For the record of the 2 BCM demo uppers I own, one is a true thermofit while the other one is barely a thermofit (I can almost slide the barrel in by hand except for the last inch or so).
Yeah, but, as the years have been rolling by since I started messing with ARs, I've been all over the place, in my head, about what really matters when assembling an upper.
I really can't say how many uppers I've put together, or stripped and reassembled with different parts. These days, I do prefer a thermal fit, because logic dictates that tight is better than loose. Maybe its just my OCD that makes me think that way.
If I'm assembling an upper and want some sort of real accuracy out of it I will use an upper that has at least a tight slip fit. If the fit is sloppy enough to allow the use of shim material between the extension and the upper, I'll relegate that upper receiver to a red dot only rig. Again, likely due to my OCD.
On uppers that are a thermal fit or a tight slip fit, I'll bed the extension with loctite 620 and call it a day.
I did make an observation that led me to question the insistence on a thermal fit. I, at one time, had a need for 2 11.5 rigs. I put them together using the same type of barrel. One receiver was a good thermal fit, so I used that one for my LPVO setup. The other was kind of on the loose side, so I just did the loctite thing.
They shot similarly to each other, but I decided to tear em down to turn the thermal fit upper into something akin to a short spr with a more accurate barrel and the other, with the more loose fit, into a lightweight setup. What struck me was that disassembling the one with the loose fit, that was bedded with loctite, was just as difficult to get apart as the thermal fit setup. The barrel did not just slide out. I had to heat it, as I did with the thermal fit setup.
Another observation I made was that, on the looser fitting upper, the loctite seemed to be evenly distributed all the way around the extension. This kind of leads me to believe that, at least to a point, tightening the barrel nut to torque spec compressed things such that every thing centers itself.
There's really no moral to the story. Just a couple of observations that made me think and consider a couple of things.
What I question now is how tight can a thermal fit be before it should be considered too tight. That was the problem I had with at least one of the BCM uppers that I got. It was so tight that I couldn't true the receiver with neither of two tools. When I went to fit the barrel to one particular upper, I had a time getting it in.
