I can confirm that margin on new guns, at least in competitive markets, is around 10%. Dealers try to make some of that up on accessories and used guns, which might be about 30%.
In the halcyon days of olde, when dealers got MSRP, margins were good. Nowadays, I can't see a shop surviving on gun sales alone. That's also why most dealers don't move on prices: to go any lower would be to lose money.
In the depth of winter I finally learned that there was in me an invincible summer.