OOC # 39
In reply to Tarja Vanska (msg # 213):
I don't have any problem with where you got it from. But 5mm is .20 caliber (an inch is 25.4 mm, one fifth of an inch is .20...) That's basically BB-gun ammo. The rounds, unless they are MASSIVELY density-enhanced, just don't have enough mass to actually carry any force to penetrate a target. That's great, if you're shooting at extremely fragile EVA suits, but not much use against anything else. The armor is firing high-velocity pellet guns, basically. One of the big selling points for a .45 ACP (sidearm) round is it has a lot of mass (as bullets go), and applying a little basic physics...Force equals Mass x Velocity. If you cut the mass down to almost nothing, you have to REALLY ramp up the velocity to compensate, or else just accept the fact that you're hitting with minimal force. The .45 is a heavy bullet, so it doesn't need to travel as fast to deliver the same force at impact. The .223/5.56 round that is currently NATO standard is what Roy described, being intended to wound, rather than kill, and they're also not very ballistically stable...there's a reason almost all the sniper rifles and competition shooting rifles are .30cal or heavier...the bullets have enough mass to remain stable against the wind resistance at trans-sonic speeds, which you need for distance shooting. The 9mm became a widely accepted utility round because it's a good combination of mass and velocity...lighter than a .45, so it doesn't need as much force to send a round down the barrel and therefore doesn't cause as much recoil, but still heavy enough to readily convince someone to lay down and die when they get hit by it.
Pre-Korea, standard US military doctrine was 'teach them how to shoot effectively with a round heavy enough to kill the enemy'. After Japanese human wave attacks in WWII, and Chinese and North Korean human waves in the Korean War, the doctrine shifted...there was less focus on marksmanship and more reliance on volume of fire (which is why US forces stopped using bolt-action and semi-auto rifles and changed over to assault rifles with full-auto options). The average infantryman wasn't trained to shoot any further than 300 yds, really, and 'spray and pray' was regarded as a viable firing tactic, with the intention being to tie up as many enemy troops as possible by either wounding them or occupying others with moving the wounded. But snipers shunned the new assault rifles and stuck with .308 rifles (generally) or converted .30cal M14s because they couldn't afford to screw around with dubious long-range ballistics and also needed their target dead with the first round.
If you're duplicating Vietnam/post-Vietnam tactics and hoping to tie up enemy forces with lots of wounded to deal with, high-velocity 5mm might work. If you're trying to make a lot of dead enemy troops on the battlefield (especially when they're in armor, or they are significantly larger than human sized, like Tank Demons, Largrans, and a lot of Creshians), 5mm is a peashooter that can't carry enough force to penetrate decent armor. Look on YouTube and see if you can find a .223 or 5.56 round that penetrates steel plate (which isn't even technically armor, because it isn't alloyed the way armor plate is), and consider that those rounds are appreciably heavier than a 5mm round would be.
That is why Roy doesn't like the 5mm (which I SWEAR I've seen listed as 4mm in posts, somewhere, but I can't find them, but it doesn't matter, the complaint is the same). The Expanse may have had some logical explanation for using it...but as a legitimate, fighting-on-the-ground caliber, it's practically useless.
