The game uses battlefield effective ranges, not laying out comfortably on a range firing at paper targets in your own time sort of ranges.
I found that my normal "range" accuracy with the SLR went
waay down when we did a "march and shoot" on the combat range (pop up targets at random ranges, running up to the top of a 20 foot tower to take the shot etc), all after a lovely ten mile hike in full combat load in broiling Vogelsang sunshine in a year so dry we weren't allowed to cook out in the field and so food was brought out in landrovers on manouvres (50DM spot fine for an open fire in 1977 money per head, not per fire).
At least since it was so hot, we did not have to do the usual "dig a trench in full noddy suit" exercise followed by "on the ranges firing in NBC". But from experience - do not expect to hit much when zipped up in impregnated carbon suits with respirators on. The game does not do NBC - but if you want to try, then probably edit all troops from 6 MP to 4MP, any HMG from 4 to 2 and any mortars to 0 would be a likely starting point, together with editing all non-vehicle weapons to half the default?.
And I gather that folks who have tried shooting on a "two way range" find that accuracy falls off considerably in those circumstances. Luckily, never had to do that myself. Just heard stories from the regular PSI staff..
So the game uses a set of
nominal ranges - all carbines are say 400m, all SMG 100-150m and the M4 is a "carbine" so it gets put in the generic useful range bucket for carbines.
Bottom line is that the game doesn't use the "I can shoot a gnat's gonads off at 1/2 mile at my local paper target range" metric of "hobbyist shooting"
. Remember that most regular troops don't have the passion for shooting that civvy hobbist types do, and conscripts even less. All they care about is a basic pass score on the annual marksmanship test (if thier army even does one).
If you disagree - unlike most games that lock you into thier "approved" dataset you have mobhack. Just remember to run the points calculator after any edits. And if playing PBEM, you may have to negotiate with your opponent on your changed OOBs.