Thanks, Peter.
Attachment 99673
I was wondering if the gaps between the Fazakerley flanged mag housing and the casing was the reason. The Sterling housing doesn’t have them and is clearly better thought-out and made.
On the Sterling made Mk4 there’s a very small indented ring in the casing just forward of the magazine housing (where the barrel flange sits inside). I am guessing the ring was put there after the inside of casing was reamed to size and then reamed to provide a smaller ID for the barrel flange to fit in?
On the Fazakerley made gun the ring is much larger and deeper. Also, it doesn’t go all the way around the casing. There’s a section missing in front of the magazine housing.
Attachment 99674
On the inside you can see the reamer took off considerably less material on the right side, making the ID of the ring offset to left (where the gap in the ring is). The barrel flange touches the right side of the ring and there’s a small space on the left. How did this happen? Was the reamer allowed to take the path of least resistance? In the Sterling made gun the barrel flange is centered in the casing.
Also, only the ring appears to have been reamed on the Fazakerley. The rest of the tube is smooth, with no tool marks. The production methods at Fazakerley must have been quite different than at Sterling.