[osr] shields shall be activated

Have the summer end rains already blessed your part of the world? I can feel them approaching in timid peeks; barely passing through the ranks of their mortal enemy, the ch3m7rails. Welcome to your comfy mood blog where we sometimes post and discuss rpg houserulings. 

I was working on a chart tonight. In my d6 pool d&d there is one rule that proved to be very cool: the shield roll. Shields basically do not add to your AC, but instead block an attack 1/3 of the time (roll a 5+ on a d6)

This means they mean much more protection than the small +1 (5%) they add to AC in B/X. But the increase is not good on itself: its a matter of taste. The good part is that it's relative importance increases as you wear less armor: the percentage of blows stopped increases as you are easier to hit. 

For example, a PC in plate armor can be hit on a 6 (17% chance), so the shield drops the chance by a third (by about 6%). The same PC in light armor is hit on a 5 or a 6 (33% chance) so the shield drops the chance by 11%. At AC four (no armor) and three (no armor and yet a level 1 fighter) the shield will be useful in 17% and 22% of the attacks received, respectivelly.

I made a chart that ports the system to B/X like this: Instead of adding a +1 to your AC, the shield can be activated once you have been hit (before damage). Roll a d20 when you do it: your AC becomes that number against that specific attack.
These are the chances against a monster with no attack bonus. Took a little time to calculate the percentages manually with the Windows Calculator, then I translated it to AC:

As you can see, the relative protection of the shield increases greatly, from their simple +1 ac on the original; being much greater in the lighter armors, but never too big as to make them useless. A fighter in leather and shield has 16 AC! just as if he wore plate. However, a one in plate and shield is just 18 AC, just one pip over his original equivalent, which I like because it mimics how plate armor got diminishing returns from the shield in real life.

Against monsters with actual attack bonuses (which is the norm) this shield boost diminishes gradually, but still getting on a median an extra AC point (around +2) at chainmail levels, with marginal benefits the greater the armor and the greater the monsters, but not reaching total zero. This way, the decision on wether or not to use one is always present.

YMMV about if the increased AC is a good thing or not. To compensate for it, I'd limit the shield activations to one per turn, which will only be relevant against monsters with multiple attacks or against multiple enemies.

------------------------------------------------------------------------------------------------------------------------------------------- EDIT #1: (afterthoughts the next morning)

One more iteration on the concept of Activated Shields. A much simpler method than the previous entry, in which the shield d20 roll replaced the current AC. 

The rule is:

Shields don't provide any passive AC bonus. Instead, they block a succesful hit on a roll of 6 on a d6 (before damage is rolled). This makes their relative AC be better or worse depending on the armor worn (as they are going to "work" more on lower ACs), according to this table.

 

AC (ascending)	% to be hit at +0 bonus	1/6 blocked by the shield	+ ac equivalent (+1 in d&d)
2	95	15'8	3'17
3	90	15	3
4	85	14'1	2'87
5	80	13'3	2'66
6	75	12'5	2'5
7	70	11'6	2'33
8	65	10'8	2'17
9	60	10	2
Unarmored (10)	55	9'16	1'83
11	50	8'3	1'66
Leather armor (12)	45	7'5	1'50
13	40	6'6	1'33
Chainmail (14)	35	5'8	1'17
15	30	5	1
Plate Armor (16)	25	4'1	0'83
17	20	3'3	0'66
18	15	2'5	0'5
19	10	1'7	0'33

 As you see, this makes shields be a liiiitle better on leather and chainmail, not enough to make a big difference, but its a little treat to those fighting man who don't get plate as soon as they can. I want to open the possibility of using less armor in order to open more encumbrance. Not sure if this little boost would be enough.

Below this lines, you can see the same table but for a shield roll that prvented hits 1/3 of the time (5 or 6 on a d6)

AC (ascending)	% to be hit at +0 bonus	2/6 blocked by the shield	+ ac equivalent (+1 in d&d)
2	95	31'6	6'33
3	90	30	6
4	85	28'2	5'66
5	80	26'6	5'33
6	75	25	5
7	70	23'2	4'66
8	65	21'6	4'33
9	60	20	4
Unarmored (10)	55	18'33	3'66
11	50	16'6	3'33
Leather armor (12)	45	15	3
13	40	13'3	2'66
Chainmail (14)	35	11'6	2'33
15	30	10	2
Plate Armor (16)	25	8'3	1'66
17	20	6'6	1'33
18	15	5	1
19	10	2'5	0'66

As you can see, the relative AC of a shield is greatly improved from the original. It is remarkable that with this rule, they can a priori block natural 20s.

The point in which shields will really shine with this rule is when fighting monsters with high attack bonuses. A fighter in plate armor and a shield, for example, when fighting a red dragon with +8 attack bonus, would defend with an equivalent AC of 9 (17 -8).

By this rules, the 16 AC plate would become an 8, and then get the bonus from the shield: +2'17 for the 1/6th version and +4'33 for the 1/3 version; making it a factual AC of 10'17 and 12'33 respectivelly.

Not sure if implement this or on how. I like that the increment in AC can make up for the fact that I want to drop the attribute scores (and with it, the Con bonuses) and it seems appropiate that the con bonuses are more important at higher levels, just as when the monsters attack bonuses are higher and raise the effectivity of the shield. So in a way, the 1/3 version of the shield might not be as OP as it looks in comparison.

On the other hand, there is something so easy on giving normal shields 1/6 of effectivity, and have magical shields (that would be +1 as per the original rules) to work at 1/3.

Non-fighters have also the choice of using shields at a reduced armor rate. Be it taking the 1/6 instead of the 1/3, or using them at disadvantage (roll 2d6, keep lowest). I'm a bit reticent to hard-coded restrictions, and though it may sound ridiculous, the game feels more "real" to me if that kind of things are just severely handicapped instead.

The great downside of the shield roll is that it, of course, adds another roll. On its defence, I'd say that it only comes up on a succesful enemy hit. Personally in my current "D6 D&D rules" I pair it with my variant of Homebrew Homunculus D&D without damage dice, and with the damage rolls gone, I don't find the shield rolls tedious at all.