OK, a few things up front.
ALL shotguns or semiautomatic rifles are subject to the "parts
compliance" rules. ALL of them -- even a Colt made AR15. ATFE has
made a list of 20 types of parts, and said that if a shogun or
semiauto rifle uses more than 10 of them, it must be built into a
configuration that would be acceptable for importation (i.e.,
whatever ATFE decides is "sporting" THAT day).
Luckily, there aren't that many parts in a MkIII based semiauto build
that have to be counted when figuring the parts compliance. (By the
way, if you build it as a registered Short Barrelled Rifle or Any
Other Weapon, or you build it as a pistol -- the "sporting"
and "imported parts" crap doesn't apply to either pistols or
registered NFA weapons.)
The parts that would apply are:
2. Receiver (you have to make a new one anyway, as the original was
destroyed before importation was authorized)
4. Trigger group housing
7. Hammer (required in a semiauto build, not present in the original)
8. Disconnector (i.e., "tripping lever")
10. Magazine body
11. Magazine follower
12. Magazine floorplate
13. Barrel trunions (AFAIK, they count both of them as being a set
that makes up one item, and if either one is imported, teh set is
Forget what you have heard about modifying a part "makes" it "US
made". ATFE has been horribly inconsistant on how much change they
think is necessary. Just go all out with replacement, as they CAN'T
quibble about that.
So, even if you managed to convert your original bolt and trunions to
work in a legal semiauto configuration, you only need to come up with
three of these parts that aren't "imported". Well, the receiver
AUTOMATICALLY is going to be "US made", as YOU are going to make it.
So you just have to figure out how to drop two more "imported" parts,
and you can ignore ATFE's ever-changing definition of "sporting".
Now, as for what ATFE wants to see in a semiautomatic Sten build so
they don't just call it a machinegun. . .
1. A reduced diameter receiver tube, so that an unmodified original
bolt WILL NOT FIT. You can lathe down your bolt, but it is hard to
maintain concentricity unless you have it locked in to a BEEFY
lathe. It also means your barrel trunions will need to be reduce in
OD, but that's easier than the bolt.
2. A repositioned cocking handle track. This prevents you from just
lathing a full-auto bolt down and dropping it in. Of course, you
could try and drill a new hole in the bolt for the bolt handle to fit
in. . .
3. A physical block in the receiver so that a lathed down full auto
bolt with a repositioned bolt handle STILL will not chamber a round.
One way they like is to weld a 1" long section of drill rod at top
dead center right next to teh chamber. NOW, in addition to lathing
down the bolt and repositioning the bolt handle, you have to mill a
slot for that block to fit in, so the bolt goes all teh way forward.
4. Jigger the fire control group so as to disable automatic
functioning even if you did all that previous work on a full auto
bolt. You can either weld teh selector in teh SEMI ("R"
for "repetitive") setting, OR you can weld a block inside the trigger
group housing that prevents the tripping lever to move forward when
the selector is set to "A" ("automatic"). If that tripping lever
cannot move forward, the sear and trigger do not move, either.
5. It MUST fire from a closed bolt. (Technically, one could argue
that it is theoretically possible to design an open bolt system that
ATFE judges to be not "readily restored" to automatic fire -- but no
one has succeeded since 1983 or so in doing so.
6. It MUST have a seperate and moving firing pin, and ignition
controlled either by a striker system (where the firing pin is held
back by teh sear) or a hammer system (where the firing pin "floats",
generally under weak counterpressure from a spring, and is smacked by
7. As a consequence of #5 & 6, you must mill the feed ribs that
strip rounds from the magazine so that they do not protrude forward
of teh bolt face where the case head sits. That is to keep you from
gluing your firing pin in the forward position. With the feed lips
flush, the rim of teh round will hang up on the side of the firing
pin and the gun jams if you tried that.
With all teh concerns that need to be met, actually using teh
original bolt is probably more difficult that making a new one (which
would help your "parts count" issue.) Likewise, the barrel trunions
are easy to make, and their replacement with US-made new trunions
would ALSO help your parts count. Between the bolt, trunions, and
receiver, you would have all the "compliance parts" you need.