It is tapered. Diameter is different at different points. Usually this question arises when someone is shopping for a muzzle brake. Those made for the SKS and Mosin/Nagant have a straight sided barrel, and will not work on Remington.
the store
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.
Depends on the load
the first projectile was a muzzle loader ball discovered in 1812 and it was a 68 caliber ball
ABOUT .72 caliber. I say about because the degree of choke on a shotgun changes the muzzle diameter.
you will have to define if you mean commercial or wildcat.
50-450 usd
Depends on the load for that cartridge. MOST 30-06 rounds run 2500 to 2900 FPS, but there IS one load (Remington Accelerator) listed as 4,500 fps. That is a .25 caliber bullet is a plastic sabot.
The .45 caliber bullet is not used in a sniper rifle.
@ 2800 fps or so
The L1A1 SLR used by the British military is in caliber 7.62 NATO, and has a muzzle velocity of about 2800 fps.
The velocity of a .223 caliber projectile can vary depending on factors such as the specific load, barrel length, and type of firearm. Generally, the velocity can range from around 2,700 to 3,300 feet per second.