A few thoughts:
We each have money that we can spend on whatever. Every time I question a purchase of hers, I get "its my money and I'll spend it as I want to." OK, but that's a 2-way street, and I've told her as much on a couple of occasions. She's more calm about it. When she isn't, like when there's no room in the gun closet for my son's clothes (it is in his room), I remind her that the shoes and other things she buys with her money are worth nearly nothing in a garage sale, whereas the guns can be sold for nearly as much or sometimes more than what I paid for them.
Besides, I have enough guns that SWMBO doesn't know one from the other, and my removal of another gun from the safe is a non-event for her (except for "you just went shooting 2 weeks ago!").
Having bought a number of milsurps, I have long been prepared with a completely honest answer: "New gun? Its absolutely NOT new, I promise." I'll never say that I've had it for a while, unless at least 6 hours have gone by (that's "a while" but not "a little while").
Now that I've started getting into casting, I'm bracing for her reaction when I put a bunch of filthy wheel weights into my new cast iron pot ($25) and cook it up with a turkey fryer in the back yard. Ooh, yeah, that one's going to be fun...BUT, I communicated to her that I was going to be casting bullets. She hates Obama with a passion, and is convinced that we're going to end up in "re-education" camps, so it didn't seem to phase her as unusual or something to object to when I casually mentioned it. Of course, I'll also be pouring molten lead into a muffin pan, and stirring up the molten lead with a big spoon. She cooks a LOT, and this is about the closest thing to sacrelige in her worldview. Yup, this is going to be interesting. And she still hasn't noticed the 175 or so pounds of WWs in the garage (they're on my side).