So I would like to share with you some tweeks I've made to combat in ACKS, and of course I'm open to suggestions and scrutiny!
In my games I use positive AC and attack rolls (just like D&D5E), I also use critical hits as double damage OR an automatic success on a combat manouver, I also treat rolls of 1 as critical miss, rolling on a Fumble Table of my doing. Double damage works by doubling the damage after rolling and after adding modifiers, so usually the damage output is greater than normally it would be.
But the main difference to ACKS standard comes to the advantages of the many types of weapons that the system offers to us. In my games there are bonuses and/or penalties depending on the weapon you are using.
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Large two-handed weapons = -1 Initiative / +2 to damage.
Shield = you can destroy your shield to avoid damage from one source (you can decide to use this after the damage roll). Magic shields are able to do that without being destroyed a number of times per day equal to their magic bonuses, so a +1 shield would do that once per day, a +2 twice and so on. Similarly magic weapons usually deny this abilities by a number equal to their own magic bonus (so yeah, you can't use a common shield to ignore the damage from a magic sword).
Small weapon = +1 initiative.
One weapon + another hand free = You gain +1 on attack rolls or +1 to your AC against melee attacks, decided on the Initiative phase. May use free hand to interact with stuff.
Two weapon fighting = If you hit you can roll the damage for both weapons and take the higher result. You can choose use 1 medium weapon and 1 small (so 1d6/1d4) or 2 small weapons (+1 Initiative, 1d4/1d4).
Ranged weapons in short range  = +1 attack roll and +1 damage.
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My initiative works a little differently with NPCs. I divide my NPCs in "Individuals" for bosses and characters of some importance, and they are subjected to the above rules and benefits... and "Groups" for the unnamed NPCs that roll initiative collectively... they can't benefit from these modifiers. So if my players are facing a group of Orcs and their mystic leader I would roll one initiative for the Orcs as a group and one roll for the singular mystic leader, who could benefit from using a different set of weapons.
I really like your shield rules - especially the nuanced part with the effects of magic arms and shields. Also lets you get away from very nasty enemy rolls by sacrificing your shield, which reduces combat "swinginess" a bit.
My initiative works a little differently with NPCs. I divide my NPCs in "Individuals" for bosses and characters of some importance, and they are subjected to the above rules and benefits... and "Groups" for the unnamed NPCs that roll initiative collectively... they can't benefit from these modifiers. So if my players are facing a group of Orcs and their mystic leader I would roll one initiative for the Orcs as a group and one roll for the singular mystic leader, who could benefit from using a different set of weapons.
That's actually how I handle initiative too. If the group of identical NPCs exceeds the size of the PC party I will split it up into multiple groups. So for instance if a party of 5 PCs encounters 10 orcs, orc shaman, and ogre, I would roll:
Typically initiative isn't just the speed of an attack it it also about the ability to recover from strikes and keep everything fluid (thus essentially a measure of effort). While attack rolls aren't just a single swing but a back and forth and how well an individual can make an opening in the defense of their target to land a more telling blow on them.
While long weapons do initially have a better chance of getting that first blow, if you cant keep the distance of the weapons optimal reach it becomes much harder to bring it into play.
All that said at the end of the day it is often done to balace the gaming system (trading speed for extra damage) and has little to do with realism. We aren't going to be able to represent real combat in RPGs without getting super complex, so at one point or another realism needs to take a step back for the game mechanics and abstraction to make things useable and more importantly enjoyable.
That said, ACKS does a reasonable job of simulating the "long weapons do initially have a better chance of getting that first blow, if you cant keep the distance of the weapons optimal reach it becomes much harder to bring it into play" thing by giving two-handed weapons an initiative penalty but allowing weapons with reach to attack first when an opponent closes to melee range.
I don’t think long weapons only have an advantage in the first clash. They can keep foes at bay and allow striking first every exchange. Weapon length is really a huge advantage in sparing.Â
That said, ACKS does a reasonable job of simulating the "long weapons do initially have a better chance of getting that first blow, if you cant keep the distance of the weapons optimal reach it becomes much harder to bring it into play" thing by giving two-handed weapons an initiative penalty but allowing weapons with reach to attack first when an opponent closes to melee range.
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Regarding the rules in the Core ACKS, whether you are using a spear set against a charge, or you are using a longer reach weapon when the enemy closes, you hit them simultaneously, on their initiative (even if you have a lower initiative).
Where did you find the rule that you hit them first?
I was thinking along the same lines, but it also occurred to me that longer weapons are going to offer a disadvantage in the close quarters of a dungeon. I had an idea, which might be more cumbersome than it is worth, to let a character swinging a Great Sword/Axe roll 1d6 per clear square in their cardinal directions and take the highest value. (Probably letting enemies in a diagonal square claim an adjacent square if it wasn’t already occupied).
Still at -1, (but swords get a +1 to initiative in my campaign. Other weapons have other advantages, as per the Fresh from the Forge zine and The Majestic Fantasy RPG Basic Rules, but those house rules are for a different comment/thread).
If the character with the Great Weapon is totally surrounded, roll 2 1d6 and take the lower value.