The goal is to: create tactical gameplay where the soldier makes meaningful decisions to his weapon-state based on his needs or his combat threat. 

This is achieved by having movement speeds affected primarily by what ready-state the gun is in (from fastest to slowest):

HOLSTERED;
UNHOLSTERED/LOW-READY;
READY;
ADS.

Additionally, the weapon type he is using will have a further, but much smaller effect on his movement speed in the three stances: UNHOLSTERED/LOW-READY; READY and ADS. 

Depending on his purpose, he will therefore need to select the right tool. Bigger guns have bigger firepower, but they make you less maneuverable. From fastest to slowest:

PISTOLS;
ASSAULT RIFLES + SNIPER RIFLES;
LMGs/GPMGs;
LAUNCHERS.

The more ready a weapon-state is (i.e in the ready position or aiming-down-sights) the slower a soldier will move. This means he will only have the weapon ready to fire when he knows he is about to use it. This also to allows recon units to determine at a glance the possible combat state of enemy players.

If a player has his weapon holstered so as to move faster, you know he is in no perceived threat; if he is unholstered or low-ready then you know he is alert; high-ready and ADS means he is vigilant or about to engage.

This is unlike the vanilla infantry experience where every soldier, all the time, has his weapon in the high ready state. Whether he's in the midst of combat or at the command tent strategising with his squad he is aiming his gun and ready to shoot. There is no meaningful choice to be made in how you hold your weapon.

Additionally, this creates a much more tense, slow-paced, tactical CQB and urban infantry fighting experience where one cannot rely on bumrush, w-key tactics to win a fight and must therefore use teamwork, strategy and the right tools for the job in these dangerous environments. 

This is also great for recon (or even immersion), as you will see players for the first time using muzzle discipline in safe zones where they want maximum speed.

Further to that, players moving cross-country will always have their weapons holstered unless they are expecting danger. For the first time, seeing a variety of weapon states in use means your recon or pointmen can gain valuable intel to determine if your enemy are alert to any threats.

All-in-all, this makes the skill-expression in infantry combat far more wide and far more deep. Indirect fire, utilities, flanks and superior teamwork will become what wins CQB or an urban firefight.

WISH-LIST
-Low-ready / unholstered states to have their own movement speeds, making low-ready have a meaningful place in the hierarchy.
-For "carbine" style rifles to have their own entity category, making them find use as more mobile urban/CQB firearms in exchange for their penalities (they have less muzzle velocity and penetration in case you didn't know!).

WANT MORE?
For an added layer of realism, see my other movement mod: Sunshine's TIMS. This mod is part of the full Sunshine tactical infantry combat suite, creating the overall authentic mil-sim infantry combat experience.
https://reforger.armaplatform.com/workshop/6728865D648BB916-SunshinesRealismMovement

Sunshines TIGMS

Sunshine's Tactical Infantry Gun Movement System (TIGMS)

Part two of a 2-part mod creating an authentic and meaningful infantry combat experience.

Find the part-1 link at the end of the description.

Sunshines TIGMS

1.0.10

1.0.9

1.0.8

1.0.7

1.0.6

1.0.5

1.0.4

1.0.3