this post was submitted on 01 Apr 2024
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Metric was too confusing for bullets, so we use both, and but neither of them are actually the diameter of the bullet, most of the time.
.223" is the same diameter as 5.56mm (which is 5.7mm across), but if you use 5.56 in a 223, it might kill you.
223 in 556 is fine, might fail to cycle.
Even the "metric" measurements for firearms ain't necessarily true measurements either. Lots of them get rounded off or simply depend on just how they made the measurement to start with, (land to land or groove to groove). In any case a bullet diameter is almost always going to be just a tiny bit larger than actual bore size for modern cartridge bullets.
then why is it called 556 if its actually 5.7?
556 was the measure of the inner diameter of the rifling of a barrel of a gun that shot 556.
Metric is confusing. That's why for most shotguns, we measure the width by the number of lead spheres of that diameter that would equal one lb, eg a 12 gauge shotgun is the diameter of a 1/12lb sphere of lead.
Nobody knows how big 18.53 mm is, but everyone knows what a 12 gauge shell looks like.
Oh, and gun powder is measured in grains, maybe early smokeless pellets were about the same size as grains of wheat.
Grains as a measure of weight comes from the Troy weight system, think Troy ounce of gold. It is a very old system that for a long time was mostly used by apothecaries and probably has its origins in Ancient Rome.
Grains Apothecary is used to measure powder charge weight is because it was a "fine" enough scale for measuring small amounts of things that if you get it even a tiny bit wrong, can kill you. So, ammunition manufacturer's looked around and scales used for accurately measuring small amounts of drugs were commonly available, so they went with that.
Cool side point: Powder charges are checked by weight and dosed out, (or thrown), by volume as it has always been done since the first gonnes were a thing.
The problem is it's impossible to tell whether you're joking or being serious
he's serious. The old casting method for round shot was to dump a measured amount of molten lead from a tower into a pool of water 40 feet below. the molten lead would form a sphere in free fall and fully set in the water, so it was convenient to define gauge diameter by fractional weight of a pound. Twelfth pound sphere fits a 12 gauge gun, etc.
The problem is it's impossible to tell whether you're joking or being serious. Throwing molten metal off the tower sounds like the most ridiculous thing ever, but apparently is a real thing.
Here's where it gets political. I learned about shot towers in passing years ago and thought that was a good idea. You learned about shot towers in passing, but then with a detailed explanation, still thought that was ridiculous. One of us is prone to rational thought and the other is not. This is a 17th century conversation happening now.
Ah no, it's just that from reading this, I imagined it being poured outside, not inside the tower.
Like, someone looking at Galileo doing his experiments dropping weights off Pisa tower, and saying:
— What if we put a bucket underneath? What a splash it'd make!
And another one going:
— Yeah! And why just weights, let's throw molten lead off! What safety concerns? Haven't heard any
That's actually fascinating. Thank you.
No problem. There's always a reason, and usually a pretty interesting one, for old odd hold overs like this, but it's been 200 years since shot towers were a thing, only history buffs and muzzleloader enthusiasts really know about these.
He's correct and showing the...quirks of the system.
say it ain't so!
Mostly.
Maybe the original was 5.56mm and some dumbass decide "nah, not enough b u l l e t, better make it 5.7mm."
OK, so there is a 5.7mm, that's the same diameter as 5.56/.223, but it's not compatible with either because of the french.
I look into it. 5.7 is shorter than a .223 and is a much smaller grain.
Kinda?
The case is both shorter and narrower than 556/223, so it won't even sit right in anything not designed for it. But FN makes quite a few guns that use it.