Are any legal forms required to purchase?

(1)a pre-ban 11.5" barrel assembly
(2)a pre-ban 14.5" M4 Type Carbine barrel assembly
(3)M16 fire control components

It is my understanding that only M16 components require the paperwork for a registered NFA weapon.

Thanks for any information and Happy Easter to all!

No - a seller is not required by law to get copies of your forms before selling any of these things, and a buyer is not required to provide them.  However, many sellers do require proof though, just as a matter of company policy and CYA tactics.

Actually mere possession of just one or more M16 fire control parts could be enough for prosecution and conviction for possession of an unregistered MG.  The statutory definition of a machinegun is far broader than that for a SBR/S.  Note the "any part" or "combination of parts" language.

26 USC 5845(b)The term ''machinegun'' means any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger. The term shall also include the frame or receiver of any such weapon, any part designed and intended solely and exclusively, or combination of parts designed and intended, for use in converting a weapon into a machinegun, and any combination of parts from which a machinegun can be assembled if such parts are in the possession or under the control of a person.