Semi Auto From A M1a1 Parts Set?
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.
Posted 24 January 2004 - 07:42 AM
Posted 24 January 2004 - 09:26 AM
Posted 24 January 2004 - 10:11 AM
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.)
Posted 24 January 2004 - 02:56 PM
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!
Posted 24 January 2004 - 03:11 PM
Posted 24 January 2004 - 06:31 PM
Posted 24 January 2004 - 07:01 PM
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.