My personal gripe was always the original Hawaii 5-O and silencers on revolvers. So much of the noise comes out from the cylinder rather than the muzzle that it would hardly make a difference.
Some guns do not fire at all without cocking, so the purpose is self-evident. Most modern guns need to be cocked for the first shot, and then cock themselves with each subsequent shot using the recoil force. In most modern handguns and rifles, the cocking for the first shot happens automatically during reloading anyway, and unless you intentionally de-cock it somehow, you don’t have to think about it.
For guns that can be fired without cocking, all this means is that the pull of the trigger provides the force required to cock the gun, rather than requiring the user to manually cock it via some other means. This means the trigger force required is rather high, which can degrade accuracy.
Many guns support both means of operation - you can cock it by pulling the trigger, which results in a very hard trigger pull, or you can cock it manually, which results in a light trigger pull.
Again, in modern guns, this applies pretty much only to the first trigger pull after you have somehow reset the weapon.
An example of a manual gun would be a bolt-action rifle, which after every shot requires the user to manually pull the bolt backwards to eject the cartridge casing, and then move it forwards to insert a new one. There are also lever-action rifles, which have the same basic concept but use a lever motion instead of a push-pull motion.
A so-called single-action-only revolver, in which the user must manually cock it after each shot, would be another example.
Most modern guns are semi-automatic: they will fire one round every time you pull the trigger, without requiring any other action. This includes almost all modern handguns, modern revolvers, and modern rifles that are not bolt-action.
An automatic weapon will continue to fire as fast as it can as long as you are pulling the trigger. For semi-automatic rifles at least, this is usually a fairly minor variation in the trigger mechanism relative to the semi-automatic version.
Criminals are stupid, overconfident, and frequently in a hurry.
Using a revolver would solve this problem as well, as it does not eject the shell casings.
This is true. Before the 1994 federal ‘Assault Weapons’ ban, I purchased a SAR-8 (HK-91) from a gun store in Los Angeles. All perfectly legal, and it was not on California’s banned list. Several years later I thought I might have it customised. The gunsmith said, ‘Sure. Just bring in your registration paperwork.’ 
It turns out that the state had expanded their ban, and it now included my SAR-8. I was an avid news-watcher, but never heard anything about it. Certainly no one ever sent me a letter or anything. Naturally, it was too late to register it. I can’t tell you how relieved I was my next trip to Washington when I crossed the border into Oregon with that rifle.
Unless you do what Lee Harvey Oswald did after shooting officer Tippet.
Another reason they do not collect brass is because it tends to bounce randomly on the ground, is small an easy to lose, and not exactly as shiny as you might like.
It can also be very hot!
There are very few “gas operated” pistols (e.g. those using a gas piston or direct impingement to cycle the action), and most of those that exist are modified versions of gas operated rifles or carbines such as AR-15 based pistols; the only purpose designed gas operated pistols that come to mind are the Wildley Magnum (which fired custom high pressure rounds largely based upon cut down rifle castings) although doubtless there are others in special applications. There are a few handguns such as the H&K P7 or the Steyr GB that used a system called gas retarded delayed blowback in which gases vented front the chamber were used to lock the slide in place until pressure dropped after the bullet exited the barrel, but the operation of the slide was by blowback action (that is, once unlocked, the slide was cycled by pressure in the chamber rather than a separate gas system. However, the vast majority of autoloading pistols are either direct blowback (slide is restrained only by its inertia and the force of the recoil spring) or short recoil operated (the barrel and slide and slide are locked together in some fashion which is released by the action of a cam or link after it moves some predetermined distance). Direct or ‘straight’ blowback is typically used with lower powered rounds such as the .25 ACP/6.35x16 mm, .32 ACP/7.65x17 mm, and .380 ACP/9x17 mm, while service-grade rounds such as the 9x19 mm, .40 S&W, .45 ACP, and similar rounds use short recoil operation to reduce the mass of slide and required strength of the recoil spring to a reasonable value.
Some autoloading pistols have to have the hammer cocked or action cycled before they are ready for use; these are mostly referred to as single action (SA, pulling of the trigger only performs one action, to release the hammer) but some striker fired weapons such as the Glock Safe Action also fall into this category. Most common service weapons made today are either combined double action/single action (DA/SA, pulling the trigger can both cock and release the hammer, but in subsequent shots the hammer will already be locked back) or double action only (DAO, hammer is only cocked by the action of the trigger). SA weapons are preferred for greater precision in trigger control as the trigger action is shorter and typically lighter, but in service use nearly all law enforcement agencies and military users (save for select special forces units) us DA/SA or DAO weapons because they are perceived to be safer in regard to an unintended discharge. There are also some actions which partially cock the action to reduce trigger pull but still provide the perceived safety of a DAO, but in practice they are essentially the same in operation as a DAO.
