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I recently played the game Mary Skelter - Nightmares, in which the protagonist after some time acquires a weapon that is called a Mary Gun - a device that shoots his own blood.

I'll change the properties this gun has in the game a bit:

  • A Blood Gun is supposed to be a gun-like device strapped to the hand/forearm of the wielder.
  • A syringe that is part of the weapon design is able to draw out blood from the wielder.
  • When pressing the trigger the gun shoots the blood of the wielder at the enemy.
  • The weapon can store a certain amount of blood so that there is enough blood readily available when needed.
  • The wielders blood is an effective way to deal damage to enemies that are hit by it - assume the blood has some magical property that damages magical creatures that get into contact with it somehow. This magical property can be ignored when making assumptions related to the blood. The gun is merely used to deliver the blood reliably over a distance of at least a few feet when needed.

Such a Blood Gun poses quite a few problems:

  • How many times can the gun be fired in a row?
    The answer to this problem is comprised of two separate problems:
    • How much blood can I drain from an average adult German male (~80kg at ~180cm) before my protagonist falls unconscious in a stress situation?
      Unconsciousness in battle would mean the end of my protagonist.
    • How much blood do I need to shoot for each shot?
      Depending on the design there may be a minimum amount of blood, for example to build up pressure. The gun should be able to fire the blood at a distance of at least a few feet.
  • How far can I shoot the blood in its liquid form?
    The farther, the better.
  • How fast can I drain the blood without my protagonist suffering any medical difficulties?
    Drawing a huge amount of blood in a matter of seconds may introduce problems for my protagonist, but the faster I can draw the blood, the better.
  • How can I make sure that the blood doesn't clump inside the gun?
    Depending on the design the blood doesn't need to be fired directly after being drawn out of the wielders body, but could instead be kept inside the gun. This could for example be used to build up pressure and to already have a "bullet" ready when it's needed, which requires anticoagulants being stored in sufficient quantity and applied to the important parts of the gun without affecting the wielder. Otherwise my gun might not work the way I want it to work in a critical situation, for example because the blood won't be able to pass through the exit.
  • How much weight is acceptable as a sort of gun that is wielded in one hand and therefore limits the amount of parts and storage of the gun?
    A design that requires the protagonist to carry multiple kilograms of equipment attached to their hand would be problematic when trying to escape danger or when trying to quickly target an enemy.
  • How can I make sure that my wielder will not die from blood loss the moment something damages the gun?
    If something hits the gun and suddenly the syringe rips open my protagonists' arteries he will surely die without any help.

All of these problems lead me to the question:
How can I build a Blood Gun with present day technology?

This question is purely about the technological considerations that I need to account for in such a weapon.

The fact that shooting your own blood doesn't look like an intelligent way to combat enemies can be ignored. Other solutions, such as preparing "blood bullets" in advance and simply having a couple cartridges with you are outside the scope of this question.

For all cases where you have to make assumptions about technology you can assume real world technology. For all assumptions about the wielder and his blood you can assume an average adult European male.

Answers should take the problems outlined above into considerations. The more problems you discover that I couldn't find the better.

This question is different from Building a Syringe Gun, which basically asks about a tranquilizer gun, because I don't want to simply fire liquids in cartridges or containers of any kind - I want to shoot blood in its liquid form, like a water pistol, with the blood taken more or less directly from the wielder of the gun when pulling the trigger.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – L.Dutch - Reinstate Monica Aug 8 '18 at 17:29
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    $\begingroup$ "How can I make sure that the blood doesn't clump inside the gun?" - Would a scab shooting gun be acceptable? Stores the blood, dehydrates(?) it, packs it into bullet shapes, shoots small super dense dried blood projectiles! $\endgroup$ – Sandy Gifford Aug 8 '18 at 18:26
  • $\begingroup$ That's a lot of questions, Secespitus.... And since people get woozy after drawing a pint, this appears pretty impractical. Why wouldn't you synthesize plasma from your blood and use it as the source? Doesn't it take hours to a day to replenish the blood? $\endgroup$ – JBH Aug 12 '18 at 3:58
  • $\begingroup$ I think if you are so desperate as to use blood for ammo, explosively spreading 100% of your blood over an area of effect is a viable tactic. $\endgroup$ – Aron Aug 14 '18 at 12:12
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This dude is shooting his own blood. Time to set niceties and practicalities aside, and maximize the grisly badassitude.

  1. Blood clotting. He prevents this by anticoagulating himself before going into battle. A shot of low molecular weight heparin would work within 15-20 minutes and last 12 hours. His blood will now not clot.

