So the only issue I have with solely using DARKSIDES method of disabling the chamber of the barrel and permanently welding the barrel to the tube is when I read this answer on the ATF website
https://www.atf.gov/firearms/qa/what-unserviceable-firearm
An unserviceable firearm is defined as one which is incapable of discharging a shot by means of an explosive and which is incapable of being readily restored to a firing condition.
An acceptable method of rendering most firearms unserviceable is to fusion weld the chamber closed and fusion weld the barrel solidly to the frame. Certain unusual firearms require other methods to render the firearms unserviceable.
An unserviceable NFA firearm is still subject to the controls of the NFA, but may be transferred tax free as a curio or ornament.
In addition, here is the NFA's definition of what qualifies as a machine gun
https://www.atf.gov/firearms/firear...-firearms-national-firearms-act-definitions-0
For the purposes of the National Firearms Act 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 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, or
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.
If I understand this correctly, than if a Sterling were a regular old semi-auto or bolt/lever action or what ever have you firearm, then his method would be perfectly fine. Where the problem comes from is on two fronts. A real Sterling SMG falls under two qualifications making it a NFA item. Its a Machine Gun, and it's a Short Barreled Rifle. The ATF's policy of "Once a Machine gun, always a Machine gun" is why we here in the US cannot import demiled firearms from overseas and can only get a hold of torch cut parts kits. Whats unclear is if one were to do as DARKSIDE suggested (or even my method for that matter) would the moment that the receiver is welded back together still make it a "Unserviceable NFA Firearm" under those definitions as either a MG or a SBR. Which is why the incredibly vague definition of "readily convertible" really sucks.
It's even more confusing when I look at my own firearms collection. Take for instance my PTR-91 which is a US made semi-auto clone of the HK G3
On the left of this photo is the full auto receiver to a HK G3 (no not mine sadly) and on right is my PTR 91. The ONLY difference between these two, and the only thing keeping the PTR from being classified as a MG, is that welded shelf that prevents one from attaching a FA trigger group (that little tab hanging out of the mag well of the G3 is just a paddle mag release. I just haven't installed one on my ptr yet). The PTR receiver still readily accepts a FA bolt carrier just fine, its just the welded shelf that in the eyes of the ATF keeps the PTR and all semi auto HKg3 clones from being machine guns.
To me, speaking from the point of view as a machinist, it would be incredibly easy remove that welded shelf and drill a pin hole for a FA trigger group. Doing so would be horrendously illegal, but the point still stands about the lack of a definition of what is and is not considered readily convertible.
Wiselite arms made a few ATF approved Semi auto sterling's available a few years ago before they went belly up. I haven't had the chance to see one in person yet (they've become expensive and kind of hard to find), but what I do know is that they were able to use original diameter receiver tubes and parts for their guns. They modified original FA bolts to be closed bolt semi auto some how, and this I suspect is the only way to get past the Machine gun definition... Unfortunately it looks like they were unable to use the original barrel lug on the front for their design for some reason. I suspect it may have been a forced modification due to whatever they did to the bolts
I'm still going to try and get a answer in writing from the tech guys at the ATF on what to do once my kit shows up in the mail and I can lay out a solid idea for them to assess