Weapon vs. Armour in Chainmail vs. AD&D

Disclaimer: Note that some of the links below are affiliate links (meaning I get a small percentage of the sale without extra cost to you or the publisher).

Also, I'd like to give a big shout out to waysoftheearth over at the odd74 forum for his insights. My original goal was simply to reproduce his calculations and to be able to compare variations myself — like I do so below.

I have recently talked about some aspects of the man-to-man combat tables in Chainmail (and even shared my calculations so that you can play around with your own variants). It occurred to me that there's a weapon vs. armour table in the AD&D Players Handbook as well (which has been shown to be wildly different from that in Chainmail) — wouldn't it be fun to compare the two?

Here is the weapon vs. AC table in AD&D. As a reminder, these are modifiers added to the to-hit roll with a d20. Armour Class (leaving Dexterity aside) is determined as follows:

For a level 1 fighting-man, the minimum required to hit these AC values is (20 minus AC). so 10+ on d20 hits an unarmoured foe, and 18+ hits someone in plate and shield.

To compare these values to those in Chainmail, we need to cut some of it. The Chainmail tables only include thirteen different weapons, and the armour categories are also different (for instance, chain, banded, studded, and splint mail all belong to the same category). To make things easier, we will only consider the following armour categories: no armour (AC 10), shield only (AC 9), leather (AC 8), leather & shield (AC 7), chain (AC 5), chain& shield (AC 4), plate (AC 3), and plate & shield (AC 2) — leaving out AC 6 for now.

Most weapons in Chainmail have a clear equivalent in AD&D, but note the following:

• for mace and flail we'll use the footman's mace and flail
• sword is interpreted as longsword
• polearms, a single entry in the Chainmail table, are left out because AD&D has different stats for each variant
• lance is similarly left out for now to make things simpler

Edit: I made a crucial mistake in my calculations, essentially flipping the modifiers in AD&D. I'd like to thank everyone who pointed this out! My conclusions are also rewritten to reflect this. Apologies for the errors in the first place.

If we calculate the minimum requires rolls for a level 1 fighting-man with these weapons vs. the discussed armour categories, we get the following:

 

 

And here are the percentile chances of hitting compared to Chainmail:

 

In AD&D the average difference in performance is minimalised; blunt weapons are generally much worse against heavy armour than in Chainmail (cf. mace 20% vs 41.67% or 25% vs 58.33%); and the two-handed sword doesn't vastly outperform all other weapons.

Now, what if we make a direct conversion from Chainmail instead? If we take the percentile values and convert them to target numbers on the d20 scale, we get the following:

Of course, lance and polearms are still too ambiguous in AD&D terms, but it's definitely a starting point. Now, for the last bit, here are the to-hit modifiers for the above, presented in the AD&D format (AC 6 left blank and up for interpretation):