3d6 vs. AC Hit Rolls

TL; DR: Table of % chances to Hit using various permutations of 3d6 vs. ascending AC 9 to 18, instead of d20.

	Av.	Ex Rng	to Hit AC 9	10	11	12	13	14	15	16	17	18
3d6	10.5	8-14	74%	63%	50%	38%	26%	16%	9%	5%	2%	*%
3d6 -1	9.5	7-13	63%	50%	38%	26%	16%	9%	5%	2%	*%	-
3d6 -2	8.5	6-12	50%	38%	26%	16%	9%	5%	2%	*%	-	-
3d6 -3	7.5	5-11	38%	26%	16%	9%	5%	2%	*%	-	-	-
3d6 +1	11.5	9-15	84%	74%	63%	50%	38%	26%	16%	9%	5%	2%
3d6 +2	12.5	10-16	91%	84%	74%	63%	50%	38%	26%	16%	9%	5%
3d6 +3	13.5	11-17	95%	91%	84%	74%	63%	50%	38%	26%	16%	9%
4d6 DL	12.2	9-15	90%	82%	73%	62%	49%	35%	23%	13%	6%	2%
4d6 DH	8.8	6-12	51%	38%	27%	18%	10%	6%	3%	1%	*%	*%
3d6 adv	12	10-14	93%	86%	75%	61%	45%	30%	18%	9%	4%	1%
3d6 dis	9	7-11	55%	39%	25%	14%	7%	3%	1%	*%	*%	*%
4d6	14	11-17	95%	90%	84%	76%	66%	56%	44%	34%	24%	16%

• DL drop the lowest.
• DH drop the highest.
• adv/dis advantage/disadvantage (reroll for a higher/lower result).
• Av. average roll, rounded to one decimal place.
• Ex Rng expected range of most rolls, based on rounded averages and standard deviation.
• ascending AC.
• *% rounds to lower than 1%.
• - cannot be achieved on an adjusted roll.
• all percentages are rounded up or down to nearest whole number (for spacing reasons).

I used a combination of AnyDice and Rumkin Die Roll Stats to compile this; any mistakes are mine, from input or interpretation.

For the homebrew D&D-adjacent system I've used and continue to tinker with, there's a 3d6 Hit Roll (4d6, drop lowest, for Fighters and monsters), vs. base AC 9 and the leather/chain/plate scale from BECMI etc. No bonuses to Hit or to AC from Ability Scores. Base damage is the margin-of-success; crits on triples (less than 3% chance). There are Parry and Dodge options. 

I currently use AC 15 (plate/Heavy) as the hard cap for the system, and there are (good?) mechanical as well as narrative reasons for adventurers to favour lighter protection. 

I've extended the AC line to cover the base ACs of most retro clones (10), plus Basic Fantasy RPG (11) and Lamentations of the Flame Princess (12), up to the maximum unadjusted roll of 18.

While primarily interested in the 3d6 and the 4d6DL rolls, I've included adjustments -3 to +3, 4d6DH, advantage/disadvantage, and straight 4d6 (but ignoring rolls above 18) for comparison.

Addendum (28/04/2020).

Link to the d20 SRD entry for Bell Curve Rolls (3d6, but also 2d10), for reference.

You could use the Dragonwarriors Armour Factor progression instead of D&D adjacent, which would bring the numbers down a bit if you don't like how strong Heavy/Plate gets: padded leather +1, hardened leather +2, ring mail +3, chain +4, plate +5.

For the tinker homebrew, shields do not contribute to AC, but there's no reason not to use them RAW for your preferred system if you're using this.

Also for the homebrew, monster ACs tend to be the same as Unarmoured, but a tough hide (Elephant) might be +2 (leather); a scaly hide (Crocodile) might be +3 (BECMI scale mail); a shell/carapace (Giant Beetle) +5 (BECMI banded), and a Living Stone Statue equal to Heavy/Plate at +6 (though could also be AC 9, but only ever takes one point of damage per hit and you risk breaking your weapon).