Wikipedia:Reference desk/Archives/Science/2020 December 20

= December 20 =

Cocking and sliding handguns
Frequently on film, characters either cock a handgun ("pull the hammer back," I think it would be referred to) or slide the top of the gun back to get a bullet from the magazine into the chamber. (I might be getting some terminology wrong here, sorry.)

But don't these things occur automatically? I mean, you don't have to cock the hammer back while the revolver is turning and putting another one of the 6 bullets into place to be shot -- that would be really slow. And similarly, when the magazine that fits into the handle of a more advanced, updated gun (a glock?) advances bullets into the gun automatically as each one that's already there gets shot, you don't have to manually advance them.

So I'm wondering if these actions are done merely for effect, to show that the character is getting ready. Or is there a real reason why these things are done. Thanks!  DRosenbach  ( Talk 01:06, 20 December 2020 (UTC)


 * The bullet has to get into the chamber somehow, when you stick the mag in all you did is put it near the chamber, you still have to do the same pulling the slide back thing that would normally be powder-powered, by recoil or gas or whatever. So if anyone accessible is trying to kill you and is putting the mag in attack him now because you can only get shot one or zero times if he can't reach the pull the slide back and let it return phase. Oops, I wasn't thinking, if there's still a round in the chamber once you insert a non-empty mag the system should be just like after you pulled the slide back after normal loading so don't get to that part cause he's now loaded with whatever's in the mag plus 1. This would be the case if he didn't shoot till there was absolutely nothing left, if the slide locks rearward that means the chamber is empty and he is reactively reloading. The magazine insertion must let the slide forwardization spring (the one that's resisting when pulling it back, don't know proper name) release somehow then you would still be completely safe from being shot until the slide goes back once then forth once. And maybe there's some rare guns that load the gun automatically with motors or something, I dunno. And the old-fashioned cowboy guns had to be cocked after every shot. No one uses them anymore but maybe there's some modern ones that still need the the hammer cocking for the first round sometimes. You have to do the ch chink thing to something to load an AR-15 too. Sagittarian Milky Way (talk) 01:58, 20 December 2020 (UTC)
 * I'm sorry -- that was a super convoluted answer. If I want to shoot 5 bad guys with a Glock, I don't need to slide that thing back 5 times -- it's automatic.  So why do they need to slide it back to begin with at all -- that's my question.  And as for the old 6-shooters, ok, maybe in the times of the cowboys they needed to manually cock the hammer, but say in the 1990s, when the NYPD was still using revolvers, wasn't it automatic, in the sense that pulling the trigger was all you needed to do?  So assuming I'm not watching some old Western film, all of the cocking and sliding is meaningless...correct or incorrect?  DRosenbach  ( Talk 02:58, 20 December 2020 (UTC)
 * Actually come to think of it it might not be physically possible to fire some or all revolvers without at least cocking the first shot, just like in the pistol some of the chemical energy of the gunpowder must be used somehow to turn the cylinder and recock the hammer or slide for you so you don't have to do it after every shot. So it is not meaningless, it is the only way to shoot the first round after the gun has been completely empty and/or decocked for extra insurance against accidental discharge. And if you did in fact pull the slide back at a time when that was meaningless cause it was already loaded and cocked (perhaps in an attempt to show that you're ready and don't need any further preparatory motions to intimidate) that would just do the same thing that happens between shots which would be to try to eject whatever's in the chamber as if it were an empty shell then reload the chamber if possible. This would throw away 1 unused round on the ground at best, maybe even make the gun unable to safely fire if the round gets stuck since it's shape is different when the bullet is still sticking out of the part that normally comes out of the chamber when you shoot (the bullet of course normally leaves by the barrel, not still stuck to the empty husk)Sagittarian Milky Way (talk) 03:04, 20 December 2020 (UTC)
 * And to be clear a 1990s hammer on a revolver would still need to be pulled back after every shot, it just doesn't have to be done by hand. Maybe the energy of the gunpowder does it, maybe you can even make it cock even the first hammer pull if you make the first trigger pull long and hard enough to have enough energy to cock the hammer with a mechanical linkage. What do I know, I'm too young to remember revolvers on TV much. Nevertheless no matter how it's done it has to be done as the hammer has to hit the cartridge hard enough to ignite the gunpowder primer and it can only do that by being pulled back against the force of a spring. Sagittarian Milky Way (talk) 05:09, 20 December 2020 (UTC)
 * Revolvers are largely mechanical, with their cylinders rotating and (for double action revolvers) the hammer pulling back all as a result of the trigger pull. Automatic weapons use the energy from the gas escaping from the chamber to cycle the loading mechanism.  Ian.thomson (talk) 05:42, 20 December 2020 (UTC)
 * Short answer: in an automatic or semiautomatic, the mechanism for setting up the gun to fire the next round is powered by the energy released by firing the previous round. When you first load the gun, it hasn't fired yet, so you have to do this manually. Iapetus (talk) 10:18, 21 December 2020 (UTC)
 * Thanks Iapetus, and everyone else who contributed! :)  DRosenbach  ( Talk 22:20, 11 January 2021 (UTC)