On the topic of length, the barrels of rifles and shotguns are cut down to carbine length or shorter in order to make them easier to handle in tight quarters or to aid in stowing or concealing them. For instance, the M-4 rifle used by the US military (the current evolution of the M-16 rifle) is has a length from chamber to muzzle of 14.5". This makes it easier to handle the rifle and egress from a personnel carrier or aircraft. Unfortunately, it also reduces the velocity and long range accuracy of the 5.56x45 NATO round, which has been problematic in long range encounters with skilled Afghan riflemen using older but full length rifles with larger calibers. Many units have what is referred to as a Designated Rifleman or Squad Designated Marksman (or some variation thereof) who is detailed to carry a larger full-bore rifle for long range contact. These aren’t snipers per se in terms of their assignments and complete skill set but they fill a role in squad defense to engage at distances longer than an assault rifle can effectively be employed and have similar marksmanship skills.
In terms of longer barrel lengths for actual sniper use, the barrel is generally made as long as can be practically carried in the field to achieve a specified accuracy (usually something like 1 MOA out to 1500 meters for a high grade .30 caliber rifle). However, while increasing the barrel length will increase bullet speed at the muzzle exit (thus increasing effective range and reducing bullet drop) there are a number of factors which give limited benefit to longer length including overall mass, balance of the weapon, ability to cover a sizable field (the longer the barrel and further forward a bipod or rest, the shorter the lateral field it can effectively cover without the sniper having to physically move his position), and the modal response of the rifle during a shot (the barrel wants to vibrate like the cantilever beam it is, and the longer the barrel the lower the frequency and greater displacement it will experience, which is harder to compensate for). Most .30 caliber/7mm sniper’s rifles have barrels that are somewhere between 24" and 28" in length (not including the muzzle brake or other attachments) and a .50 BMG will be somewhere around 30" in length. A 120" barrel will be completely unwieldy and long past the point of gaining useful performance from any existing rifle cartridge.
It is sometimes possible to identify the type or make of weapon by characteristic marking on the expended cartridge, but it is rarely if ever possible to conclusively link a cartridge to a gun by cartridge marking alone unless the weapon is unique or has some kind of defect that leaves a particular mark, and anyway, casings can often be strewn about far and wide of the shooter. It is sometimes possible to match marks left on the undeformed base of a recovered bullet to characteristic scratches or other features in the rifling (striations) of a weapon, but this isn’t universally true (many hammer forged barrels use polygonal rifling which leaves little in the way of unique markings), the marks a weapon leaves may change over time with use, and bullets frequently fragment or deform making identification of characteristic rifling marks difficult or impossible. Unlike how television and movie forensics equate rifling marks with fingerprints, rifling marks often do not provide a conclusive link between bullet and weapon, and may often be technically disputed. In fact there isn’t even a universal method for identifying and coding rifling marks used across law enforcement agencies or a standard for determining the degree of uniqueness of an observed pattern of striations, so even with direct comparison between a bullet in evidence and one fired from the weapon alleged to be used in a crime, there is a degree of interpretation of the evidentiary value of direct comparison.
I think the other questions have been answered adequately and I don’t have anything further to contribute on them.
Stranger
The British had a humungous surplus of. 303 ammo at the end of world war 1 (1918) and it was decreed that the stock be used up prior to new designs and requisitions. So during the battle of Britain (1940) the spitfires and hurricanes mounted 8 .303 machine guns instead of 4 .50 cal or 2 20mm cannons. The ammo used were meant for the luftwaffe pilots’ fathers.
Some do.
Watching the police show The First 48, which follows homicide units from the start of a homicide investigation. They had an episode where a criminal put his hand and pistol inside a plastic shopping bag to retain ejected cases.
Use a revolver. Not only will there be no ejected cases, but you can still use the sights. 
I must say, these responses have been phenomenally interesting and informative.
Thank you!
I was under the impression that technically speaking a double action revolver is not a “semiautomatic” in that the mechanical placing of the next round into battery (not “loading and chambering” as it’s a circle of preloaded chambers that move into line with the barrel) is not driven by recoil energy or gas pressure, but entirely by the shooter’s muscular effort. In a standard revolver, after firing, the hammer is left resting on the spent cartridge. If it’s single-action, you then manually cock the hammer and that simultaneously ratchets the cylinder to advance the next chamber and cartridge into place, and the trigger simply releases the hammer. If it’s double-action, the press of the trigger first performs both those actions but again, it’s the shooter’s hand strength that’s moving the spent round out of the way and the next one into battery.
