Did you use a lighter spring too? Maybe a reduced diameter bolt/tube sleeve with a light spring would work?Well I found this thread on a search for something else.
I did a lot of prototype work on a .22 STEN conversion, actually had one firing.
Still have some of the pieces laying around.
Forget using a STEN bolt, what more or less worked was a full length aluminum sleeve, new bolt and barrel.
The hard part was getting the bolt light enough to travel far enough back to use the original sear.
Wound up using a plastic bolt body and a steel bolt face.
What really killed it was the 9mm ejector getting in the way.
The Sterling ejector is easily removed so more practical to convert.
Got busy with other projects and never got back to the .22 conversions.
I may eventually just use the parts I have to make a dedicated reduced diameter receiver .22 STEN.
Richard @ BWE Firearms said that he started to work on a .22lr conversion for the Sterling but is waiting on getting caught up with his other work before he starts pursuing it further.