An inventor, inspired by hydrophobic sprays, has created a spray that temporarily removes the friction of whatever it's coated on. Curiously, he decided to coat a 10mm Auto cartridge with it. With great difficulty, he loaded it in on a Glock pistol. He then fired it straight on a 1/2 inch steel plate, angled at 45 degrees.

What will happen to the bullet? Will it exit the barrel at a faster speed? Will it spin too fast and disintegrate instead? Will it even penetrate the steel plate, or the frictionless bullet will actually ricochet off instead?

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    What shape is the bullet?, i.e. the part that is ejected from the cartridge and travels through the air. – chasly from UK Dec 7 at 19:43
  • @Separatrix Hilarious! Never thought of that simply falling out. But, wouldn't the rifling at the very least slow the bullet down as it spins inside the barrel? Then again, end result would still the same. – Basher Dec 7 at 19:51
  • @Separatrix ytechnically, the bullet will slide out the shell casing. Unless they put a groove around the inside of the casing, and squeeze some of the bullet into it. But it will slow down the bullet, or increase chance of casing blowing up. – Bald Bear Dec 7 at 19:53
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    the problem of course is all the spray will run off the bullet and drip out the bottom of the gun almost immediately, since there is nothing to hold it on the bullet. – John Dec 7 at 20:57
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    What is your objective ? WB is not really for exploring wild ideas and if you explain what your objective is in making a bullet frictionless we may be better able to help you. – StephenG Dec 7 at 23:13
up vote 4 down vote accepted

This is a tricky question, because there is no "friction" force. Rather friction is more a class of forces which we identify by their properties. Real friction is cause by things like microscopic surfaces or electrostatic attractions which are tremendously difficult to calculate. However, when you sum them all together, we see the traditional properties of friction:

  • Proportional to the normal force pushing the two surfaces together.
  • Directed along the surface.
  • Opposing the direction of motion

So this spray is really going to negate the sum forces which function this way.

So the first thing we'll see is that the bullet falls out of the case. A 10mm Auto case is a friction fit. However, we can fix this with a pair of pliers. Crimping the case into the bullet creates an indent. Now this is useful because this can hold the bullet in place. The forces we need are normal to the surface, so they hold the bullet in place. Now at this point we should be terrified because it's highly dis-advised to modify a cartridge in this way. You can increase the pressure in during firing and that can lead to a barrel explosion. Needless to say that's bad news. Never modify a cartridge unless you have the knowhow to make sure it is safe!

Now, air forces. These are going to matter for flight, and they're also going to matter for the gunpowder's effects. These pressures are mostly normal to the bullet surface, so they will not be affected. We can also expect the rifling to work for the same reason that the crimped case worked. The bullet is deformed to fit the rifling during firing. It's not just a friction fit. However, it may be less effective, depending on the depth of your Glock's rifling. For some reason, there's not much information on how well Glock rifling performs on fricitonless bullets for me to reference.

While flying, drag forces are very different from friction forces. There is some friction from air moving along the bullet, so that will be made better by this spray. Most of the effects, however, are air pressure and viscosity issues. You still have to push the air out of the way. You still have a vacuum in your wake. You still have effects like wave drag.

Now for the part you were interested in: the impact!

When you impact, friction may vanish, but momentum is still there. If you want the bullet to slide off your steel plate, you still need to impart enough horizontal momentum to make that happen.

So what is your steel plate like? Too soft, and it will deform as the bullet hits while imparting that horizontal momentum. This can lead to the bullet getting trapped like a ball in a baseball mitt. Once this happens, momentum will tear that cute little lead slug apart.

If the surface is perfectly hard, then you will be applying all of the momentum to the leading edge of the bullet. This will cause deformation in the lead, letting the bullet rotate like a fluid in a blender. The material properties of the bullet will matter here. My expectation is that the lead will deform fast enough to cause the bullet to spin tail first in the same direction as friction would have caused it to spin -- just for a different reason.

However, if this was a different material, the story could be different. The bullet could be made out of an unobtanium that is perfectly rigid. In this case, the bullet will start to slide, and will be rotated by the torque from this impact location. Most likely (depending on bullet geometry), this torque will cause the front of the bullet to slide along the plate, like when your front foot slips out from in front of you on an icy patch.

This creates an interesting situation where the bullet is spinning "the wrong way." But that's fine because we have a frctionless material making sure everything is fine.

If you had a minnie ball, which was perfectly spherical, the force would be in line with the CG of the bullet, so the result would be a perfect "slide." That is, of coruse, assuming the bullet doesn't glance off. There's an entire set of math left to analyzign that question. Bullets indeed do often glance off.

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    A Minni ball was not spherical. – Andon Dec 7 at 23:47
  • @Andon: and his name was Claude-Étienne Minié, not "Minni". – AlexP Dec 8 at 7:20

Bullet will exit the barrel faster, but probably not by a lot - it is limited by speed of gunpowder exploding.

I assume the coating will still be on when it leaves the barrel. Perhaps you can spray coating on the bullet as it exits

It will fly a further and straighter than regular bullet, but only a little. I do not think cancelling friction automatically cancels air resistance. Your bullet still needs to compress air to fly through it.

Upon hitting a 45-degree plate, bullet will be more likely to glance/or ricochet, and less likely to deform as it does so.

