Comments by "William Cox" (@WildBillCox13) on "The Drydock - Episode 053" video.
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A most interesting question. Here's my spin on it:
First? My understanding of topic.
Reduce cross section of penetrator and you increase penetration for an equal weight/mass round (and propellant charge*). Smaller impact area transfers more energy into a smaller point, concentrating stress on a hard, but brittle, surface (face hardened plate is the similar to Tempered Glass for its method of impact resistance). That's the good part.
Unfortunately, artillery rounds in flight are affected by aerodynamic/fluid dynamic factors the same as any Mach2-3 Jet. There is a "perfect" length versus diameter plus optimal ogive for every particular caliber of gun. Make an artillery round too long (L:W) and it tends to nutate severely in flight, which loses accuracy, and absorbs muzzle energy through greatly increased drag. That's the bad part.
Now for a bit of hard evidence.
Planet Slo-Mo has a video posted where they fire both the 12.2cm P Obr 42/L46 AND the M4 (L) Tank's 76mm M1A1/L51, and follow the rounds in flight. Notice that even these, supposedly "perfect" shapes are prone to nutation throughout their flight. Longer L:W rounds are far worse.
https://www.youtube.com/watch?v=xpJ8EoGmLuE
So-o, why not? Diminishing returns.
Disclaimer: I am no military expert. I am old and have spent most of my life reading military history and technological examinations of military gear. For absolute understanding, let me recommend Ian Hogg.
*Reducing barrel diameter makes you increase the heat and pressure in your chamber, too. You'll need a reinforced Breech,, Chamber and Barrel to push the same mass the same speed through a smaller barrel.
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