POSSIBLY the .14-222. This is a .222 cartridge necked down to .14 caliber. MV is about 4500 fps. The Remington 30-06 Accelerator is a FACTORY loaded saboted 30-06 (.25 caliber bullet) that is close behind it. There have been stories of wildcats that exceed 5000 fps, but at that speed, bullets tend to break apart from the speed of the spin. The extremely high velocity cannon on modern tanks overcome this problem by being smoothbore- no spin on the projectile.
you will have to define if you mean commercial or wildcat.
Depends on the load
The L1A1 SLR used by the British military is in caliber 7.62 NATO, and has a muzzle velocity of about 2800 fps.
800-1000 fps
The .45 caliber bullet is not used in a sniper rifle.
The muzzle velocity of a .270 caliber rifle can vary depending on the specific ammunition being used. On average, a .270 Winchester round can have a muzzle velocity of around 3,000-3,200 feet per second. It's always best to check the specific manufacturer's data for the most accurate information.
Some of the fastest- the Ruger .204, the .220 Swift, and the 30-06- but only if the 30-06 is shooting the Remington Accelerator. That is a sabot cartridge that fires a small .25 caliber bullet at roughly 4,500 fps.
It can vary from 900-1300 FPS depending on the weight and the firearm. Generally a smaller slug can have a greater velocity and a greater energy.
Between 830 and 1300 feet per second, depending on the load and the barrel length.
Can't be answered without knowing the projectile weight and muzzle velocity
No one answer. There are dozens of different .44 caliber cartridges, and hundreds of different loadings of those cartridges. A rifle will have a different velocity than a handgun. This is sort of like "how fast can a V8 car go?"
"Deadlier" is subjective. The .223 has more velocity on average, but the .308 has considerably higher muzzle energy.