It seems to me that the answer to your question is one word in your post.
That being "asking"
I have not been aware that shop dealers were so staunch in their pricing that they would be offended at an offer, made in earnest.
Perhaps the phrasing of the offer could go something like----
"I would very much like to order a gun from you and would do so, but have a locked in budget." If he/she wishes to entertain an offer, you will be asked what that limit is. This is the "go ahead" to be sensible and fair.
It may well be [possible to add some extras to the order that the dealer could include as a courtesy. That might be better than a 5/10% discount.
Hope these thoughts help.
Best,
John
