If your project is older the version 2.4.18 then you’ll need to follow the DIY Guide linked below before following this guide.
https://defusestudios.com/sgk-v2-improved-weapon-ammo-chambering/

This guide will show you how to remove the loading of ammo into magazines and instead allow ammo to be loaded directly into the weapon.

Open your weapons holdable blueprint I will be using the included BP_M4A4 then in the Class Defaults search for Max Ammo and set that value to what you want the max ammo of your gun to be. Then search for the Chamber Amount and set that to the same as your Max Ammo setting’s value.

Next if your weapon has a magazine attachment by default that can be removed by removing the magazine item from the Default Attachments setting like the video below, this also shows how to remove the magazine attachment slot from the weapon.

Next search for Chamber in the Class Defaults and make sure Allow Chambering Reload is ticked on.

Next you need to have a Chamber Character Animation setting set to your reload montage, by default the BP_M4A4 already has one called AM_Chamber_Rifle. If your creating one for your weapon it needs to use the UpperBodyPostAOGroup.UpperBodyPostAO montage slot and you need to add the BP_ChamberWeaponAmmo notify, if you need an example you can look at the included AM_Chamber_Rifle montage.

You can also set the Weapon Chamber Animation this is the animation the weapon mesh will play when reloading, there isnt an included one for the M4A4 but this is the setting you’ll need to use for your own weapons.

You should now be able to pickup your gun and its ammo in game and press R to load the ammo directly into your weapon from your inventory. I suggest making sure your not loading a save game that was made before these changes when testing this as its weapons may still have the magazine attachment added to them which may cause problems.

Magazine Mesh

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This next step is optional, you may still want your magazine to be visiblely attached to the weapon if yours isnt built into the weapons mesh, to do this open the weapons holdable blueprint and copy the video below. If you dont have the magazine socket in the parent socket list then you’ll need to add that to your weapons skeleton on the magazine bone.