Re: using spaceships for battletech-style tactical mecha combat
 

Well, that significantly overestimates the randomness. Impact angle is pretty consistent at any given range, particularly against deck armor (the target's facing affects impact angle against belt armor, the turret's facing affects angle against turret armor).

Re: using spaceships for battletech-style tactical mecha combat
 

There are a lot of random factors, especially at sea. Even a one foot swell could cause a couple of degree change in the angle of the placement of a shell.

Re: using spaceships for battletech-style tactical mecha combat
 

Well, there is pitch and roll, and roll can be quite significant in bad weather and if it target is more interested in not being hit than in holding a steady course to aid its own fire control. Small warships and some of the less stable larger ones could roll quite a lot when turning hard, or even when turning not hard at all in some cases.

USS Mobile Bay (CG-53) turning hard to starboard.

Then of course there's listing from flooding on one side of the vessel.

These can all be significant enough to affect penetration, though they are not going to have as much of an effect as moving from bow-on to broadside-on will on hits to the belt.

Re: using spaceships for battletech-style tactical mecha combat
 

Couple degrees won't matter much until you're hitting angles where you are probably going to just outright miss the ship (to drop from 6/die to 3.5/die is going from 90 degrees to 36 degrees; 1/die is 10 degrees).

Re: using spaceships for battletech-style tactical mecha combat
 

I suppose my question was is the 70 DR figured to resist square hits by shells or average hits by shells?

Re: using spaceships for battletech-style tactical mecha combat
 

Square hits, though in some cases it might not be at point blank range. The idea is that you can take standard statistics for weapons (weapon X has a penetration of Ymm RHA at distance Z) and transform them directly into game statistics, and those numbers are generally given for impact normal to the surface.

Re: using spaceships for battletech-style tactical mecha combat
 

And then you find out that they were for an angled plate and the source didn't mention this (annoyingly common in secondary sources), or that the entity doing the testing had a different measure of 'penetration' from what you were expecting (quite common - just about every army and navy had a different definition, some more useful to us than others).

Re: using spaceships for battletech-style tactical mecha combat
 

In addition, a kill was defined as different things by different militaries and different vehicles. The old Oliver Hazard Perry frigate was well known for being able to take multiple hits from different 'ship-killing' weapons and still taking half a day to sink. The ability for it to function long after it was technically dead meant that it could continue to fight long after the point where targeting it was technically overkill, and it was thought that the only way to stop it from fighting back, even while it was sinking, was blowing apart its weaponry or killing all of its crew.