Air guns and air rifles come in many sizes. BB guns are .177. Pellet guns & rifles are made in .177, .20, .22, .25, 9MM, 357 Cal, 45 Cal and 50 Caliber. The last four sizes are usually restricted to the PCP (PreCharged Pneumatic) class of air guns. (Big Boar)
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.
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.
No such thing. You can have small caliber but not low caliber.
Some guns yes, but most guns would require gunsmithing to change caliber.
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.
To find BB guns for sale, one can look in various department stores such as Walmart or Target. Or you can look on online websites that sell BB guns such as Amazon or "Air Gun Depot". The price of the BB gun depend on the caliber of the gun. 0.22 caliber BB guns are typically the most powerful, and therefore, more expensive.
The 308 caliber guns are more expensive than 5.56 assault rifles.
JC Higgins sold about a dozen different .22 cal air guns that were made by Crosman and Daisy. Which model are you asking about?
This is just an opinion of the airguns that I have owned. I can only assume you mean both guns and rifles, not just guns (Like you said in the question). Most collectors and longtime users consider the Sheridan "C" Series air rifles and the Benjamin 392 or 397 the standard that all pump air guns are measured by. They are so well made that they last generations and have a considerable amount of power. The Sheridan C9 is a .20 caliber The Benjamin 397 is a .177 caliber The Benjamin 392 is a .22 caliber. You can not go wrong with any of these models.
I would do a search on the web for "Air Gun Repair," there are shops that make a living repairing air guns. Or see the link below.