One way you can force them to use another weapon is to trade or steal all the ammo for the gun they're using, and give them ammo only for the gun you want them to use. The only problem with this is it that it goes by what the game considers 'best' and doesn't allow you to manually choose. All you have to do is trade or steal/plant it on them and give them the right ammo, then just tell them to equip their best weapon.
If you talk to NPCs, they'll tell you what kinds of weapons they can use. And with the free Fallout 1 giveaway here, a lot of people are probably interested in this.
Know this is an old post, but this is the first result that shows on Google and there's a lot of inaccuracies in the replies.
