Bit difficult to say whats wrong without any files to check...try downloading my weapon mod
http://ufoai.org/forum/index.php/topic,8001.0.html its for 2.5 but the file layout is identical
And look at the revolver entries in research_ajg.ufo (the 3rd, 4th and 5th tech entries) and revolver.ufo for weapon and weapon mod (ammo) set up. The revolver has 2 ammo types and as its on its own in the file its easier to see whats going on.
If you have copied and pasted from the original files it's probably a missed bracket or typo so carefully check everything.
Hope this helps a bit.
*Edit*
I just noticed you dont mention the new ammo model you MUST have a model and I seem to remember you can not just point it at an existing model. However you can copy and rename the normal clip .md2 file (say to assault_ap) and point the weapon model data to that file.