Why not just go to Apple's "NUMBERS" program (part of "IWORKS"). Here is a screen shot of one of their templates. A little bit of changing and you can convert this from a rooster to an inventory. Being a spreadsheet type program you can also do the sorting and so forth to customize it to your needs.
We seem to have a good difference of thought on the worth of iWorks 09.
If you are comparing it to the 1990-1995 era versions, yes that was not real useful. When I use the 2009 version, it does everything I need and makes it quite a bit simpler to use. I have moved more of my things over to the iWorks 09 applications.
There are still some applications that I use Excel for. These are mostly the things I put together back 10 years or so ago and I am comfortable with them but if I had to write them new today, and I didn't have to share all of their features with other users, they would be done in iWorks 09.
If you write them in Numbers (or Pages or Keynote) you can save them in Excel (or many other) formats but some of the features will not pass through to the other formats.
Now if you are talking about Enterprise type applications, then I would suggest a stronger program. I don't know how many firearms you may want to inventory but, unless you are a dealer and have the large volume and detailed record keeping needs of a dealer, Numbers would probably do the job.
capvan - I just used AppleWorks and designed my own Data Base format to build an inventory that covers the whole house. The intent was to have a document that would provide insurance people with data on our property in case of fire or a major theft. I did not consider photos in the data base but do have photos saved on a Zip Disk. Both the Zip and the inventory are kept in a separate place as well as the computer covering the event of a total destruction of the home. Fortunately, like hiding a key on the frame of vehicles, I've not needed the data - yet! It may be basic but works quite well.