Assuming you are talking about shooting a patched round ball, then .490 would be the most normal ball size for a 50 caliber. Of course other types of projectile might be used, such as maxi-balls or bullets with sabots, depending upon the type of gun and the purpose of shooting.
209. Double check with a gunsmith.
You need the services of a gunsmith
Typically BBs are .177 caliber. BB stains for ball bearing, which can be ANY size, those popular for shooting are 177.
Size and relative power. A .40 caliber bullet is .40 of an inch and the entire cartridge is 28.8 millimeters long. A .50 caliber (handgun) bullet is .50 of an inch and the entire cartridge is 40.99 millimeters long. The .50 caliber has more powder, more velocity, and more muzzle energy.
I have a Ranson in ,45 calibre, the nipple on my rifle is M6x.75. Some Italian rifles have M6x100 threads.
A 22 caliber rifle is a firearm and a BB gun is an air gun (Not a firearm) Now if you are asking about a .22 caliber air gun then the difference is in the size of the projectile. A BB is a hard round .175 caliber ball. The .22 caliber is a larger size pellet.
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.
Yes the difference in size is minimal you can cut off excess matirial after you start the ball in the bore if you want.
No! 9mm = .35 caliber - .354 caliber!
.22-.50
By 870 I assume you mean the Model 870 shotgun? If so, there would be no purpose for that type of conversion. Shotshell primers are of the 209 size. The 209 "conversion" is done with muzzle loaders. The idea is to replace the muzzle loader nipple (that holds the cap) with a conversion unit that uses the 209 shotshell primer to get more positive ignition of the black powder.
Its the size of the bullet that the gun is made for. its the caliber that a handgun uses