Holster

Season 15 added Holster Leg Mods for all weapon types that have more than one round in the chamber by default (i.e. no tube grenade launcher, rocket launcher, or sword mods).

The description says:

Gradually reloads your stowed <weapon_type> over time. Multiple copies of this perk stack to reduce the time taken to fully reload.

The various mods work similarly across weapon types, the only difference is the number of round loaded for each time the mod procs.

One Holster Mod

If you have one Holster mod equipped it will proc and partially load the magazine first at 4 seconds and then every 6 seconds thereafter.

If you have a single Fusion Rifle Holster mod equipped, it will reload a single shot each time the mod procs. If you empty a 6 shot fusion rifle and switch to your kinetic weapon, it will load the first round after 4 seconds, the second round after 10 seconds, and be fully loaded after 34 seconds (4 + 6 + 6 + 6 + 6 + 6).

If you switch back to the fusion rifle before it is fully loaded, the timer will stop and you’ll need to stow it again for 4 seconds to load the next round.

This is especially good on those weapon types that benefit from beign fully loaded (like those with the Chain Reaction perk).

Two Holster Mods

Having two Holster mods has diminishing returns. The mod will still wait 4 seconds after stowing before the first proc. All subsequent procs will also happen after 4 seconds. So a 6 shot fusion rifle will load the first round after 4 seconds, the second after 8 seconds, and be fully loaded after 24 seconds (4 + 4 + 4 + 4 + 4 + 4)

Rounds Loaded For Each Weapon Type

The only difference between weapon mods is the number of rounds loaded for that weapon type. Pulse Rifles are weird in that some will load a single “burst” and some will load 2 “bursts” on each proc.

WeaponRounds Loaded On Proc
Fusion Rifle1 round
Linear Fusion Rifle1 round
Heavy Grenade Launcher1 round
Shotgun1 round
Sniper Rifle1 round
Auto Rifle6 rounds
Scout Rifle2 rounds
Submachine Gun4 rounds
Trace Rifle10 rounds
Hand cannon2 rounds
Sidearm2 rounds
Pulse Rifle1-2 “bursts”
Bad Juju3 rounds (1 burst)
Vigilance Wing5 rounds (1 burst)
No Time To Explain3 rounds (1 burst)
Legal Action II3 rounds (1 burst)
Graviton Lance2 rounds (1 burst)
Outbreak Perfected6 rounds (2 bursts)
Chattering Bone6 rounds (2 bursts)
The Third Axiom6 rounds (2 bursts)

I have not tested any 4-burst pulse rifles, none are actively dropping in the game as of Season 15.