No1Mk111 Receiver Holding Fixture

[Son]

Please excuse me if I stuff this up, I am 24 hours post op and still under the influence of some wonderful chemicals....
As far as a repeatable datum is concerned I think you need to reproduce the Wing Gauge part of the Gauge Parrallel that went into the boltway and the machined recess between the trigger guard lugs. Naturally you won't have concern yourself with the wing part as this is only needed for indexing the barrel with the gauge parallel.
At the back you can leave a bit of length on it to either incorporate your three flat sides on this part or a shaft and a centre hole, or both. You will need to come up with a set up for the front that screws into the barrel threads and then can lock into the front end of the wing gauge part through the receiver ring... Perhaps make up a collar out of a barrel stubb and bore it for a neat fitting through bolt, screwing into the end of the wing gauge.
I see this as giving you an upright datum at 90 degrees rotation from the top of the receiver on the "leg" of the wing gauge that goes to the bottom at the rear trigger guard mount. The barrel stub screws in to bottom on the inner front surface of the receiver ring, and a few thou gap from that rear face of the barrel stub to the front face of the wing gauge that can be shimmed if tightening the through bolt distorts the receiver at all.
You can also then machine three sided driving flats on the barrel stub too, but it won,t give you repeatable possitioning like the wing gauge should.
I am away from my pics at the moment, but I am sure if you aren't familiar with the shape of the wing gauge, someone can post a pic... If not I will stick one up when I get home.

Attachment 71282
Attachment 71283

With the gauge/ mandrel inserted into the receiver from the back, the alignment lug that goes into the rear trigger guard space gives you a repeatable datum from the centre line of the receiver about the boltway bore/ barrel bore. Making it extended to the front of the receiver to join up with a method of locking it in place like I suggested above to a centre hole for mounting. Leave some length at the rear to cut your three sided driving lugs concentric to the bore and mount in a 3 jaw chuck in an indexing head on the machine table.

I assume your main aim is to machine the half dovetail on the side of the receiver charger bridge for accepting the rear base mount for a HT (which has to be a very precice distance from the centre of the bore) and from that mark out the center of the receiver ring to accept the front base mount.

Because of the slight differences in the outside sizes of receivers, Looking at a HT without mounts, it is easy to work out the first machining step was to create the flat to work from. Once the flat is at the correct angle and distance from the center, then it is as simple as cutting the half dovetail and drilling from a jig the mounting screw holes.

Attachment 71285
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