The oddball revolvers are the likes of the Mateba (known to readers of the *Ghost in the Shell *comic) which are recoil operated.
for example, Glock pistols (which are all striker-fired) will leave a rectangular indentation on the primer due to the shape of the striker. but as you say that won’t tie an expended casing to a particular weapon.
Yes, you’re right, in terms of categorizing the way the action works, a double-action revolver is much more similar to a bolt action or lever action rifle than a semi-automatic pistol, since all the energy to cycle the action comes from the user. From the perspective of the user though, all they have to do is pull the trigger, just like a semi-auto pistol.
The OP said that he didn’t know much at all about guns. And for simplicity I would call the blowback of the slide as a gas operation, because that is what it is.
If we were answering a more detailed question your nits would be a more appropriate target. As usual, you have over complicated a simple answer. The slide and action is indeed operated by the gasses of the preceding round and thus explains the lack of a need to cock for each shot.
You are perhaps overlooking the Desert Eagle, a purpose-designed gas-operated pistol which is fairly famous as the largest and most powerful semi-automatic pistol in production, in no small part because it has appeared in many video games and Hollywood movies.
I knew I’d forget something, hence the qualifier. However, the IMI Desert Eagle certainly isn’t a common or convenient weapon unless you are Arnold Schwarzenegger.
Straight or delayed blowback are not “gas operation”; the term “gas operation” is universally understood to mean the operation of a separate mechanism to cycle the action by use of gas pressure. And again, most service-class pistols use short recoil action to lock the barrel and slide together. I’m sorry that you regard that explanation as having “As usual…over complicated a simple answer” but perhaps you can go whine to a mod or start a Pit thread to whinge about it in an appropriate forum.
Stranger
Most of the questions have been reasonably answered except sawed off shotguns. While some states or jurisdictions have banned them, they are classified as an NFA weapon at the federal level and can be purchased or manufactured after the appropriate background check and tax is paid. Indiana just reversed their ban.
The reason for them is maneuverability in close quarters, especially in home defense. The spread is minimized in the close range encounters you would expect in this scenario and knocking someone 10’ back only happens in Hollywood.
Hopefully I didn’t double answer anyone; I scanned the thread.
Main categories:
Pistols:
Revolver
Semi-auto (called “automatic,” especially in detective stories)
Automatic (rare)
Some bolt-action pistols exist but not very popular and old
Rifles/shotguns:
Bolt-action (including single shot, don’t know if any shotguns exist)
Lever action
Pump action (rare these days for rifles)
Semi-automatic
Fully automatic
Revolver-style rifles existed at one point but were mostly experimental.
Bolt, pump, and lever work basically the same way, just the way you operate them differs. These are “manual guns” but that term is not used. Lever is often used for certain calibers, like .30-30, .45-70 Government, or pistol-caliber carbines (.357/.38 and .44 Mag/Spl).
Shortened barrels (either through sawing or made that way) are legal (in most states) but restricted. The NFA that covers fully automatic covers sawed off/short barrel shotguns, short barrel rifles, silencers/suppressors, “Any other weapons” (e.g. pen guns), etc. The same restrictions apply, but automatic are furthermore more expensive due to 1986 GCA. The laws are Byzantine; for example the “always a pistol” rule applies: you can buy short barreled “AR-15s” that are considered pistols without many restrictions, and technically you can switch to a rifle barrel. But once a gun is classified as a rifle, it can’t legally be shortened without lots of paperwork. The penalties are steep.
Not just the barrel length, but also that the distance between rear and front sights is longer, allowing more a more accurate sight picture.
The rest is already answered, the longer barrel add weight and lowers maneuverability, but doesn’t add power, accuracy, or can even decrease them. But check out punt guns!
Basically forever, if stored properly. Older military ammo, especially European, will be corrosive. The US military moved out of corrosive ammo earlier. This generally means that you have to be extra diligent in cleaning ASAP, but isn’t bad for your gun. I have used stuff from the 40s. IIRC corrosive ammo (the primer compound, really) was used because it was more reliable in extreme temperatures and cheaper at the time and with the technology available.
