Your info from MG Board is partly incorrect!
If you have a Full Auto Lower assembly in a box in your closet along with info on how to convert your semi auto into a Full Auto or just a bunch of parts for Full auto [Auto Sears etc] and the ATF happens to have cause to Raid your House then yes you may have a problem.
You as a Private Person, Non Manufacturer or /FFL can legally Build any Firearm you want. [as long as it is not Full Auto. And the parts used are converted to meet ATF Regulations. [This means adhering the Full Auto rules and the The Imported Parts Count used on your gun].
You can purchase a Semi Cetme or HK [ptr91] Receiver and a Complete Parts kit /Minus the intact Full Auto Upper Receiver and assemble it for yourself.
You can Hand make your own Receiver, your own design, as long as it is not Full auto and has no possibility to be Readily Converted to a Full auto /with no Serial Number. [as long as you own it] [if you want to give it to your Son it then needs to be serialized per ATF Regulations]
If you want to lean toward caution, just copy the conversation info from My Build Thread in the "Sticky". When the Full Auto Lower arrives, just remove the Auto Sear and Trip Lever and throw them in the Dumpster.
And place the info in the box with your parted Lower.
If you are going to do the work, just lay out [scribe and center punch] the parts and or cut the shelf in the Trigger Frame which will remove the Sear Pin Hole.
Or just Place the parts in a box with a letter of intention and send them off to a builder that can update the parts for your gun.
LEGAL INFO
Copied from IGF.com
The following is the written response I received from the B.A.T.F. regarding various methods of converting a G3 trigger pack to semi auto only. The main question I had was if it would be legal to weld over or weld a piece of round stock into the existing auto sear pin hole (with the auto parts removed of course) which would eliminate the possibility of installing the automatic fire control parts.
Department of the Treasury
Bureau of Alcohol, Tobacco, and Firearms
Washington, DC 20226
Sept 28, 2001
Mr. xxxx
xx, AR 72653
Dear Mr. xxxx
This is in reply to your letter in which you have requested proper procedure for modifying a Heckler and Koch G-3 machinegun trigger, into a semiautomatic trigger.
To properly convert a G-3 machinegun trigger into a semiautomatic only trigger there are several modifications needed. The modifications are listed as follows:
1. Remove the automatic sear.
2. Grind off the automatic sear notch from the face of the hammer
3. Cut away the front corner of the trigger housing where the automatic sear is mounted, .410-inch from the bottom and .540 inch from the front towards the rear.
4. A new hole has to be drilled to accommodate the trigger spring. The hole dimension is .200 inch drill that hole .165 inch from the front of the new corner and .250 from the bottom of the new corner. These dimensions are for the center of the hole.
5. The trigger spring should be replaced with a semiautomatic trigger spring but the G-3 trigger spring will work.
6. The trigger over travel stop has to be moved towards the rear tang of the trigger .150-inch
-2-
Mr.xxx
7. It will also be necessary to modify the trigger housing. Modify the housing by cutting off the tabs for the front assembly pin. Then adjust the fit of the housing into the receiver.
Title 26, United States code, Chapter 53, section 5845 (b), defines the term machinegun as any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot without manual reloading with 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 from which a machinegun can be assembled if such parts are in possession or under control of a person.
If you do not properly modify the G-3 machinegun trigger pack and install it in your semi automatic rifle, and that weapon fires more than one shot automatically with a single function of the trigger, you would be in possession of an unregistered machinegun.
We trust that the foregoing has been responsive to your inquiry. If we can be of any further assistance, please contact us.
Sincerely yours,
Curtis Bartlett
Chief Firearms Technology Branch