Who knows. I haven't been shooting with anything fancier than an assault rifle in RL (not counting mounted guns), so I wouldn't know the reality about 0.50 or whatever Sinon's rifle is. Although in Youtube you have videos of Americans shooting bigger rifles however they want. However, in Arma, where I tend to like sniper rifles on suitable missions, they aren't good for clearing houses, despite the game not placing any weird, unrealistic restrictions on them. In the end a big gun that only has for example five bullets in the mag, and only a semi-auto, is a poor choice for such conditions, regardless of the sight. I don't know how many fricking times I've died in the game because I missed the first shot and the enemy returned a healthy burst from an assault rifle. So, it doesn't need any weird hip fire penalties to be the wrong gun for such tasks (the long barrel should in fact make it easier to see where it's pointing). Obviously the main scope is totally useless in close quarters.
But the fact is you don't have to make them suck. Because they suck naturally at short ranges. The shooter games just are so haphazard generally that it might be hard to notice. It makes it all the stranger that this GGO in fact makes them suck less, what with the tiny girl handling the big rifle so easily and the scope being so magical (you don't need an extra sight for short distances if the main scope covers everything, unlike in RL). The only remaining effect is the strange aiming inaccuracy that probably affects every gun from a slingshot to a homing missile... I'd assume it doesn't conveniently affect Kirito's sword at all, though.