Dude, you are making phenomenal progress! Seriously, getting the inventory and widgets moving like a breeze on a brand new framework as a beginner is a massive achievement. Don't sweat the complex parts of ALS it intimidates experienced developers all the time because it is packed with dense systems.
Let's keep things super simple so you don't burn out or get lost in code:
1. Match the Ammo Variables
Since your ALS project already has a Mag Ammo and Bag Ammo variable, you just need to feed my inventory numbers straight into them. Think of it like connecting a water pipe:
Find the blueprint code where ALS fires a weapon.
Right before it subtracts a bullet, use a Set node to make ALS's Mag Ammo equal my asset's Ammo In Mag variable.
When your character triggers a reload, have the ALS Bag Ammo variable check the total bullet count inside my asset's Inventory Component.
Don't worry about splitting ammo types right now. Treat all ammo as generic until you get the gun shooting.
2. Bypass the ALS Body Slots (The Simple Attach)
You mentioned you don't care about seeing weapons holstered on the back or hip. That saves you a massive headache.
Instead of messing with the complex ALS slot system, just use the Attach Actor to Component node inside your character blueprint when you equip a weapon.
Force-attach the gun directly to the hand socket you adjusted earlier. This completely skips the ALS body slot logic and forces the gun into the hand.
3. The Slide Loop Fix
You hit the nail on the head! Once you open that montage, create your section for the slide being pulled back, and check that Loop box, it will stay locked back perfectly when your ammo hits zero.
Since you are working with ALS socket attachment, seeing it done visually helps a ton. Check out this excellent video guide:
It shows exactly how to copy, paste, and align hand sockets between a weapon system and the ALS skeleton.