In Halo the human's technology has evolved to include cyborgs and giant spacecraft capable of hyperspace.
Yet in the year 2555 the shotgun and the sniper rifle seems to have gotten through the ages unchanged?
The shotgun (with very minor cosmetic changes) looks like the new Remington "Super 870" Riot Shot Gun that we use. With the folding stock extended of course. Though some on my squad think Master Chief is using an Ithaca Police 12
The sniper rifle is a Barret .50. Plain and simple. Though the game took the liberty of placing a computer thingie on top of the gun where our optic sight would be.
The Assualt Rifle doesn't look like anything we have now, but H&K has a few models coming out that are being tested for the military. Those resemble the Halo Assualt Rifle.