My son made a play gun at preschool one day out of Duplos, and was playing cops and robbers or something with another boy. They scolded him and said that guns were bad to play with. I had an issue with that, because play guns are perfectly fine in my opinion, and my son was doing fine-- but I would not of course have had a problem with them telling him not to use play guns there.
It would just be bad manners, in my opinion, to plan a gun-themed birthday party, in light of different views on play violence. I would probably let my own son go to such a party, but if one of his good friends couldn't go due to a parent's views, would probably just line up a play date with that friend in place of the party.
