system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why are some guns more deadly than others? Is it something to do with the speed at which bullets come out, or the size of the bullet? Like why is a revolver taken to be a stronger weapon than a glock/pistol?
gpt
Oh dear, you're already getting a lot of terrible information, so let me help you out:

There's a lot of different factors that determine how "deadly" a firearm is.  The most important is probably the type of round it fires.  The keys to a deadly round are how much force it carries, and how effectively it transfers that force to the target.

There's two major factors in a firearm cartridge to consider: size of the bullet, and size of the casing.  Bigger (wider/longer) bullets will obviously carry more force simply by virtue of having more mass.  Longer casings will provide the bullet with more velocity because they hold more gunpowder.  Generally speaking, a higher combination of mass and velocity is more deadly because it carries more force to the target.  However, that only gets us part of the way there.

How that force is delivered also makes a difference.  The goal of a bullet is not to simply penetrate a target, but to damage it.  In this case, shape and composition of the bullet come into play.  Rounds that have more solid bullets (such as full metal jacket) and/or more pointed ends are more likely to cleanly penetrate a target.  Cleaner penetration means the shock will be concentrated to a smaller area.  Also, if the bullet passes *through* the target, then some of its energy will not have been transferred.

Rounds that have flatter/softer tips, on the other hand (such as soft-point or hollow-point) will expand on impact, increasing the area in which the force is delivered in the body.  This means force is delivered to a wider area, causing greater shock to the target.  The expansion also decreases the likelihood that the bullet will escape the body, ensuring that *all* of its kinetic energy is transferred.

So, generally speaking, you want a heavy, fast (but not so fast that it over penetrates) round with a soft, expanding tip.  Right?  Well...maybe.  Another important fact about firearms is that they are very situational.  They are a form of tool, and there's a right tool for every job.  While you might want a medium-velocity hollow-point round to shoot a regular target, what about someone behind cover?  In that case, a more pointed round (such as a spitzer bullet) with a higher velocity would be better.  What if you're shooting at very close range, possibly with poor visibility?  You may not want a bullet at all, but rather a shell loaded with shot pellets.  

As for the firearms themselves, it's also very situational.  Most people would probably pick a modern automatic or semi-automatic rifle as "more/most deadly," but it depends on the use.  If your target is very far away, you might hit them with a bolt-action rifle when your AR-15 can't even fire that far.  If they are close, a shotgun would be more effective, while a rifle round would probably fly right through them.  Infinite other questions also affect the answer.  Is the target shooting at you?  Are they wearing armor?  What is the terrain/climate like?  Is being immobile an option?  Is the target aware of your presence?  

An interesting example comes from the war in Afghanistan.  The standard issue rifle for the US military is the AR-15 (M16).  This has long been considered the modern gold standard for military rifles.  However, in Afghanistan, Taliban fighters were using less fancy rifles, but with higher-power rounds.  Due to the open terrain of the country, they could take up position far away and shoot at US soldiers, while the AR-15s didn't have the range to reach them back.  The military then started re-issuing the M14.  Although it's an outdated firearm (taken out of service in 1964), it fired a round that could travel the distance and properly engage the Taliban.

TL;DR: Mass + velocity are generally good, but otherwise everything is situational.