I know that when you give a follower a new weapon you have to give them ammo... If you give them a weapon that is the same caliber as their default... do you still have to give them ammo...

My understanding is that follower weapons are a different ID than a world weapon of the same type, therefore using different properties so if you give them a world weapon, it's gonna need world ammo.

I am glad someone has asked this as I was also wondering how it works. I gave Piper an upgraded and suppressed 10mm and it didn't seem like she was using it even after I selected it.

Am I correct that settlers are different as if you give them a weapon, you only have to give one of the ammo type (a bullet, missile etc) and it will give them unlimited use?

Nope, there is no unlimited use trick in this game, the instant any NPC is given a gun beyond a default, it needs and consumes ammo.

So far my settlers have not used up any ammo and have been in several firefights.

Ahh.. sorry Remmus, that's not quite correct. Settlers are different than followers, all you need is a full clip, and they don't need any more after that (some people say you only need like 10/13 rounds or so, but I just make sure there's a full clip plus a lil extra just in case) After multiple raider, super mutant attacks on my settlements, not a single one of them has had a single round go missing. (they all have either combat rifles, assault rifles, laser rifles, or auto shotguns) so it's easy for me to keep track of the ammo, also, TAKE THEIR CRAPPY WEAPONS off of them and put em to use.. scrap those svckers at the WB.

Also, don't forget to actually EQUIP the new weapon on them... if you give them the ammo for the weapon first, then give them the weapon, it should show the ammo auto equip for them.

Followers: TOTALLY different animal. Give them anything and they'll need ammo (did the above trick for Piper, gave her a legendary laser rifle I'd upgraded, 138 fusion cells for it, and then a while later I noticed her shots went from the laser rifle back to her pistol. So I opened her inventory, re-equipped the laser rifle, and she promptly switched back, until I gave her more ammo. Now she's back to pew pewing instead of bang banging

So a follower would still need ammo if their default weapon is a 10mm pistol... and you gave them a modded 10mm pistol...

Yes, in that case you have changed to a different weapon, it will require ammunition