Microsoft Ad Center does not allow advertising that promotes firearms:

Advertising that promotes firearms or weapons of any kind is not allowed.

Neither does yahoo:

We will not show results on pages that contain problematic content, including but not limited to:
….
Weaponry, ammunition, fireworks or explosives

Nor does google:

Sales or promotion of weapons or ammunition (e.g., firearms, fighting knives, stun guns)

Google’s restriction is only restricted to sales but this has not stopped them from banning gun bloggers.

Unfortunately gun bloggers are out of luck with Microsoft, Yahoo and Google: the mainstream advertising network.