There were two series with Firearm in the title, each with a 0 issue. As far as I know there were no variant covers (there are UPC variants, but the cover art and logos are the same for those books), so here is a visual guide:
Firearm: 0-18
Codename Firearm: 0-5
Here's the trick. If you're collecting these comics you are one of the few, so don't go crazy. Just be patient, look for large lots on the cheap and keep your eyes on auction sites. But if you see a gold hologram cover, grab it!
Ultraverse Flashback 
July 1st, 2011 at 12:29 am
Correct about no variants for CODENAME: FIREARM, but with the FIREARM book, it depends on what you consider a “variant.”
Issue #1 came plain with a $1.95 price tag, or in a newstand edition with a UPC, packed in a polybag, included a trading card and had a $2.50 price tag.
Issue #5 came with a UPC on the cover, and without one.
July 1st, 2011 at 8:29 am
Also #1 was released as an Ultra-Limited 5000 Silver Foil Edition as well.
July 1st, 2011 at 10:13 am
I’m trying to get scans of all the polybagged price variants for a future post (I’m missing three). But with the UPCs I’m starting to think there are UPC variants for nearly every Ultraverse comic from the first year or two. For the main Firearm series alone I have seen nine different comics that have UPC codes. I have a hunch that there are many more.
Since I covered the Ultra 5000s in a different post I left them off here; I was just thinking of variants as books with different cover art or logos. Maybe I need to tweak the wording a little …
Thanks for the help, fellas. 🙂