I'll jump in on a couple of points here.
Watershed year in the US was 1986, that's when the closed the books on manufacture of new machine guns in the US for civilian ownership. There are 'post-86 dealer samples' that can be owned by a dealer, or brought in by a dealer with proper paperwork indicating that they are for a demonstration for a police/military display with them looking to sell.
Every other machine gun in the USA right now was registered prior to 1986. It's a fixed market, there are no more new ones, so the prices on the existing will only go up.
Stepping over to the AR-15. Yes, there ARE in fact fully automatic AR-15's. I've held one. It was at the Warminster Infantry Training Center in the UK. It was, quite clearly, an AR-15, but it was also Full auto. It was manufactured that way, and was legal to do so in the early 1960's when it was made.
The current generation of AR-15's have a number of internal manufacturing differences from the M-16/C-7 FOW (Family of Weapons).
The basic differences REQUIRED between a full-auto and a semi-auto AR-15 are:
- -Auto Sear Lever not installed
- -Auto Sear Axis Pin hole not drilled
- -Semi-Auto Selector lever installed
There are a number of other internal differences that have been made over the years. These changes include:
-Inserting a tool steel block where the auto sear would fit
-Machined hook on hammer
-Ramp slot cut on bottom of bolt
-Larger diameter firing pin ring to engage with hook on hammer through slot on bolt
-Removal of auto sear relief cut on upper receiver
-Semi-Auto Trigger and disconnector not machined to function with Full Auto Selector
I have had the occasion over the years to be inside the guts of a *LOT* of AR-15's, C-7's, M-16's, etc, etc, and am very familiar with the internal workings of them. I have assembled AR-15's, I've built barrels, conversion kits, etc.
There exists a thing called a "RDIAS" a 'Registered Drop In Auto Sear' that will serve to convert a semi-automatic AR-15 into a Full Auto, with no receiver modifications. These require specific parts, and a specific lower receiver internal profile from an early 1980's Colt AR-15. The tool-steel inserts that Colt started putting into their lowers in the 80's/90's precluded the insertion of these devices. Note as well the "REGISTERED" part. The RDIAS must have been registered with the BATF in 1986 or prior to be legal for use. Don't have an unregistered one in your possession in the US with an AR-15 in your possession otherwise they'll arrest you for 'constructive intent.'
Now, moving ahead with home built firearms. I have in my collection the 3rd, legally registered in Canada Semi-Automatic Sten Sub-machine gun.
Yup. I own a Sten. I could never get it to fire more than 3 rounds in a row though.
I built it myself. Took me about 100 hours of machine time using a Sherline Micro-mill and various hand-tools including getting some help with the welding. It's been inspected 3 times by the CFO for NS, and there are several differences between it and a real Sten that would prevent it from ever functioning in Full Auto.
If I had to build another, it'd be much better, and much quicker on the 2nd time around. I now have a much larger milling machine that would enable much faster work on some of the bits that took me a long time with the mini-mill.
It IS legal to build yourself a firearm in Canada. I've done it. The rules are fairly easy....once you intend to build a firearm, if it is restricted, it must be registered. Once you complete the frame/receiver, it's registered as a 'frame or receiver only.' Once you receive the registration certificate, you may then go ahead and complete the assembly of the firearm, and you then have 30 days (from the time it's complete) to contact the RCMP/CFC and have them change the status from Frame/Receiver to a completed firearm with barrel length, etc added to the new certificate. Once you've received the new certificate, then you can take the firearm to a shooting range to test it.
Oh, and the whole 3D printing thing? I've been watching it for several years where it comes to guns, and the US Military has recently developed the ability to 3D print a 40mm grenade launcher, AND PROJECTILES. So, yeah, there's that too.
I'll do a supplementary response to this to show some of the differences between an AR-15 and C-7/M-16 in pictures.
Hope that helps clarify things a bit.
NS