@Fire-Fox, yeah if I have the time, I will try that combination out. I am not sure if I will get to it anytime soon though
I agree with redmoon3580 on explaining why the shims help out. Like he said, I think the bb will push up on the hop-up arm when it begins to move. If the shims lock the arm in place, then the arm will be able to maintain its position in the chamber during the shot. This will do two things, one it will apply more pressure over the bucking than if it were to "give away", and two, it will hold the bb back for a tad bit longer allowing pressure to build up somewhat behind it. If I am right on these two ideas, I think 30 FPS should be about correct. I am sure it will vary a little bit from rifle to rifle though.
You did bring up a good point though. Perhaps the TM chamber does have a better fit around the bucking which allowed the PDI o-ring nozzle to seal more effectively. It could have been that the combination of the TM shells, and shims to hold everything in place, is what helped me realize that 30 FPS boost.
However, I do recall one test that confirms that shims helped in boosting FPS. There was a time when I was running an un-shimmed TM chamber and my modified JG hop-up arm. With maximum hop-up, my muzzle fps would average right at 490. One day I decided the shim the TM chamber and test the rifle to see if consistency improved. Well I took a few shots and the bbs would just sky rocket at maximum hop-up. The shimmed TM chamber and JG arm was over-hopping the bbs like never before. When I chrono'd it, the muzzle FPS was averaging at 521. That experiment is where I remember that the shims added 30 FPS. My local field only permitted 500 muzzle FPS, and even with hop-up on its lowest setting, the bbs would still over-hop. I never chrono'd the rifle at lowest hop-up...I wish I had...but I didnt bother because of the over-hop, the arm had to be modified. I compared the TM arm to the JG arm and noticed that the length of the teeth in the JG arm were a bit longer. I believed that if I reduced the length of the teeth, I could lower the FPS and prevent the rifle from over-hopping. I was correct in this idea and was able to fix the FPS/Hop-up issue by filing down the teeth of the JG arm. However, in an attempt to get the rifle exactly at 500 FPS, I ended up screwing up one of the teeth beyond repair. I resorted to shimming the TM arm and trying that out un-modified. To my luck, the TM arm shot exactly at 500 FPS on max hop-up. You have no idea how happy I was lol.
So I know that there were a lot of variables being changed during these trials and I wish I had recorded all of the data. The thing is, I wasn't testing the shims for FPS specifically at the time; that was a discovery made by accident. Further, I am certain that compression does play a vital role in muzzle FPS. However, at least in the example above, the muzzle compression was the same. I noticed the increased FPS when I shimmed the TM chamber with JG arm. It is that observation that makes me certain that shimming alone can increase FPS.