The largest contributing factor to BUE (built-up edge) on reamers and other cutters is speed. When speeds are too high, BUE forms, heat breaks down the hardness of the cutting edge, etc. When I'm teaching machining, I harp on RPM/speed being too high as being THE primary cause of cutting tool breakdown, especially with HSS tooling.
There are other contributing factors to BUE, especially if we talk about materials other than 4140 or 4xx stainless steels, but speed is the largest factor in barrel steel machining for BUE.
The rule I was taught by senior machinists about reamers (in general, not chamber reamers specifically) is "half the speed, twice the feed" - ie, put the reamer under load, but then halve the RPM from what you'd use for another cutting tool, such as a drill bit.
When I chamber, I'm typically turning at about 90 RPM, using high-sulphur lube and clearing chips every 0.050" to 0.100" of depth.
Rubbing copper on the reamer, and pennies: The year that we changed from actual copper in pennies to copper-flashed zinc was 1982, I think. If you find pennies from before 1982, you've got an actual copper penny. Hang on to them - they're typically worth more than a $0.01 in melt value now.