[scratches head] Please explain that comparison.
I think the purpose is to demonstrate how much money is spent by gun-owners. Sort of a buy-cott. I don't know that I'll get one, but I don't see any problem with the idea.
Who is going to look at it? All the cashier cares about is the dollar value of the bill, and whether or not it passes the magic pen test for counterfeit money. Even IF a cashier happened to notice the stamp, they're not going to tabulate how many they saw. They're not going to report to the supervisor at the end of their shift, "Hey, boss, 0.000037 percent of my sales today were paid for with 'I support guns' bills."
And managers or owners don't routinely check each cash drawer when it's turned in. There's probably a back office, low level apprentice bookkeeper who does that. Again, does that person care if a bill has a pro-gun stamp on it? Heck, no. The bean counters in the back room probably just pick up a stack of bills and run through them, flipping up the corners to see the denomination and make a count. That's all.
Besides, virtually nobody (other than me) uses money today. Most transactions are paid for with debit cards, and most of the rest are paid for with credit cards. Very few of us use real money. How do you gun stamp a card swipe?