[nataz]

You have to go one layer deeper and think beyond the projectile (bullet) and include the cartridge (bullet, accelerant, primer, case).

Basically how fast some thing shoots (how much accelerant) is just as important as what you are shooting (bullet characteristics).

Each of those variables also have weight and dimension penalties which determine how much you can reasonably carry.

An interesting relatively recent example is the development of small caliber armor piercing rounds. NATO needed something to deal with the rise of body armor. Same requirements, two different solutions to get there in the fn 5.7 and hk 4.6x30 (simplifying enormously here). Basically these are engineering and manufacturing challenges.

[collegeburner]

For anybody who hasn't heard of them and wants a rough overview of the differences, the 5.7 and 4.6 mostly work on being smaller-caliber at higher velocity. 9mm parabellum usually has muzzle velocity anywhere between 1000 and 1500 ft/s where 4.6 is closer to 2300 ft/s. The 5.7 is closer to 2800. They're also usually steel core.

[ggreer]

I'm pretty sure all the steel core 5.7x28mm is considered armor piercing handgun ammo and is therefore limited to law enforcement.[1] I've never seen steel core versions of it sold at any gun store.

1. https://www.atf.gov/firearms/docs/open-letter/national-jan20...

[collegeburner]

Sadly yeah, it's one in a massive list of stupid laws but FOPA states anything that "may be used in a handgun" if I'm remembering the wording right. I believe they submitted the duty round for the NATO RFP tho and that's steel penetrator. If i remember right blacktip .223 is similarly banned because of AR pistols, you can basically only get AP in full rifle rounds (.30-06) or something like .50 BMG.