Anyone know if you can mod up your companions original weapon and have it still have infinite ammo when you give it back to them?

No. the companions original gun is in a hidden list. You could try giving a companion a gun that uses the same ammunition - haven't tried it myself, though.

After seeing Cait for the first time with a baseball bat in her hands I made her a really good one and gave it to her.....she never used it and instead used a double barreled shotgun the entire time....I dumped her after Piper refused to do a threesome.

Unless they changed it since Fallout 3 and NV, companion specific weapons use special "magic" ammo no other weapon can use.

You have to tell them which weapon to equip otherwise they default to their standard weapon. Cait uses whatever I tell her to use.

I did. Piper uses a 10mm pistol so I did a tricked out 10mm for her and guess what. When I tell he to equip it she does... but then when she goes to fire she switches back to the hidden gun. The ONLY way to get her to use the tricked out 10mm is to give her a bunch of 10mm ammo which defeats the purpose.

That makes perfect sense, actually. The 10mm pistol she uses by default is "chambered" for Magical Companion Ammo (and I'm pretty sure that that is it's actual real name). The 10mm pistol you give her is "chambered" for 10mm rounds. Because she has no ammo for the one you gave her, she uses her default weapon. Once you give her ammo, she starts using the one you gave her because it's better.

Yeah, I do understand it... I was just saying it was a shame she doesn't use companion ammo. The gun I gave her (Piper's Pisto) is silenced too so that it helps to keep startling the Ferals. Unfortunately she ran out of ammo for that in the middle of a whole bunch of them the one time... Scared the crap out of me when she started firing loud ammo and I nearly turned around and popped her too (thinking there was an enemy raider or something behind me).

Well imagine if you made a super modded up weapon and then your companion used it with unlimited ammo. Makes sense that they limited this.

I do have Cait using modded up melee weapons only. You just have to tell her to equip it and it's all she'll use. She's a beast ... and I don't have to worry about her shooting me in the back.

rofl

Since I got strong I haven't changed companions at all. He can carry tons of stuff and is ridiculous with melee weapons. I gave him a grognaks axe

I wish we could at least mod their default weapon.

This is most probably the case. Any other weapon you give them will use ammo. In New Vegas at least it actually was called Magical Companion Ammo

Once the tools are out you can mark their weapons as "playable", which should allow you to remove them, though due to them being special, i doubt they can be modded. But it shouldn't be hard to mod the mods so a mod allowing them to modded can be modded

its best to give them your really good melee weapons I have cait dressed up as a barbarian and swinging an axe

Used a mod to remove and change main weapon for companions in NLV I'm sure it'll come along.

Me too. He is a beast with Grognaks Axe and I like to let him run into battle so I can wait for enemies to swarm him and toss a grenade. Sure, Strong is also hurt but he gets over it pretty quick.