All you have to do is to show him ARMA 2. In fact, make him complete ARMA 2.
But because that isn't much of an answer, I'll just include a few details, because if I were to go into everything, I'd be here all day.
1) Ballistics and firearm damage are modelled completely wrong. The cartridge dictates range, accuracy and what not a lot more than the gun does. An FN FAL is considerably more powerful than an M16, and there shouldn't be a huge difference in traits between 5.56mm rifles.
2) Dual-wielded pistols don't work, particularly Desert Eagles.
3) The only thing that dual-wielded shotguns are useful for is the humour factor when you break both of your wrists.
4) Desert Eagles are terrible, terrible guns, and nobody uses them in combat unless they're complete fools.
5) Unless you're a superhuman, you cannot carry a rifle calibrated in .50 BMG on your shoulder while standing up, and you definitely can't fire it without the sort of accuracy that would make people who can't hit the broad side of a barn laugh with derision.
6) Shotguns firing buckshot have effective ranges far exceeding 20 metres.
7) Grenades are a lot more effective - and dangerous to both sides.
8) It takes a lot more than a second to take out, use and sheath a knife, and the reason that most soldiers carry a knife is for utility purposes, and because of the morale boost that an extra weapon gives you, regardless of its utility on most aspects of the battlefield.
9) Russian soldiers shouldn't be using Tavor TAR-21 rifles, as most of the Israeli army haven't had them fully rolled out yet. Conversely, militia soldiers shouldn't be using Winchester Model 1887 shotguns (or more accurately, Model 1901 shotguns, as the Model 1887 is a black-powder gun without the provision to fire smokeless cartridges).
10) Soldiers should be using dedicated marksman weapons for short-range sniping, rather than bolt-action weapons or the like. The Intervention, Barrett M82 and Walther WA2000 don't belong in the game at such short ranges.
11) In fact, as only a few hundred WA2000s were made, all of them being very expensive and notably unreliable thanks to their internals being made for accuracy rather than battlefield conditions, it shouldn't be in the game at all.
12) The M240, a general-purpose machine gun firing the 7.62mm NATO round, should not have substantially lower damage than machine guns firing 5.56mm NATO rounds, like the L86 LSW and Steyr AUG HBAR.
