Lapping

[tommag]

2+3 doesn't equal 4, evidently.
I like the BCM upers because they're machined slightly undersized, so you have to thermo fit them to the barrel. I dug out my lapping tool and discovered it would be an interference fit in a BCM. Thought about warming th BCM but I'd have to keep it warm or the lapping tool would seize in there and keeping the heat on would also enlarge the tool.

While I'm only putting a Faxon barrel in it, I do like to make everything fit as precisely as possible even if it's not a lilja or bartlien.

[MontanaLon]

The expansion rates differ between aluminum and steel are enough that even when you thermo fit them, you can get them apart by applying heat to the aluminum receiver alone or even heating the whole thing together. The expansion rate of aluminum is about double that of steel. Assuming a 0.002 inch interference fit you would need to raise the temperature of the assembly about 310 degrees fahrenheit for them to pull apart easily.

Put on an oven mitt heat the receiver to the temp needed to accept the barrel and lap it quickly. If you do much lapping you are going to lose the tight fit though. 220 grit lapping compound measures about 0.0025 of an inch. The total press fit on the BCM uppers is between 1 and 2 thousandths. So, the grit may just remove any of the press fit you started with.