What do you use to build your upper?

[Wheelhorse]

I believe that if I had an upper/barrel that wouldn't slip together, I would put the upper in an oven at a low setting for an hour or so while the barrel was in a freezer.
This would heat the upper more evenly than a heat gun or hair dryer.
This is the method I use on chainsaw crankshaft bearings and they fall right in.