OK, so the 3rd try with thread lock was successful, and I'll be careful not to screw the shroud on very tight until I take the extension off and cut a screwdriver slot. I went ahead and put the 700mm liner in, and did a very brief test with NSA 30gr .217 slugs, and JSB 25.36 monster redesigned pellets. At first glance (without trying any different settings), I can't see any significant improvement over the 600mm liner results. I'm getting about an inch groups at 70 yds. It was interesting to note that the JSB was nearly all left to right, where the NSA slugs were spread evenly up/down and left/right. There was some wind, but not much. I didn't have time to chrono them, but I'm sure they're flying a bit faster.
Now a sanity check request. If you have a 600mm barrel with the stock FX suppressor, can you measure the suppressor for me? I swear I remember taking off one or two sections of this to make it fit the case, but now I can't find any extra sections. Mine is 125mm, made up of 3 segments. The center part is about 75mm, and the end caps are 25mm each. I'm nearly certain there was one or two more sections, but I can't imagine where they would be hiding.
Thanks,
Rusty