Some guns yes, but most guns would require gunsmithing to change caliber.
Pellet guns tend to come in two standard caliber sizes. The first smaller caliber is the most common for pellets guns and is .177 caliber. The second is .22 caliber that can be found on some higher powered pellet gun models.
7.62 mm (or 30 caliber) and 5.56 mm (or 223 caliber) are common rifle cartridges. Pistols, machine guns, and artillery will of course be different from these.
The caliber
.68. Almost all guns are this caliber.
The 308 caliber guns are more expensive than 5.56 assault rifles.
44-40 ?
Mostly all of the paintball guns made today are .68 caliber. Though with the recent changes in paintball some guns are being made with .50 caliber. Though by the year of this gun, it is 100% of .68 caliber.
Artillery
In current standard production, no.
Depends on the caliber.
Which model?