Semi-Automatic Sterling questions

The return spring cup used in the Mk6 and Mk7C is slightly longer than the one used in the Mk4/L2A3. Obviously there's a reason for this or they would have just used the Mk4/L2A3 return spring cup. Can anyone tell me what the reason is?

Next question: Anyone know where I can find one of these semi-automatic return springs cups? I ordered one, but when it came it was for a Mk4/L2A3. There are plenty of those available.

One more question: Do all semi-automatic Sterlings use the same internal parts, breech block, firing pin, firing pin spring retainer, springs, etc?

I read through the "Closing the Bolt" chapter of the GOD book. Great book BTW.

I'm not sure that there IS an obvious clear reason for using a different return spring cup unless the diameter of the inner return spring on the Mk6 is a different diameter. There are MANY variables of these semi guns......... Some changes were made simply in order to satisfy the local authorities, such as the BATF that the internal parts weren't interchangeable.

Is the 'longer' return spring cup longer externally AND internally or just externally but spaced internally? Why don't you just try the standard L2A3 cup, return spring and see if it operates?

As I said earlier, there were many variables in these Mk6-on guns for several reasons so the answer is that as a general rule, the parts are interchangeable but on some guns, certain features, such as the mechanical safety plate were omitted and on others the trigger mechanism blocking plate (that prevents the insertion of a rock and roll trigger assembly) differed

Attachment 34671

The 'longer' semi-automatic return spring cup is both longer externally and internally.

Attachment 34672

I have tried the L2A3 cup. (It's the one in the middle. Top is Mk6 and bottom is L2A3.) It does work. Doesn't feel like the factory semi-automatic.

I tried shortening the large return spring. It didn't help. Made it feel even more choppy.

Looks like I will have to fabricate the longer cup.

I have an unrelated question about firing pins. Was there ever an issue with the tip of the firing pin breaking off?