"Why do weapons jam?" == Grdzero
Classic question. Why do any moving parts eventually fail ? Friction plus wear. Dirt contributes to
both friction and wear. What you are calling a miss-loaded round, I assume to mean a "stove-piped" round.
Proper design of the chamber and bullet carry mechanism should take care of all but the worst of dirty weapons
and improper pressure in the loads. (Which causes the auto function of the chambering action to fail to function
as designed. Simply use proper pressure loads and keep the action clean.)
The overheat problem for automatic weapons deserves a separate response.
You will probably get multiple definitions for head-space. Rather than give you one, let me explain my understanding
that has kept my rounds safe through years and thousands of rounds of reloading. There are multiple cartridge types
thus different definitions. What it means to me is that in a properly loaded round, upon firing of that round, the bolt face
and the base of the cartridge should not show relative motion in a perfect firarm. Ergo, the case of the bullet should not
have to streach back until it is engaged by the bolt face when fired. This can have drastic implications in most rifle cartridges but
not so much so in non-magnum pistols. (They do not generate enough pressure to place the brass in a near liquid flow condition.)
In older firearms, such as my old .303 Enfield, time and wear have combined so that upon firing, the .303 case will streatch back
along the anealing line which can rupture the case and scatter bad stuff back to the shooter. That means the case no longer
fully contacts the bolt when closed, and moves back to it when fired, but the front part of the case expands to grab the barrel metal
and the anealing line (about half an inch from the rim on .303 cases) is where this grabbing action stops, and only that part of the case
moves back, which thins the case along that line enough to be very dangerous. I can talk more to a specific round and case and firearm
if that was the intent of your question. If you shoot a lot and/or reload, these concepts are absolute SAFETY requirements.
The shotgun gauge question is easy. Try this reference
Airguns, an entirely separate topic as well, but you can hunt with them. Here is a sample .50 caliber airgun that is a serious hunting arm
Note it is recommended to use a SCUBA tank to refill the air chamber and these weapons are NOT silent. They are however rather
distinctive sound makers.