I'm going through the same thing.
I bought www.ammosphere.com - a small online ammo retailer that was advertised here, as well as on other sites. Previous owner was using authorize.net as the gateway and merchantwarehouse.com for the processing. apparently Authorize.net will no longer accept vendors who sell ammo or firearms.
I just finished my paperwork with Payment Alliance www.paymentallinaceintl.com. I found them on the NRA website. Rates seem to be inline with everyone else, though merchantwarehouse.com was cheaper.
Hope to be doing business by the end of the week.
