-- REPEATER --

Vanilla Repeaters are an abomination, their mechanics are completely dumb and have no place in a strategy game. ABB removes the insta-kill mechanic and replaces it with a bonus to the RNG mechanic that is already a huge part of the game: crit damage. Repeaters give a +1/+2/+3 bonus to crit damage, depending on tier.

-- SCOPE --

On my everlasting crusade against unconditional aim bonuses, the scope was the first to fall. Pretty much mandatory in vanilla, the aim bonus gets moved to Stocks (but with a condition attached, see below) and Scopes offer access to dodge prevention and Covering Fire instead. The progression of Scopes is not strictly linear, it looks like this now:
Basic Scope becomes "Smart Scope". Smart Scopes prevents your shots from becoming grazes.
Tier2 Scopes become "Reflex Scope". They provide the Covering Fire perk.
Tier3 Scopes become "Smart Reflex Scope". They combine the bonus from tier 1 and tier 2.

-- STOCK --

To offer a way to increase your aim with attachments, the Stock now gives acces to a new ability called "Stabilized Shot". Stabilized Shots cost two actions, but apply an +5/+10/+15 aim bonus dependant on the tier of Stock. Note that these can effectively be used without drawback on weapons that already always use two actions (Sniper Rifles, Cannons). This is intentional.
With Shotguns and Rifles, you can use either. You always get both the standard shot and the stabilized shot as options, even on your first action. This is because its relevant in some cases, the Skirmishers ability to fire multiple standard shots being the most important one.