For those not playing on PC and don't use console commands.

• Sometimes it can be very easy, sometimes very hard to pickpocket ammo. The easy way to do it if you keep failing to pickpocket someone is to reverse-pickpocket one round of the same ammo type INTO the NPC, then you'll have a 100% chance of pickpocketing the whole stack of ammo from that person.
• Sometimes you want a weapon the NPC is carrying but don't want to kill the NPC for it, and the weapon is hard to pickpocket even if you take all his ammo. What you need to do is shoot the weapon once (in VATS or so) with a high powered weapon, then holster your weapon away before the NPC becomes hostile. If the weapon is at 0% condition, the NPC will eventually unequip it when you leave the area and when you come back you have a 100% chance of pickpocketing the broken weapon from him.
• Want to give an NPC grenades without killing him? Reverse-pickpocket any grenade into him (it'll go live), then quickly reverse-pickpocket any grenade you want him to have, then quickly leave the area (going through a nearby door works, or maybe fast travel away can work), and when you come back the NPC will still be alive and have the grenades you gave him in his inventory.