Its for reasons like this that Dead and Back has its simple ammunition rolls. Despite my lack of hunting, police, or military training - I'm pretty familiar with all this, but its rather unreasonable to expect everyone interested in table-top games is as well. Furthermore, trying to keep track of every single bullet can be rather complicated, and not particularly fun.
Dead and back's combat system only tracks effecive hits - its a matter of description rather than mechanics if the hit is twelve rounds tearing thrugh the torso or one bullet going thrugh the left eye socket - while a filed roll means etheir a clean miss, only stirkes to armored areas, or no shot presented itself and no bullets fired. Hence, another reason for the simply ammo management is that the number of rounds fired isn't really counted. A reload is simply enough ready ammo to fill the gun back up.
The upshot of knowing about ammunition differences (excuse the pun) is that it gives the GM a of moderating tension within the rules. At the lowest level one simply has generic reloads by type - ie rifle, pistol, and gauss ammo. However, the GM could rachet up the tension by being more specfic with ammuniton loads - by claiber or even by gun type (a SIG P-226 and Glock -17 share the same bullets but not the same magazines - so ready ammo can't be exchanged between them in a firefight). This in turn means people might get weighted down with multiple types of weapons to use every bullet found, or find they have plenty relaods but no weapon to put them in. Bullets, bullets every where and not a shot to fire...
To mix things up a little more, the players might be offered the option of partial clips. They treat the ammo die as a size smaller then normal, but still don't require actually counting bullets. This allows something between the standard OK or no relaods availble options of the rules.
The Website Modern Firearms is an invaluble resource for learning about the types and histories of many weapons in current and prior military use.
Although the actual guns don't really matter under the D&B system, this is a good way to establish character. Just as a car or clothing style can speak volumes about a person, their choice of weapon means something as well. Going for every acessory possible or prefreing something lighter and less battery intensive, Chosing the most recent manufacture possible, or holding on to a family member's war-trophy from decades back. Wanting a WW-II Lee-Enfield because its cool isn't going to hurt them, and a 2048 M-42 gauss rifle isn't going to make the character invincible - so let it be a point of intrest for the character.
*For those of you wondering the difference is:
- 7.62x25mm: Pistol round - used in Soviet WWII sub-machineguns and TT-33.
- 7.62x39mm: Assault Rifle Round of AKM & SKS
- 7.62x51mm: NATO ammunition for machine-guns, and a rifles like the FN-FAL
- 7.62x54R: Russian from from 1880s, yet still used in the SVD, Machine-guns 120 years later!
- .308: Reduced power 7.62x51 for hunting - probably won't cycle a military automatic, and the NATO round may be over-pressure for a hunting rifle.
- .30-06: WW -I & II high powered rifle round