You're saying when you insert (from the top) the Hahn block it's tight? Have you tried loosening the roller  (not sure what they call it) w/the Allen wrench that came w/the Hahn block?

Maybe I'm an idiot, I just poke around the 9mm forum because it interests me but....

Arent Hahns for COLT systems, not OLY's?  
Also, arent they for use with modified UZI mags, not STEN?

Good luck either way bud.

My bad, I thought it was the Colt type.

Are you seating the mags empty or with rounds in them?  What you're describing seems like the block is seating the mag high, at the extreme end of tolerance and it is edging up right against the bolt/carrier, which in turns pushes it down snug against the release lever.  To test this, see if it seats easily when the upper is on and the bolt is locked back.  And when you release the bolt, does it slide easily over the mag or does it scrape against it?

Also, you might try with different sten mags to see if the tightness is the same as there is often variation between mags.  For example, I have some stens that have trouble seating completely in my Bushy magwell due to the overinsertion tabs being in slightly different positions.

I would rather have it seat too high than too low as it seems the release would wear in a little.  However, if it is consistently so tight that you can't pull back the charging handle and chamber a round, then you may have to look into filing down the catch just a hair (although this would be my last choice).  Edited to add that I agree with Dehammer's idea that you should file on the $5 sten mag before messing with the mag block.

Quoted: www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=21676&title=AR-15%2fCAV-15+MAGAZINE+ADAPTERS

Sten mags vary quite a bit.......modify your mags if necessary.......the mag catch is in the right position.......it is NOT too high........there is a difference between having the correct feeding position (lips, bolt, chamber) and just having the mag 'fit'.

Most Sten mags work pretty well.....some are junk......check out the Hahn Precision website for details on the adapter and Sten mags.