It depends on the upper & barrel, BCM uppers are a pain in the ass to get barrels to seat into.
I’ve done pretty much every sketchy process building ARs that you can find videos of on the web, a few you probably won’t find on there as well lol!
Now that I have the proper tools I will do 2 things to get the barrel to seat into “snug” upper, you can stick the whole barrel into the freezer over night and then before you get the barrel out of the freezer use a hair dryer to heat up the threaded area of the upper, a proper harbor freight heat gun is much faster though, I would not suggest using a propane torch because you can get things too hot and bad stuff “can” happen, but I’m sure someone here has gone that route.
Once you have the area where the barrel seats heated up go grab the barrel and then before you seat everything go over it with the heat for a couple of minutes and then seat everything, the barrel will hold the “cold” much longer than the upper will hold the heat so having the barrel out of the freezer for a few minutes won’t bother anything.
If you do not have a chest freezer large enough to stick the barrel in you can do the following.
Get a ziplock bag or something similar and cover the barrel extension, it doesn’t have to be super tight but you want things to be fairly water proof (ish).
Get a large bowel or a bucket and put a layer of ice on the bottom and then spread salt (ice cream salt is best but table salt works fine), stick the plastic covered barrel extension into the bucket and continue to layer ice & Salt enough to fully cover the barrel extension, the ice and water won’t hurt a thing but adding the salt will make things get much colder, much faster and as we all know salt water is bad for most metals and finishes, just keep that in mind when you’re doing everything and do your best to keep the salty ice/water off the metal, a few drops won’t be a disaster but you have to clean it off well and neutralize the corrosive properties of the salt before you seat the barrel, having it sit between the upper & barrel extension is no bueno, it normally takes 10-15 minutes for the steel to get cold enough to contract a micron or two, the upper will expand a little more than that so “most” of the time just applying heat to the upper will do the trick, if not use the heat & freezer/ice trick and you’re golden.
hope that makes sense and it helps.