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Semi Auto From A M1a1 Parts Set?

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#1 Rifleman1903


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Posted 24 January 2004 - 12:56 AM

This is my first post on this forum and I was wondering if there is anyone here that has built a semi auto Thompson out of a parts set? I have been considering this for a while but have never pulled apart and examined a commercial semi auto thompson so I do not know a semi auto reciever differs from a full auto one other than it needing to fire from a closed bolt and needing a positive disconnect.

I am a more than capable machinist so the fabrication/modification of the required parts will not be a problem.Any advice Or hints on how to go about this or where to turn to find out would be greatly appreciated.

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#2 full auto 45

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Posted 24 January 2004 - 07:42 AM

The FA lower will not fit on a semi upper. And if you did, you must have a ,I think it's a class 2 manufacturers license. Or it's old wedding bells for Bubba in the pokey. See the post on building a dummy gun. I believe the same will apply. In other words, don't do it. Buy a semi gun and put the old wood on it, and maybe do a short barrel. I don't want to chase you off like some will try. But it's a no-no.
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#3 deerslayer


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Posted 24 January 2004 - 09:26 AM

Didn't Devlins project X gun (built by PK) use a full auto lower frame? The selector had the bolt hold open feature. As long as you don't use any of the full auto receiver parts (once a machine gun, always a machine gun). I'd say talk to PK there must be an acceptable way to build the thing the way he did.

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#4 Bill in VA

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Posted 24 January 2004 - 10:11 AM

This has been covered ad naseum on the boards and (if it isn't already) ought to be included in the FAQ. There are a host of legal issues to be dealt with as well as a number of logistical and engineering issues to overcome.

First: the logistical/mechanical side... There are a number of critical dimensions that differ between the semi-auto 1927A1 rifles and the full-auto machineguns. This is a design feature intended to preclude the ready use of full-auto parts and to assuage the fears of those policy-making career bureaucrats in Washington.

Few parts will interchange. For one, the semi-auto is about a tenth of an inch shorter than the full-auto receiver, so a full-auto bolt will not interchange with a semi-auto bolt. Moreover, the internal mechanism of the receiver is totally different (using a series of 3 springs, a steel "pellet", and a "Z" shapped striker instead of a simple heavy bolt and single spring. (The M1A1 is amazingly simple principle.) Without going into too much detail, aside from a variey of minor parts such as screws, springs, sights, magazines (requiring minor modification), the buttstock, and the barrel (legal issues) there isn't much from a full-auto parts kit that's usable on a semi.

Legal issues: Barrels will interchange, but swapping the 16" bbl for a 10.5" SMG bbl requires approval since you'd be constructing a short-barrelled rifle (controlled under the auspices of the 1934 NFA.) Similar problems could also arise from adding a detachable buttstock to your semi. Trying to shorten an M1A1 bolt by milling it down would most likely be construed as attempting to manufacture a machinegun (big no-no since May 19, 1986 and the passage of 922(o).)

In short, save your money and time. get yourself a WH or Kahr semi-auto and file a Form 1 to make and register an SBR (or leave the 16" bbl on it.)

My .02
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#5 Ron A

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Posted 24 January 2004 - 02:56 PM

This question has been asked so many times on this board - it just proves no one reads the "old" posts.

PK does take the "lower" 28 receiver and mate it to the Kahr or AO semi upper. YOU can't use a real 28 upper and bolt to make a semi....

I have one of PK's conversions and think its great!
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#6 Sgt


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Posted 24 January 2004 - 03:11 PM

In case there is any question about the attachable stock on a SBR semi, I actually added that as a description on my SBR papers. I had no problems getting that through.
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Posted 24 January 2004 - 06:31 PM

Rifleman, you can use several parts from your M1/M1A1 Parts kit to build up a SA Rifle. These Parts would include the following: Frame, Rocker Pivot, Safety, Stock Assembly, Magazine Catch/Spring, Trip, Pivot Plate, Rear Grip, Rear Grip Screw. Front Sight/Pin, Grip Mount, Rear Sight, Ejector, Extractor, Bolt Handle, Foregrip/Screw, and Frame Latch. If you have ATF Approval you can also use the Barrel. The M1 Bolt could be modified into a SA Bolt, but it would probably be cheaper buying just an all new Kahr SA Bolt. The Lower would need modification to fit a SA Receiver. I had a '21 Style SA Gun built by Forum Member PK that used a modified '28 Frame. This Gun was known as Project "X". If your serious about building one look him up. In your spare time you may want to do a search on the old Message Board for "Project X". You may also want to order a Catalog ($5.00) from Doug Richardson.
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#8 PK.


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Posted 24 January 2004 - 07:01 PM

There are few parts from an M1 SMG kit that are compatible with the approved semi design gun. Some of them are; the wood, grip mount, safety, mag catch, some screws, the barrel (with form 1 approval form BATF to build an SBR), sights- that’s all that comes to mind at the moment. Some other parts can be modified into semi auto parts

The SMG fame can be adapted to the semi receiver and there is no legal issue here, but it must be used with approved semi auto bolt. Most of the FA fire control components therein are not usable; you would need to get new semi auto fire control parts, some of which would require modification to function properly. Except for the receiver interface issues, the frame of the semi gun and M1 are very similar, the most notable difference being the hole for the pivot in the M1; this can be utilized as an auxiliary bolt stop.

Adaptation of the 28 frame to the 27a1 model is more popular as it will allow the use of the detachable stock. This is not a legal issue. Again, there is work that must be accomplished to allow the FA frame to be used on the semi receiver.

Doug Richardson publishes a paper on the adaptation of FA frames to Semi guns, I suggest you get a copy.

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