All of this talk of energy is ignoring some of the basic laws of armor penetration. A strike from a knight's lance at full gallop has dozens of times more kinetic energy and momentum than an armor piercing .50 BMG from a barret anti-materiel rifle. (which is entirely legal to own and purchase across most of the US with no more regulation than any other semi-automatic rifle) A knight's lance in a joust doesn't penetrate the plate armor of the other knight in the joust. An armor-piercing .50 BMG will
absolutely penetrate the plate armor of the same knight that just walked away from a lance to the chest, and leave his torso with a massive hole in it. Hell, even just a standard full metal jacket .50 BMG would easily penetrate a knight's plate armor.
How concentrated the force is on the point of impact and the span of time that force is applied matters more than the energy of the projectile itself.
This is before you consider that the shield charm might be a
reactive defense. Meaning, it detects an incoming threat and applies a counterforce to stop it.
What if there is a small reaction delay? One that doesn't matter against arrows and spells, but makes the shield charm completely useless against supersonic projectiles? If
@Fencer wanted to, he could go 'magic stoopid hur dur' and make shield charms
completely useless against guns and still be entirely compliant with the 'can stop a charging knight at full gallop' argument.
Now, the OP obviously wants shield charms to be somewhat useful agaisnt guns but not no sell them, and to my knowledge of ballistics and armor penetration, and from all the ways the various posters in the thread have talked about shield charms, the OP can basically have guns behave basically however he wants against shields and be more or less compliant with almost every argument presented. Ballistics and armor penetration are
extremely complicated subjects with a lot of nuance to them, such that against something a wacky as magical shields it really could go either way.
For reference, there are kind of 5ish categories that I put small arms into. Pistols, pistol caliber carbines/submachine guns, assault rifles, battle rifles, and anti-materiel rifles. A note on this: this categorization is focused on the ballistics of the cartridges, and less on the function of the rifles themselves, as the whole question here is on armor/shield penetration.
Pistols have the least penetration, as they tend to have relatively large bullet diameters and low muzzle velocities. Pistol caliber carbines and submachineguns have the same cartridgesas pistols, but they have longer barrels which give higher muzzle velocities and thus aomewhat higher penetration. Assault rifles have 'intermediate cartridges' which is less powerful than a battle rifle (to make them easier to control in full auto) and typically have much better armor penetration than pistols or PCCs/SMGs. Examples of this are ar-15/M16 rifles, G36 rifles and ak74s. The intermediate cartridges typically have a very small diameter, light bullet (usually lighter bullets than even pistols) and
really high muzzle velocities. Battle rifles use 'full power' rifle cartridges, 7.62x51mm, .30-06, 6.5mm creedmoor, basically big game hunting cartridges. Battle rifles have much better armor penetration than assault rifles, but they are heavier and have much more recoil, making them much harder to control in full auto. Not impossible, and some battle rifles have some creative recoil management systems that make it easier, but the bullets are bigger and there is far more energy in the system than there is in an assault rifle.
Finally, we have the anti-materiel rifles. These are basically rifles that use the same cartridges as .50 caliber heavy machine guns, but are designed to be light enough to be carried and used by a single person. To clarify, these rifles aren't usually used on people. The clue is in the name:
anti-materiel rifles. These are designed to put a hole through the engine block of a vehicle, and penetrate lightly armored vehicle armor. These can put a hole through a thick brick wall with relative ease. However, they are near the upper limit of what a single person can comfortably use from the shoulder, and they are far too powerful for full auto. They are also pretty expensive, the semiautomatic .50 BMG rifles are usually several thousand dollars, though they aren't any more tightly regulated than any other semiautomatic rifles.
They, given the OP's impression of how guns should work against shield charms, can likely penetrate on the first shot, unless the wizard has particularly good shields, then 2 or 3 shots should penetrate. These are the real wizard-killers. The explosive ammo can literally blow you in half.
Probably the funniest part about them is that as far as I'm aware they are entirely legal to own in England. So long as their magazine is 3 shots or less they are treated the same as any hunting rifle. There are plenty of cheaper .50 BMG rifles out there that are single-shot bolt action rifles, and they are only a few hundred dollars IIRC. Taylor (with an adult doing the purchase as she's still a minor) could just buy a .50 BMG rifle and own it (through the adult) entirely legally in England. They would need to wait a few months for the gun license to be approved, but that is the only real obstacle there.
