For starters, how heavy are they? I may need a caddy..
Depends on which one, they can range from 3 or 4 kilos to maybe 8 or 9 depending on type, cartridge and length.
It's the Barret .50 BMG (Browning Machine Gun, which is the same round the guns on the P-51 WWII aircraft shot. 50 caliber, or 1/2 and inch) sniper rifle. The muzzle brake ("arrow shaped end") is used to disperse the gas pressure behind the bullet prior to its exiting the entire barrel. Without it, it would be like dropping a 30 kilo weight on your shoulder, where with it makes it 1/3 or that.
Also, the weight depends on the sniper rifle. The one you showed is a very large rifle and weighs a lot, but it is not the exact same as the one in MGS4, although that is also a .50.
To compare a .50 Browning rifle round with some other rifle rounds, the 4th one in is the 7.62X39mm which the AK-47 is chambered for, and the 5th one in is the 5.56X45mm which the M-16/M4 is chambered for.
Now the .50 BMG is all the way left.
what about pistols ?... magnum, desert eagle, berreta ? which one is better ?
For what purpose? Dropping a target, power, accuracy, ammo capacity, fire rate?