 * For revolvers, it depends on whether they're single or double action. In either case, pulling the trigger rotates the cylinder to the next round (ensuring that a bullet is going to be in the right position), and releases a cocked hammer for whatever purpose it serves in the firing mechanism (this part does vary throughout history).  Single action (which was more of a thing in the cowboy days and still a thing in many rifles and shotguns), you need to pull the hammer back every time (which is why you'll see cowboys "fanning" the hammer, repeatedly slapping it back).  (A cowboy using a single action revolver might also keep the hammer cocked on an empty chamber, so that if the hammer gets knocked it'll just click but they can pull the trigger to cycle the next round).  Double action, pulling the trigger does pull the hammer back but pulling the hammer back ahead of time reduces the amount of pressure the trigger requires.  This makes the first shot slightly faster and more accurate.
 * For automatic pistols, if you take an empty pistol and load a magazine into it, the chamber is probably still empty (having it automatically load that first round presents a variety of safety issues, though ones I would not be amazed if some company somewhere has insist they fixed). Automatic pistols use the gas from the previously fired round to push the slide back: which opens the chamber, ejects the spent casing, loads the next round, and cocks the hammer all in one action.  As the first round had no previous round to open the chamber, the first round would need to be loaded manually.  This also results in most automatic weapons having a capacity of X+1 (for the chamber), since you could load the chamber and then load a magazine (leading to scenes in action movies where someone counts the number of bullets in the magazine and forgets the one in the chamber).  Ian.thomson (talk) 05:42, 20 December 2020 (UTC)
 * And not all people will know that you are using the "automatic feeding" definition of automatic and not the "machine gun" one. You still have to pull the trigger before each shot in the "automatic feeding" definition of automatic which is called semi-automatic when a less slightly old-fashioned or jargony name is desired while if you can rapidly empty a cocked and loaded gun by simply squeezing the once and never unsqueezing that would be fully-automatic. Sagittarian Milky Way (talk) 06:45, 20 December 2020 (UTC)


