It would be fairly easy to give the Covert Ops (the only class that gets the sniper rifle in ET) higher accuracy than the Soldier (who gets most of the other weapons) and everyone else lower accuracy
on every shot and add inaccuracy of each weapon to that for
every shot. It'd be a bit harder to make everything fire bullets on ballistic trajectories at decreasing velocity and have them all tumble as they fall back through the sound barrier (except the .45, .22SSS and 9mm from the perforated barrel of a silenced Sten, all of which emerge subsonic and therefore don't have that problem) but with enough RAM and enough processor speed you could do it.
The shotgun in particular should lose total power over range as well as having its power more and more spread out. Beyond a certain distance, it becomes a matter of chance whether any of the pellets hit you even if you're within the spread area.
Google has a gunwiki article cached [http://webcache.googleusercontent.com/search?q=cache:8Xv1hUqLO4sJ:thegunwiki.com/Gunwiki/FactorsOfEffectiveRange+ammunition+lethal+range&cd=10&hl=en&ct=clnk&gl=uk&client=firefox-a] (hence the long link) that quotes some ranges:
* Absolute maximum effective range: This the "this round is not considered lethal after crossing this threshold" distance. Neither of the other two common "maximum range" values will be greater than this. Purportedly, NATO defines this as the point at which the projectile's kinetic energy dips below 85 joules (62.7 foot-pounds). This is typically claimed when recounting that the P90's effective range is 400 meters on unarmored targets, as classified by NATO. It's worth noting that while the P90 looks neater than the civilian PS90, the extra barrel length increases the muzzle velocity and thus the civilian model actually has a longer absolute max effective range.
* Maximum effective range on a point target: This is the maximum range at which an average shooter can hit a human-sized target 50% of the time. "Point target" is basically a euphemism for hitting a human torso sized area in this context. If this range were greater than the absolute maximum, the absolute maximum would be quoted (a non-lethal hit may be accurate, but it's not effective).
* Maximum effective range on an area target: This is the maximum range at which an average shooter can hit a vehicle-sized target 50% of the time. In other words, this is the maximum distance at which it would make sense to open fire on a group or vehicle, etc. If this range were greater than the absolute maximum, the absolute maximum would be quoted (a non-lethal hit may be accurate, but it's not effective).
M9 Pistol 9mm 1800m 50m ?
AK-47 7.62x39mm ? ~400m ?
AK-74 5.45x39mm ? ? 800m
M-14 7.62x51mm ??? 460m ???
M-16A2 20" 5.56mm 3600m 550m 800m
M2 ("Ma Deuce") 50 BMG 6764m 1500m 1830m
M-4 14.5" 5.56mm 3600m 500m 600m
M-9 (Beretta design) 9mm 1800m 50m ???
* Sniper rifle effective range: Sniper rifles are judged by entirely different criteria. A sniper rifle's effective range is judged based upon the range at which one shot, carefully fired by an expert marksman, is guaranteed to strike the target. Sniper weapons tend not to list point or area effective ranges, as sniper rifle effectiveness is not calculated with 50/50 hit ratios.
5.56mm 400m 600m
7.62x51 NATO 800m 1000m
338 Lapua 1200m 1500m
50 BMG 1500m 2000m
Not the horribly short effective ranges on the pistols. It's not that it's ineffective beyond 50m. It's just rather unlikely to hit your target beyond that range ... unless it's that sniper pistol from HALO or the sniper pistol from Half Life or the sniper pistol from ............
Starting from that, I'm inclined to take the "effective range of fire" on the Modern Firearms [http://world.guns.ru/assault/as00-e.htm] Assault Rifle pages, where it's provided, as being "point target" range.
Some examples:
Steyr AUG: 450-500m
SA80IW: about 500m with SUSAT
Galil: 500-600m 7.62mm with bipod, 450m 5.56mm without, 300m 5.56mm short version, 150-200m 5.56mm SMG-style
M16A1 (US 5.56mm, 1:12 twist): 460m
M16A2 (Euro 5.56mm, 1:7 twist): 550m
Colt M4: 360m
The sniper rifle pages mostly assume it's as good as the shooter holding it. Some quote accuracy in the 0.5 to 1.0 minute of arc range rather than range but the L96 page quotes 800m for 7.62mm and 1100+m for big Magnum calibres.
Then you get the Steyr 15.2mm AMR, which'll blow clear through a modern APC at 1km, the RT20 20x110mm "effective" to about 1800m, the NTW-20 20mm good to 1500m and NTW-20 14.5mm good to 2300m, the Barrett M95 12.7mm good to 1800m and all those other weapons you wouldn't normally issue.
As I said before, even the 360m range (to 50% on-target for an average shooter not leaping around and spraying) of the Colt M4 is probably all the way across the map for most online FPS games, so it would make sense to scale the ranges down, making the pistol and SMGs horribly inaccurate beyond 25m, the carbines not very accurate beyond 125m, the assault rifles in single-shot mode only 50% accurate
for the soldier class at 200m and so on and maybe also to allow people to set the range on their sights then curve the bullets' trajectory and reduce power over range (google your own ballistics charts for that).
What a lot of games seem to do instead is have really ineffectual weapons, so someone can run up the road despite it being watched by someone with an MG, knowing that the ten hits they'll take from that gun won't be enough to stop them, whereas one sniper shot could drop them instantly.