Purchased and downloaded the Gun Runner's Arsenal. I noticed that is deleted my saves for my game. I used backup to reload them. Playing game and I can see and purchase the new weapons and mods. However I am not able to upgrade any weapons with the new mods. I have a Anti-Material Rifle and I purchased the upgraded bolt and receiver parts for the weapon. However, when I go to my inventory screen and hit "x" to modify this weapon, nothing shows up (neither the bolt or the receiver parts). I tried modifying some weapon systems but that did not work either. I still seem to be able to to upgrades on the original weapons but do not seem to be able to do any of the newer ones? Does this DLC only work for new games?

To circumvent a problem that would occur code-wise, only GRA weapons can equip GRA mods.

Purchase the Anti-materiel (GRA), then try to mod it.

I think protonmasher's got it in one. The DLC doesn't add mods for existing weapons, it adds new weapons (often copies of existing ones) with the "GRA" text appended. Only these will accept GRA mods.

Want an explanation for that? My theory is that most of the weapons that you use are actually pre-war, but weren't designed for mods. So what the Gun Runners are doing is taking the old weapons and rebuilding them to be "mod compatible" then designing mods that these weapons can accept. So you've got an old gun, swap it for a spiffy GRA one!

Game mechanics -wise that was propably done to avoid bugs. The non-unique GRA weapons are exactly the same as the non-GRA ones, with the exception that they can be modified.

I think it's because of the modular nature of DLC. If you were to uninstall GRA with GRA mods on a non-GRA weapon then it'd glitch something AWFUL.

Yeah, it's a load order issue, they had to make duplicate GRA version weapon sets to make the mods work with them.

GRA ammo (both hand loaded and purchased) however, can be used on all weapons such as the 12ga Dragonbreath and Flechette Rounds and the 20mm and 40mm Plasma Grenade Rounds.

IIRC the exception is the vanilla Fatman, the GRA mini-nukes will only work with the GRA Fatman and Esther.

To be sure, this doubling does have its advantages. Chet now sells full-condition GRA Laser and Plasma pistols at the start of the game, which makes life MUCH easier for energy weapons Couriers.