2A. Blood delivery. He has an arterial line leading from his wrist, entering his radial artery and continuing up to the aortic valve. His gun shoots blood at arterial systolic pressure, in spurts with the beating of his heart. He can open and close the valve with the same hand the line is in, turning a stopcock with thumb and forefinger.

2B: Blood delivery. Normal systolic blood pressure is only about 2.5 psi. He takes drugs before battle to raise his blood pressure up to 210/120 and increase spurt distance. This gives him a headache, bloodshot eyes too and possibly a nosebleed, which will help if this is an anime. If a normal arterial blood spurt can go 18 inches laterally this should get his range up to about 4 feet. For longer shots, parabolas are his friend.

3A. Blood supply. He is a blood doper like the professional bike riders, using erythropoietin to run a hemoglobin of 19-20 as opposed to a normal 14-15.

3B. Blood supply. He already has an arterial line in place. He has bags of saline and sugar (and more drugs) in his back. During a lull in the battle he can force a few of these down his arterial line into his heart. This will replete volume and is also good for the anime, as the red catheter from his wrist will turn the color of the fluids as he forces them back. He will need to overcome his own systolic blood pressure to get the fluids in, so it will take some force.

As he runs low on blood but keeps his pressure up with drugs and fluids, he will get paler and paler. This will be noticeable in his skin and also his bloodshot eyes. He may have an insatiable craving for ice after the battle, chawing down great handfuls of it.

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    $\begingroup$ Probably also a craving for red meat, blood sausage, and similar foods with large amounts of easily-absorbable iron. Expect him to develop an aneurysm at some point. $\endgroup$ – Perkins Aug 7 '18 at 17:40
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    $\begingroup$ I can't help but think that heading to a fight with Rat Poison upto your eyeballs is a sure way to bleed to death. $\endgroup$ – Aron Aug 8 '18 at 3:18
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    $\begingroup$ Point1 runs risk to the shooter too - failure to clot a wound could lead to a loss of ammunition, somewhat like a ripped ammo pouch and all the cartridges dropping out. $\endgroup$ – Criggie Aug 8 '18 at 4:44
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    $\begingroup$ Anticoagulating before a battle doesn't sound smart. A small cut will leave him bloody bloodless... $\endgroup$ – L.Dutch - Reinstate Monica Aug 8 '18 at 5:54
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    $\begingroup$ Re anticoagulation - the whole scheme is totally crazy! Shooting fresh blood from your aorta? Blood pressure of 210? I figure the sort of manga with a blood gun needs unsafe battle strategies so I piled them on. $\endgroup$ – Willk Aug 8 '18 at 12:35
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All you need is a normal water gun hooked up to a tube in your arm and you have an auto-filling blood gun with a few pints of ammo.


According to the professional vampires at the Red Cross, a normal person can safely donate a pint of blood every 8 weeks. However, that is just to have large safety margin, as the red blood cells are replaced in as little as four weeks:

How long will it take to replenish the pint of blood I donate?

The plasma from your donation is replaced within about 24 hours. Red cells need about four to six weeks for complete replacement. That’s why at least eight weeks are required between whole blood donations.

As with a normal blood donation, blood pressure alone will fill up the gun's reservoir in a few minutes:

Donation

We will cleanse an area on your arm and insert a brand–new, sterile needle for the blood draw. This feels like a quick pinch and is over in seconds.

You will have some time to relax while the bag is filling. (For a whole blood donation, it is about 8-10 minutes.

Since a pint of blood can be drawn in 8-10 minutes with blood pressure at a resting heart rate, we can assume the process will go much faster when heart rate is up during a fight. Even quicker if the gun includes a pump to manually pull the blood.

So, at a minimum you can safely have an auto-filling blood gun with a pint of ammo. However, you can definitely do more than that without dying:

Hypovolemia is the technical term for 'bad stuff from blood loss', and is something we want to avoid. According to this paper a person may enter hemorrhagic shock(described as 'rapidly fatal') after losing 40% of their blood in a short period of time, but anything less than 30% has relatively minor issues such as anxiety and slightly reduced oxygen delivery to muscles and organs. That paper also claims that the average person has 70 ml of blood per kg of body weight, so your hero has about 5600ml/11 pints of blood total. This means the hero can safely lose 1 liter/2 pints of blood without much issue, 1.5 liters with some lethargy and anxiety problems, or 2 liters with a serious risk of death.