 * The best answer for why this is done "frequently on film" is for dramatic effect.
 * A slight correction to Ian.thomson's answer above regarding single-action revolvers: Unlike double-action revolvers, the SA revolver trigger pull doesn't advance (partially rotate) the cylinder -- all it does is release an already cocked hammer. It is the hammer cock which advances the cylinder.
 * Additional, there is no utility in keeping a hammer cocked on an empty cylinder with an SA revolver, as the trigger pull will not advance the cylinder. The hammer would have be released (via a trigger pull) and the revolver cocked again to prepare the it for firing.
 * As mentioned in Safety (firearms) and in Revolver, older SA revolvers which were not drop safe may be carried with the uncocked hammer over an empty cylinder, reducing the capacity by one, but preventing a accidental discharge were the gun to be dropped or the hammer inadvertently struck.
 * Some revolvers are double-action only (DAO) -- particularly the so-called hammerless types -- but most DA revolvers allow single-action operation as well, and manually cocking the hammer allows for an easier trigger pull (and in the movies is used to signify that the shooter means business -- see TV Tropes:Dramatic Gun Cock).
 * A semi-automatic pistol may be carried with an empty chamber (Condition 3), typically for reasons of perceived safety (but search on "Israeli carry" or "carry with an empty chamber" for extensive discussions on the subject), and in that case the slide would have to be racked before it could be used. Were one with a chambered round to be racked, the ejection of the unspent cartridge may lessen the dramatic effect.
 * Drifting away from handguns, if you search on "cruiser ready" or "cruiser safe" you will find many suggestions that home defense pump-action shotguns be kept in condition 3 (with the tube magazine fully loaded but an empty chamber). Racking such a shotgun would be necessary to bring it into battery, and it certainly produces a dramatic noise. -- ToE 10:10, 20 December 2020 (UTC)
 * Ah yes, I forgot about shotguns, at the risk of confusing OP further or giving unwanted information some people still carry shotguns for defense where you still have to pull a thingy back before every shot. (this thingy is called a pump) Perhaps this is because full-powered shotgun recoil is very powerful and you might not want to shoot it faster than that anyway, given that semi-automotic shotguns are less certain to work (especially if you mix ammo types), more expensive and shotguns allow partial misses to be a thing which can stun the bad guy long enough to pump the shotgun between shots without him shooting at you. All in all pulling a thingy back every shot is still very much a thing in modern guns, it's just sometimes done automatically. Sagittarian Milky Way (talk) 14:25, 20 December 2020 (UTC)
 * Shotgun discusses the various actions available on shotguns. And yes, there are semi-automatic shotguns.  The slide on a pump-action shotgun cycles the action, ejecting the spent shell on the back stroke, chambering a new shell from the tubular magazine under the barrel on the fore stroke, and cocking the hammer or striker in the process.  It is not intended to slow the home defense shooter down, and the complexity of rapidly cycling the action to be prepared for a follow-up shot if necessary requires practice to master.  But in the US, at least, pump-action shotguns are more common than semi-automatic ones for a variety reasons as described in this Quora thread, including cost, reliability, simplicity, ease of care and maintenance, compatibility with a wide range of loads, and tradition. -- ToE 17:18, 20 December 2020 (UTC)
 * Of course if semiautomatic shotguns were at least as good as pump-action in all aspects no one except a strong traditionalist would use them. But if someone needs to move the gun back on target after shooting because of recoil, and they have practiced enough to be able to finish pumping before finishing reaiming, then the extra pumping time would make little or no difference in how fast they can shoot at targets. Sagittarian Milky Way (talk) 18:34, 20 December 2020 (UTC)


 * I can imagine a pistol designed in such a way that the first cartridge in the magazine is in line with the barrel and the firing pin; but then, because the firing chamber has no lower wall, the pressure of the burning powder would deform the shell, wasting some energy and likely doing damage to the magazine. So instead, we're stuck with manually moving the first cartridge from the magazine into the firing chamber. —Tamfang (talk) 02:08, 26 December 2020 (UTC)
 * It is possible to design a semi-automatic pistol to fire from the open bolt, but, without the concerns of cook-off, the disadvantages outweigh the remaining advantages, particularly for a pistol. See MAC-10 for the fate of one such firearm. -- ToE 14:41, 26 December 2020 (UTC)
 * Why are our most knowledgeable contributors on firearms all British? And yes being illegal (even in USA) cause machine gun conversion fears is a disincentive to manufacture, among other disadvantages. Sagittarian Milky Way (talk) 14:49, 26 December 2020 (UTC)

Free atom radius of hydrogen
My 2004 Sargent-Welch periodic table gives a value of 0.79 Å. The source is given as "Quantum mechanical value for free atom".

We list the empirical covalent radius as 0.25; calculated radius as 0.53; and Van der Waals radius as 1.20.

Another S-W example is 0.91 for C.

We list the empirical covalent radius as 0.70; calculated radius as 0.67; and Van der Waals radius as 1.70.

Or Zn 1.53.

We list the empirical covalent radius as 1.35; calculated radius as 1.42; and Van der Waals radius as 1.39.

Does anyone know the actual source of those S-W values? Sandbh (talk) 04:23, 20 December 2020 (UTC)
 * An atom is not a tangible thing, in the sense of having a well-defined radius. You've already found at least 4 different radii for Hydrogen.  Of course Wikipedia has Atomic radii of the elements (data page), which lists more than what you have, but also does not include the 0.79 value.  The closest to a "free atom" radius, i.e. a gas-phase single atom, would be the Van der Waals radius, which is how the gas phase atom behaves if considered a solid sphere (i.e. what is the radius where it "collides" with other atoms).  I'm afraid I am being stumped at finding a source for the 0.79 angstrom value, though I have found several places that state it, such as here and here, but they do not cite their sources.  -- Jayron 32 12:55, 21 December 2020 (UTC)