I ordered 2 of the exact same parts 1081179 for my Baikal and my pal's (we both bought 3.5" 28"-bbl synthetic).
I just got them today, opened mine and screwed it on the end of my original magazine without forcing. Thread is exactly the same, not loose at all. Only thing is that the inner female thread on the extension is 3 millimeters shorter than the male one on the end of the magazine. I think I'll go buy a spacer (1" inner diameter) tomorrow and that will be perfect. I'll take picture later today or tomorrow and show you what it's like.
I will test it tomorrow to see if it feeds correctly (only with 2 shells in the mag as it is restricted by law here, and I don't have yet the proper authorization to use an unrestricted magazine). So yes my extension is all show for the time being...
But it fits and I think it will cycle ok!
(By the way, the minimum length for barrels is written on their website http://www.tacstar.com/faqs.php
. They recommend 26" for the +5 extension. Mine is about 1 inch shorter than the 28" barrel with the provided choke installed)