In short, your gun can pull about 2 liters/4.25 pints of blood max before killing the user, so let's say your gun has 1 liter of ammo at a time to be safe. The first water gun I found online holds about 22 ounces/650 ml/1 pint of water and can shoot several blasts of water up to 34 feet/10 meters away, so you can definitely make a blood gun with a 1 liter reservoir and a range of several meters.

You'll need about a month to recover each pint/half liter of blood you use, so try not to get in too many fights.


Addendum on preventing clotting: Something I didn't consider until it was pointed out in a comment: these gross blood guns are going to clog from coagulating blood. The fresh stuff in the reservoir should be good for a bit, but the barrel will clog pretty quickly and the rest will soon follow.

To solve this problem all you need is a tank dripping anticoagulants into the blood reservoir, essentially turning it into a blood bag. There are many types of anticoagulants to choose from, so as long as you have a steady supply your gun should last longer than your body.

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    $\begingroup$ Can you add some text supporting the assertion that normal blood pressure would be sufficient, maybe by comparing it to the operating pressure of the average water gun? $\endgroup$ – Bear Aug 7 '18 at 16:46
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    $\begingroup$ @Bear: I just added some more info to clear up what I meant, but basically blood pressure is all that's needed to load the gun, and the normal pump system on a water gun is what pressurizes the loaded blood enough to shoot it. $\endgroup$ – Giter Aug 7 '18 at 17:16
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    $\begingroup$ What about clotting? It seems like the blood would start to coagulate and clog this gun very quickly $\endgroup$ – divibisan Aug 7 '18 at 17:57
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    $\begingroup$ I dispute your assertion that you can pull 2 liters out. Pulling out that much blood would severely limit the performance of the combatant. When someone is actively trying to kill you, having the fitness of an asthmatic otaku, you aren't going to survive long. $\endgroup$ – Aron Aug 8 '18 at 3:36
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    $\begingroup$ I donate blood, after a pint you definitely feel it and the clinic I go to mandates that everyone spends ~15 minutes sitting down eating cookies after the donation. I woudn't recommend any exertion during that period. $\endgroup$ – Borgh Aug 8 '18 at 14:19
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This is more of an attempt to state the obvious.

The most simple approach to this problem is not to use a syringe that actively draws blood and shoots it. This is horribly impractical, slow, inefficient, painful, unwieldly, and their is no control against drawing too much blood.

It makes a lot more sense to draw the blood out before hand, and keep it in a blood bag. Keep the blood active in the same way that blood banks do. Collect blood into a bag that has CPDA-1. The solution has Citrate (an anticoagulant), Phosphate, Dextrose, and Adenine. Have an attached heater to keep the blood around 40C so that it stays viable for up to 35 days.

https://www.ncbi.nlm.nih.gov/pubmed/7147334

Now you have addressed the blood part.

Next feed this blood into a high pressure water gun, or power washer. Now the ballistic part of this system is resolved.

The only thing you have left to worry about is running out of blood.

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    $\begingroup$ Awesome answer! The only critique I have is that, while the blood itself may be phyiologically viable, it may lose its thaumic viability more rapidly. In other words, the magic may not work unless the blood is shot more or less direct from the weapon's wielder. $\endgroup$ – elemtilas Aug 7 '18 at 16:16
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    $\begingroup$ Ah yes, just what I want going into a fight that will require athletics: a piece of metal sticking into my artery. Fortunately, such syringes never move around or wiggle or anything under any circumstances. $\endgroup$ – David Thornley Aug 8 '18 at 19:26
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    $\begingroup$ I think 40 C is a typo, as the blood will cook - blood is kept at 4 to 8 C for storage $\endgroup$ – astaines Aug 8 '18 at 20:01
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Sounds like blood bullets are essentially paintball bullets. So pretty much the exact same rules apply.

But let's get slightly more sci-fi. In theory, you could have a gun that extracts blood, freezes it instantly with liquid nitrogen, then fires it with some sort of gas propellant (like CO2).

The "bullet" would hit like a low speed musket round. Accuracy and range would be trouble, but it could definitely penetrate an unarmored opponent. A fancy enough gun could pump the blood into a mold before it's frozen into a bullet shape, then loaded and fired. You might even be able to use some rifling (but this generates heat, which would melt the ball)

This is a pretty worthless gun overall though unless your plan is to spread disease or something.

-Edit

As pointed out by the comments. You could use the nitrogen that froze the blood to also fire it, giving it far more power. Enough to actually function like a real bullet. Your accuracy would still be limited, but your range and hit power would be huge and most likely lethal.

