Kutter/Jim Yes the part in the photo is the one I am trying to use; the magazine tube guide (for lack of a better term) prevents it from turning more than 1/2 turn--as it is now. It seems like part of the rib would have to be removed then reinstalled later(after the barrel ext. is on) for it to make it all the way around--however many turns it would need. Am I looking at it correctly?
Yes, you have it right.
The first standing post (a block actually about 3/4" long) at the breech end of the bbl for the rib attachment is what is blocking the bbl extension from being unscrewed from the bbl itself.
The 1/2 turn is all you can do as the lower portion strikes that block as it comes around.
The rib itself slides off of the post(s) once the cross attachment pin(s) are removed.
But the block and all the other upright posts are silver solder/silver brazed into place.
You can unsolder that offending block. The usual method of putting these in place was to mill a shallow slot for the bloc to sit in in the top of the bbl.
So removing it and then replacing it is generally not difficult as far as proper placement (mark it for forward facing before removal is helpful)
Once off,,the bbl extension can be turned into place and clocked to proper 12 o'clock position.
It's actually better to do all the new refitting before re-soldering the rib block back on just incase things aren't quite right and you need to go back and re-adjust or remove the bbl extension again.
Removing and installing te Bbl Extension is NOT a job for a big cresent wrench.
The part is very fragile especially on the lower area where the op-rod cut is.
Trying to bring it around the last few degrees of turn will in most all instances collapse that magazine tube section if you don't support the Extension w/a proper shaped tool.
There are (or were) factory style wrenches for this when the guns were more popular.
You can make a simple wrench with flat steel, some turned pieces to fit both the chamber and the mag tube ID's.
Here's the one I made. Crude looking but it works and have never ruined an extension.
It's set up for 410 M42 now in the pic.
The handle is a simple piece of flat steel,,probably 18" long.
The working end has the chamber plug. You need to turn plugs of the different gauges you need. They need to slip fit into the chamber of the bbl the extension is being removed/installed on.
Simple threaded stud on the back end and a nut and washe attachment to the handle.
I wrap the chamber plug with a turn of news print to avoid any marks in the chamber
Below the chamber plug is a similar Magazine support plug.
Same idea as the above but this plug fits the gauge specific mag support on the bbl extension.
The slotted handle allows for these different gauge plugs to be adjusted to fit the different size Bbl Extensions.
You can use a lead hammer on the end of the handle if need to loosen or tighten the Extension w/o any fear of twisting or damage to the part.
The magazine plugs are turned for a close fit as well and then need aa little hand file fitting to put some slight 'flats' on the side to fit correctly into the hole.
Set everything up into the Extension after hand turning the Bbl extension onto the bbl.
The Bbl should be secured in a real Bbl Vise w/proper fitting jaws.
I use wood,,I use cast lead. I always use rosin.
It'll take a a few on and off trys to get it to clock up correctly.
I take any matrl off of the front face of the Extension,,a nice broard flat surface and it cuts cleanly with a sharp med or fine cut file.
All this high temp braze work destroys the bluing of course, so polish and reblue is a must.
The work from Simmons and others always came back reblued. Some polishing better than others.
Solid ribs on these bbls are soft soldered on. so Rust blue only for those. But hot salt blue is fine for the VR bbl assemblys.
Factory work was done on parts in the white, so then the Factory polishers went to work and the work was much better.
Factory proofing was done when all work was finished. Proof mark stamped 'thru' the bluing. VR and SR bbl proof mark on the left side of the bbl by the breech unless factory engraved. Then the proof is generally out at the end of any scroll work engraving (if any)done on the bbl'
If the bbl being done is for an Xtra bbl for an existing M12,,then the Original Bbl Chamber Ring mentioned above that is already in the frame should be set for Headspace with that Original Factory barrel as it came from New Haven.
Should check with a gauge anyway.
In fitting up this as an Extra Bbl, you do not want to change that Chamber Ring. Doing that will change the HS on the Original Bbl assembly.
To fit this new Bbl assembly for headspace AND to keep any original Bbl assembly in spec with the Chamber ring that is still in the gun,,,the HS on this new Extra Bbl assembly would need
to be set by reaming the chamber in the bbl itself to a proper HS spec.
The extra bbl assembly should then be ser#'d to the frame. That shows it's fitted and HS to that frame.
That way both Bbl sets will HS properly when checked on the same Frame and the same Chamber Ring in place.
There may be no need to change, adjust for any HS at all. Often the HS will be just fine.
A gauge will tell. These were close fitted parts but they still made allowances for hand fitting so such changes could be made
If this new bbl assembly is simply being fitted to an orpan frame, then HS can be set by adj/reaming/replacement of the Chamber ring or by reaming the bbl chamber back edge itself.
The proper way would be the former as you are working on the replacable part,,the chamber ring.
But lots of things have been done to M12 parts and bbl assemblys over time so you never know what you have till you start to assemble things.
Kind of like '..The transmission was a '53. And the motor turned out to be a '73. And when we tried to put in the bolts all the holes were gone.'