99% of ammo is mostly lead, often with a copper jacket that reduces lead “crud” buildup in the barrel. Non-toxic solutions are mostly copper alloy, AFAIK. But your barrel is steel; steel ammo is the same hardness and will be hell on the rifling. Worn rifling = damaged accuracy.
Shotguns used for waterfowl or in protected areas can use steel because there is no rifling. But steel isn’t a good substitute for lead: it doesn’t fly or deform right. But the other alternatives like bismuth or tungsten alloys are a lot more expensive than steel, so steel is often used.
I’m sure that the show was unrealistic, but some revolvers can be “silenced.” I offer you the silenced Nagant. It is special because the cylinder moves forward, sealing the gas. This works best if you use the proper bizarro ammo (on the right). Many revolvers can use multiple calibers, as the weaker caliber is basically a shortened, lower powder version of the main cartridge.
Note: I ain’t trying to start a fight; Lurking is for all rights and purposes, correct. But since the OP has been enjoying this I decided to run it a little further.
OK - this is picking nits but for pistols bolt, hinged, and rolling block actions were quite popular among hunters for a long time and while that has waned somewhat it still exists and isn’t very old. As an example there is the TC Contender allowing someone to shoot a multitude of different rounds on the same platform
which is an advantage over single caliber platforms like the Remington
(I have owned both at different points in my life, as well as a Jurras, and really enjoyed the TC. Lots of fun to be had there.)
One could also argue that the lever action for pistols had never been more popular than it is today. I would offer as evidence the ones made by Henry
but there are a fair amount of others as well. You almost never see them as antiques or older modified weapons; almost every one out there is modern and many very much so - say within the last 5 years.
On shotguns - bolt action (both single shot and repeaters) are pretty common and fairly cheap although not too many makers are still producing them. Browning does and maybe a few others as well. When the interchangeable choke systems started to become common around the 1980s they started to fade somewhat but still have a strong following among turkey hunters, waterfowlers and some big game people.
Pump rifles are still pretty common because of the popularity of the Remingtons and Henrys. Falling blocks, trapdoors, rolling blocks and some others are making a big come-back as well. Pump rifles I never understood much (they kick like mules and you really can’t get the second shot off aimed and fast so why bother?) but enough people like a challenge when shooting to make the different single-shot actions very popular. Plus, at the cost of ammo these days -----------
Last line first and this is also a nitpick but – rare indeed is the revolver than can be used for different calibers. By this I mean something like a platform that can be used for say .44 mag and .32. They do exist but very few and hardly quick-change.
What I think you mean is more the example of the .38Special/.357 mag. In that case the caliber is identical (both are caliber .357), the difference is the length of the chamber. Caliber is best thought of as the inside diameter of the barrel. From there the different descriptives are more about the maker, the velocity and chamber than anything else.
Revolvers are, for the most part, straight cases. In other words, they do not have the shoulders/tapers found on many rifle cartriges. It makes it easier to mix and match shells although you have to be careful in some of the more antique versions. But while the shells may have different names, the caliber has to remain the same or you become an accident waiting for a place to happen.
On the Nagant another small round of picking nits. First is silencers don’t exist; what we are really talking about is suppressors. But do they really suppress much even in the Nagant? I would argue not really. Note that they have the mic behind the gun and not beside, or in front of, it. And as he flinches a little on a couple of the shots you can hear the noise level change. It helps but not to the same level that it would on a semi-automatic platform using the same shell.
(The Nagant really is a freakazoid but firearms are ripe with examples of freakazoids. My favorite was always the semi-automatic revolver with the Webley-Fosbery being my go-to example. A self-cocking revolver so that your follow-up shots can be as accurate as the first and a lot faster? Sign me up!)
While picking nits:
Sorry, I mean these designs aren’t popular enough that *multiple *manufacturers still make these designs in great numbers. T-C is filling a niche, but it’s not one that many other manufacturers wish to approach.
To be clear: I’m talking about sales and manufacture of new guns.
The mare’s leg is based on the firearm on the TV show Wanted: Dead or Alive which is a cut down barrel/stock lever action rifle. But as noted this is pretty illegal unless you are willing to jump through hoops. But manufacturers are able to get around the loophole by manufacturing them from the start with a short barrel. This is a “lever action pistol” only because it has to be.
Which is why I said that you can shoot cartridges that are for all intents and purposed a shorter version of another cartridge (or the latter is a lengthened version - sometimes the parent cartridge is smaller, sometimes larger). I didn’t say that any old caliber can be put in. Some exist, e.g. .22LR/.22 Mag revolvers, but these usually require you to switch cylinders.