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    $\begingroup$ If you're going this route, I'd point out that if you have the LN2 for freezing the blood, you can just use it as your launching gas as well, with the advantage that it will easily generate the thousands of PSI necessary to launch at firearm-like speeds instead of the measily 800ish PSI you can get from CO2. $\endgroup$ – Perkins Aug 7 '18 at 17:28
  • $\begingroup$ @Perkins Good point, the gun could freeze and fire at the same time using the pressure. $\endgroup$ – Trevor Aug 7 '18 at 17:30
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    $\begingroup$ Broad design: You have a tube, blood valve in one end, conical-endcap at the other. Fill it with blood, freeze with liquid nitrogen, open the endcap. Blast high-pressure nitrogen gas into the back end. Your ice-stake will be a pretty nightmarishly powerful flechette. reasonably able to punch through body armour at close ranges. You'd want to make the stake quite small though, so you could alternately design it to mould the ice-spike around a metal core for extra mass. then fire your blood-popsicle with a more or less conventional gun mechanism. $\endgroup$ – Ruadhan Aug 8 '18 at 10:12
  • $\begingroup$ To expand on this, here's a functional concept of a "tear-gun" that Yi-Fen Chen built in 2016 - it extracts her tears, freezes them into projectiles and then fires them: ignant.com/2017/10/20/tear-gun-by-yi-fei-chen $\endgroup$ – Aaron Lavers Aug 9 '18 at 6:42
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For starter, I think your first concern is to avoid at any cost massive uptake of blood. Since during the peak of the fight it's easy to forget such details, I would go against "charging while fighting", and rather opt for a "charging before fighting".

Basically your guy would pump out his blood before the fight, maybe while still in his HQ, store it into a suitable vessel with anticoagulants, and then use it when needed. In this way your hero can also partly recover the removed blood before the fight, being fitter.

It's worth nothing that:

Class I Hemorrhage involves up to 15% of blood volume. There is typically no change in vital signs and fluid resuscitation is not usually necessary.

Class II Hemorrhage involves 15-30% of total blood volume. A patient is often tachycardic (rapid heart beat) with a reduction in the difference between the systolic and diastolic blood pressures. The body attempts to compensate with peripheral vasoconstriction. Skin may start to look pale and be cool to the touch. The patient may exhibit slight changes in behavior. Volume resuscitation with crystalloids (Saline solution or Lactated Ringer's solution) is all that is typically required. Blood transfusion is not usually required.

Class III Hemorrhage involves loss of 30-40% of circulating blood volume. The patient's blood pressure drops, the heart rate increases, peripheral hypoperfusion (shock) with diminished capillary refill occurs, and the mental status worsens. Fluid resuscitation with crystalloid and blood transfusion are usually necessary.

Class IV Hemorrhage involves loss of >40% of circulating blood volume. The limit of the body's compensation is reached and aggressive resuscitation is required to prevent death.

Since you want your hero to be active during the fight, I would stop at class I, meaning 15% of about 5 liters, that is 0.75 liters.

I would then use something like a paintball gun, to shot blood loaded projectiles which splash on impact, increasing the chances of hitting the skin.

With 0.75 liters of blood you can fill a fair amount of projectile, and maybe even more if you can slightly dilute the blood.

If you want to shot just liquid blood, than have a suction system connected to the vessel, and shot as long as you can use those 0.75 liters. But that would limit the maximum reach and also the pace of fire.

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    $\begingroup$ "dilute the blood" is an excellent point - can OP's putative weapon somehow use a few micrograms of "active ingredient" but top up the fired load with something else, a neutral carrier or even blood from another animal ? $\endgroup$ – Criggie Aug 8 '18 at 4:45
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Don't use a squirt gun

I'm honestly surprised this hasn't been mentioned yet - squirting large amounts of blood mid-battle sounds like a horrible idea. The water gun suggestion in particular will only last for maybe a couple dozen shots - and tyical water guns have a comparatively slow shot speed which, along with the inaccuracy of the fluid jet due to turbulence, makes dodging relatively easy (think of fights using these guns during a hot summer - even in a relatively cramped garden, where combatants are mostly prioritising offense over defense and dodging, a non-negligible portion of shots will still miss). And then you have to recover for a month!

The "paintball" (bloodball?) idea is much more manageable. According to Wikipedia, paintballs are typically around 17cm in diameter, giving a volume of around 2.57 cubic centimeters, or about 390 paintballs in a single liter. Much better, as you have almost 400 shots per month, but you're still going to be feeling slightly weak most of the time.

