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 .223 Remington rifle cartridge has a muzzle velocity of approximately 3600 feet per second.
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.
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.
50-450 usd
you will have to define if you mean commercial or wildcat.
@ 2800 fps or so
The .45 caliber bullet is not used in a sniper rifle.
The L1A1 SLR used by the British military is in caliber 7.62 NATO, and has a muzzle velocity of about 2800 fps.