I think you know, that this DLC doesn't replace some weapons, like Assault Carbine, but adds the copie of this weapons (with "GRA" index) and provides with upgrades and special ammo only for Gun Runners variant. How can I combine original weapons with his GRA variant? I tried to do this (through the G.E.C.K., of course) manually, but I have one problem: I can't set up modified model. I tried to copy modified model from GRA weapon to original, but this field is blocked, i can't paste it. Maybe there is other way to do this?

You can use any ammo in any gun. You have to use the "2" key to swap between ammo, I think the gun needs to be readied for this too.

It is only some of the vanilla weapons that can't be modded. This is why GRA added them as copies, so they could be modded. If you can't add mod slots to the vanilla weapons (which is probably why they were added as copies, then clone the GRA weapons and replace the stock ones with that.