Since you mentioned that "any amount" of blood will do, I think we can do a lot better.

Aerosol

I wasn't able to find good data online, but if you could install a mechanism to properly spray a very fine mist of blood, you could cover a significant area around you. It would have shorter range than a directed shot, but would be amazing in close quarters against multiple enemies - you'd only use somewhere on the order of 1ml of blood to reliably hit everything in your immediate vicinity.

This could also be useful for defense: spray a fine mist all around you, or - even better - at a choke point, and until the suspension settles (which could take a couple of minutes if the mist is fine enough, or even longer) this becomes an impassable area that kills any enemy that tries to go through.

The disadvantage is that this would be of limited effectiveness in windy conditions. Although slight air currents, if favourable, could actually help spread the mist - rendering it less useful for setting up defensive blocks, but more effective at spreading to kill everything around you. I don't know if this would actually be feasible in practice, but spraying some aerosol into an indoor ventilation system might perhaps be a great way to mostly clear out a building.

Aerosol bullets

Spraying blood mist is very limited in range but covers a wide area. Bloodballs/pellets have excellent range but need careful aiming, and a miss means wasted blood: you can't really sustain any sort of automatic firing. (Squirt guns, again, have neither and should really not be used.) How about a capsule filled with pressurized gas and blood, that upon impact will shatter like a paintball and cover everything in a wide area around it in a fine layer of blood? Or even exploding bullets - it's hard to find real-life examples of these since various treaties ban their use on low-caliber weapons, so these mechanisms are usually found on anti-vehicle armour-piercing ammunition, but the fact that they were deemed worthy of a ban as early as 1868 makes me think they're very feasible to construct. Add a payload of blood, and you only need to hit some hard surface (such as a wall or floor) near the enemies to spray them.

This makes shooting require significantly less accuracy - meaning less wasted blood. Moreover, you can hit several enemies in one shot, and - if the resulting spray is fine enough to function as a proper aerosol - this can be used to set up killzones mentioned above at range, and render an area deadly (restricting the movement of your enemies) even on a complete miss.

Poisoned bullet

Instead of making a thick bullet with a complicated explosive mechanism, you could simply fire blood-delivering bullets. Generally that means something like a hollow bullet filled with blood.

While less effective than aerosol-spraying ammunition (since you lose the benefits of AoE), this sounds much simpler, and while I don't know much about bullet ballistics I feel like this would make it more feasible to, say, deliver blood through a sniper rifle - or even make an armour piercing blood bullet. It also allows you to very carefully control the amount of blood delivered - you could place a tiny droplet into a tiny dimple on the bullet, and you'd be sure it would touch its target.

The only issue I can see is with clothing absorbing the blood before the bullet impacts the wearer, if your enemies wear that.

Conclusion

The main things I focused on was reducing the amount of blood needed per shot, and increasing the efficacy of each shot. Since there's no mininum amount of blood needed for effectiveness, any blood not strictly meant to come into contact with a living enemy is wasted. Using these methods will probably use little enough blood that you could engage in intensive combat regularly with almost no negative side-effects.

Mind you, these solutions are technologically advanced - you'll need a proper aerosol mechanism that creates as fine a mist as possible over as long a range as you can achieve (a non-trivial engineering task), and you'll probably need your own very specialised ammunition for the explosive bullets that would allow you to fill them on demand - or a hyper-specialised gun to coat bullets with blood in a precise way that uses the minimum amount of blood while still ensuring it's reliably delivered. This isn't something you could jury rig together in your backyard from a needle and a hose pipe. This also departs significantly from your initial image of "squirt some blood". So these methods may not suit your tastes - but they're all technologically feasible given adequate resources and achieve what you asked in your question in a very efficient way.

A note on connecting to the blood supply

A simple way would be to install a blood plug with a fail-shut valve. The gun would connect to the plug, which would open the valve. Simply make the coupling easy to disconnect, and any impact or injury will dislodge the connection (shutting the valve) before damaging any of the gun's parts. Sure, this might result in accidental disconnects in the middle of heated combat, but that's better than bleeding out. The connector could even be magnetic to make it extremely fast and easy to simply pop back into place when it gets dislodged.

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  • $\begingroup$ Welcome to WorldBuilding.SE! Extensive first answer and very nice idea for connecting to the blood supply. Good job. I am looking forward to your future contributions. In case you have any questions about how the site works check out the tour, help center and Worldbuilding Meta. Have fun! $\endgroup$ – Sec SE - clear Monica's name Aug 8 '18 at 12:34

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