So you can use your bullet for ricochet shots. You will still need a computer or great skill to aim such shots.

IMHO, better use of frictionless spray is to lubricate moving parts of your gun (so faster firing rate, and less jams). Moreover, you can lubricate your vehicles (tanks wear out their gear box quite fast). Or to sabotage enemy vehicles & transportation.

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    To prevent duplicates, I actually searched for the term frictionless and also found an interesting discussion about using frictionless lubricant for machineries and concluded that it won't actually work that well: worldbuilding.stackexchange.com/questions/88447/… – Basher Dec 7 at 20:44

TL;DR

A frictionless bullet will behave almost exactly the same as a real bullet.

Introduction

The word "friction" includes many different related phenomena. For example, air resistance is a kind of friction. Part of the friction of air resistance comes from air moving against other air. No coating you place on a bullet will reduce the friction of air on other air, or the drag from the front of the bullet pushing air aside.

When I think about a "frictionless spray" I think of a spray that, when applied to the surface of a bullet, will keep the bullet's surface perfectly smooth. The spray need not preserve the bullet's original shape. Such a spray will not have a significant effect on air resistance because most of the friction from air resistance does not come from a sheer force between the bullet and the air.

There are two things we have to examine: what happens inside the gun and then what happens when the bullet hits the steel plate.

Inside of the gun

Broadly-speaking, there are two kinds of guns barrels: rifled and smooth. Rifled barrels are grooved to give their bullets a spin. This spin helps stabalize the bullets in flight thus improving accuracy. Glock rifles its handguns' barrels.

A frictionless bullet would be affected by rifling. That's because bullets are slightly bigger than the barrel they're fired through. A frictionless bullet travelling through a rifled barrel will be forced slightly into the shape of the barrel it's fired through. Once the bullet is this shape, the rifling will force the bullet to spin. No friction is necessary.

Without friction against the barrel, the frictionless bullet will travel slightly faster than an ordinary bullet. A typical firearm loses only about 2% of its energy due to friction against the barrel. This is not significant. The frictionless will not spin so fast that it breaks apart in flight.

Impacting the target

There isn't enough information to answer this question because there are many different kinds of steel. For some I expect the answer is yes, for others the answer is no. Distance from gun to target also matters.

Instead, I'm going to focus on the heart of your question: the effect of friction on a bullet impacting a steel plate.

When a bullet impacts a target at 45 degrees the bullet's momentum can be treated as two vectors, one parallel to the plate and one perpendicular to the plate. The vector perpendicular to the place determines the bullet's penetrative power and is unrelated to friction. The bullet's penetrative impulse comes from this vector and this vector is unaffected by friction.

That said, a frictionless anything will find itself easier to penetrate anything else, which makes the bullet slightly better at penetrating the steel plate. I think this effect is of minor importance because steel--even steel that's been shot with a bullet--tends to be pretty smooth and to have a low coefficient of friction.

Summary

In general, a frictionless bullet will be a little bit better at penetrating a steel plate than an ordinary bullet. That's because friction (at least the kind you get from the bullet's less-than-perfectly-smooth surface) isn't a significant part of how bullets work. Far more important is what kind of steel you're shooting at and how far away from the target you are.

Based on answers by others and with some extra thought.

It's impossible. If the spray is frictionless then when you try to spray it onto the bullet it will just fall off. On the other hand, if it sticks to the bullet then it's going to stick to the inside of the barrel as well. In other words it will act like glue.

In the first case it will have zero effect in the second case the barrel will explode.

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    It's obviously a two-step process, whereby a precursor reacts with the metal in the bullet to produce the frictionless surface ;) – Michael Dec 7 at 22:55
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    Except that the OP says it "removes the friction of whatever it's coated on." How can it react with every possible substance in existence? – chasly from UK Dec 7 at 23:11
  • well i would assume it isn't made of quarks and leptons but some kind of stranger matter which binds to readily available such particles, and then through binding tension forms a smooth surface on the other side which screens force carriers, eliminating friction both from disconformity and EM interactions. – Michael Dec 7 at 23:15
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    This is like the old joke about how they get Teflon to stick to pans? Either the spray is initially not frictionless and undergoes a chemical change during some curing process. Or it grips little imperfections in the surface mechanically during its drying process. – Schwern Dec 8 at 2:15

Your bullet would travel faster in a vacuum because there would be no drag from the sides of the barrel. However the main force stopping a bullet from coming out of a gun fired in air is actually atmospheric pressure opposing it.

When it emerges from the barrel into air then it's not clear what 'no friction' means. The bullet still has to displace a certain volume of air to the sides as it travels. This requires energy. It is more a question of the shape of the bullet that counts.

It is likely to tumble more because the rifling in the barrel won't make it spin so efficiently or at all.

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    Actually, bullet is typically squeezed into the grooves by the pressure of the powder, so it will still spin – Bald Bear Dec 7 at 19:56
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    If that's true, the frictionless spray will simply be squeezed out of the way. The whole bullet would have to be made of frictionless material. – chasly from UK Dec 7 at 19:57
  • that's an effect i hadn't considered, i'd accepted the premise that the frictionless material could adhere, undergoing some kind of transition on bonding. If the material itself is frictionless that would appear to be impossible. – Giu Piete Dec 7 at 20:34

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