Relate it to her in terms she can understand:
'Now honey, it's like when you buy a new dress. You need to accessorize for it - shoes, watch and hand bag. Well, my new rifle is kinda the same. I have my rifle - but I need the proper accessories for it. That would be a sling, scope & rings and a butt cuff.'
Better yet, FIND SOMETHING ON SALE!!! Then you can explain, even though you SPENT $$$, you actually SAVED $$$, because you would have needed to buy these things anyway and they would have cost you more.
Put some $$$ in the 'cash kitty' so you can take her out to a nice dinner or send her away for a 'spa day' - that way she feels spoiled and it was an 'even' deal.
Works like a charm, Brother!
;D ;D ;D