist "NO" infringement doesn't hold up to close scrutiny.