My favorite example (conceptually - I wouldn’t buy one) is the .460 Smith & Wesson Magnum. It can shoot .460 S&W Magnum, or .454 Casull*, or .45 Colt (aka .45 Long Colt), or .45 Schofield.
*Cartridge names are nominal and generally a rule of thumb/intended to invoke another cartridge of a similar size. All these cartridges are the same actual diameter, usually a .452" bullet.
Which is why I used “silencer/suppressor” earlier, switching to the former solely because most people won’t understand what you mean if you use the latter alone.
People flinch even when there isn’t a sound because they anticipate it and it is a big detriment to accuracy. Some people will load a magazine semi-randomly with live ammo and snap caps (fake rounds), and keep practicing until they stop flinching to the caps.
The point is that a suppressor will greatly help reduce Nagant noise levels, but is not very useful on 99% of revolvers because while the barrel gasses are muffled, the cylinder gasses are not.
(post shortened)
Since you knew nothing about guns, it’s best to start with a basic understanding of how firearms operate, and what constitutes a round of ammunition (which the military referes to as “ball” ammo).
A round of ammo consists of a bullet, brass or steel case, propellent, and a primer. The firing pin crushes the primer which creats a flame. The flame enters the case converting the propellent from a solid to a gas, and gas pressure pushes the bullet out of the case and down the barrel.
Given a particular bullet’s caliber, heavier bullets will exit the same firearm’s muzzel at lower speeds than lighter bullets. The ogive and length of a bullet’s side wall creates friction between the bullet and the barrel’s rifling. Longer bullets creates more friction which requires more gas pressure to equal the velocity of a shorter bullet, or you can settle for slower velocity with the same gas pressure.
The longer the barrel, the faster the bullet will exit the muzzle. Shortening the barrel reduces muzzle velocity due to a reduction in the time the force of the gas pressure has to excellerate the bullet.
Bullets begin to lose velocity as soon as they exit the muzzle, and gravity immediately draws the bullet downward.
Primers come in several sizes and “power” loads. Small and large pistol primers, small and large rifle primers, magnum primers in all sizes, shotgun primers, bench rest quality primers, and military-grade primers. Bench rest primers have a very consistent flame rate, and military primers require more firing pin force before they can be crushed. Primer selection is matched to the round and it’s expected pressure.
You asked about “cocking a gun”. The act of “cocking a gun” stores energy in the cocking spring (the names vary).
Assuming there is a hammer, pulling the trigger releases the hammer which a) drives the firing pin into the primer, or b) strikes a transfer bar which drives the firing pin into the primer.
Firearms with no hammer have a cocking piece (the names vary) and spring which, when release by the trigger, drive the firing pin into the primer.
A “single-action” firearm refers to a firearm which needs seperate actions to fire. First you cock the gun. Then you pull the trigger. A “double-action” firearm can both cock the hammer and release the hammer with a single pull of the trigger.
You asked about sawed-off shotguns. Removing inches from the barrel will make the shotgun easier to conceal but will reduce the ft/lbs of energy of any projectile. Depending on how much you shorten the barrel, you could lose half of your velocity.
The formula for ft/lbs of energy is mass times velocity squared. Reduce the mass by half and you lose half your ft/lbs. Reduce the velocity by half and you lose 3/4 of your ft/lbs.
437.5 grain slug at 1200fps = 1399 ft/lbs
218.75 gr slug at 1200fps = 699ft/lbs
437.5 gr slug at 600fps = 349 ft/lbs
Removing most of the stock will also make the shotgun easy to conceal but it will make it more difficult to control (hit your target). Reducing the shotgun’s weight increases felt recoil.
p.s. Don’t believe the horseshit you see on damned forensic shows. They need to solve the crime with 30 minutes. 
Nobody’s brought up the open bolt vs closed bolt difference. Open bolt systems have the bolt retracted when ready to fire. Pulling the trigger releases the bolt which picks up a round from the magazine on it’s way forward. When the bolt is in position the firing pin strikes the round causing it to fire. The action locks the bolt back again after the shot. Closed bolt weapons have the bolt forward with the round chambered ready to fire. Pulling the trigger merely releases the firing pin. The action after firing cycles the bolt back to eject the casing and pick up the new round on it’s way forward before the next shot.
Open bolt is considered more beneficial in fully automatic weapons since you get air in to the chamber that presumably is hotter from firing more often. Since 1982 BATF considers all open bolt weapons (except grandfather ones) to be automatic. That makes it less common in civilian hands but some weapons with open bolt systems show up in entertainment.