I've worked up my own penetration simulator based off a few equations here.
http://alternatewars.com/BBOW/Ballis...Marre_Calc.htm
It implements the following formulas:
1.) Franz Kosar's Panzerabwehrkanonen 1916-1977 on page 15 reproduces a "DeMarre" Formula.
2.) The Soviet "DeMarre" Formula reproduced in Курс артиллерии. Кн. 1. Общие сведения. М., 1941 (Course of artillery. Book 1. General information. M., 1941).
3.) The ARMET-TR-17002 Linear Velocity Decay Formula from The 76mm Gun M1A2 and M1A2: An Analysis of US Anti-Tank Capabilities during World War II to rapidly calculate striking velocities at arbitrary distances.
The Kosar and Soviet "DeMarre" formulas are near identical, the only difference being that the Soviet formula adds a parameter to incorporate armor slope LOS effects.
There's a lot of working parts; and I'll write a user's guide one day. But you can use it to figure out hypothetical guns on paper, or obscure guns that little information is known about.
No, it's not as reliable or as accurate as some of the CFD penetration simulations that are linked here that the youtubers are doing -- but you can run them on limited information (no accurate scaled drawings/models of shell needed) and rapidly (results within a second, rather than hours).
EDIT: The biggest thing that screws up a lot of simple armor penetration formulas (like my attempt) are slope effects.
Slope effects are far more than just a simple "increased LOS thickness" SIN(angle) or COS(angle) calculation -- sloping increases the energy required to penetrate the same amount of armor; and scaling laws don't work too well -- at around 45 degree slope -- 76mm HVAP (K=2418) and 90mm HVAP (K=2268) diverge; with the 90mm shell having an easier time penetrating armour than the 76mm shell, presumably having to do with increased areal density, or the shell being large enough to "turn into" the slope, like the Soviet 122mm shell was notorious for.