Heating the bullets to achive better coating has not proven to work (unless for cleaning to expand the jacket metal to better remove lubricating residu from forming an drawing), for better or vorse. That is unless your, media, tumbling container and the room temperature be close to 100-150 degrees or more.

A word of caution on selection & use of a resealable container – make sure it’s virtually unbreakable and that you’re confident the lid won’t come off during tumbling. (Go ahead…ask me how I learned this!)

I use well-cleaned peanut butter jars with screw-on lids secured by gaffer tape.

Noting myself that heat helps bullet coating, when I was coating a LOT of bullets a few years ago during the winter, I built a device out of a steel garbage can & hair dryer that would allow my tumbler to reach quite warm temps for the duration of the process. Adding an electric baseboard thermostat allowed fairly precise high end temp limit; I think a conventional gas furnace fan thermostat (NOT what goes on your house walls!) would also work if rated for 110 VAC.

This also served me well for the few times I went to the range in the winter, as long as power were available. I could keep my ammo at summer temps for reliable testing in the dead (here that can be anywhere from 32 to -20 F) of winter.

The individual grains of powder are coated. This coating (and kernel size) controls burning rate. Tumbling may change that. Read a can of powder, it says do not shake (for that reason).

Note: RCBS specifically warns against tumbling live ammo.

Here is a discussion from http://www.frfrogspad.com/miscellc.htm#tumbling :

Q. Is tumbling loaded ammunition dangerous?
A. Short term tumbling will have no real affect on loaded ammunition, but extensive tumbling can cause the breakdown of the powder grains. This would have two major effects. First, smaller grains will ignite more quickly than larger grains, and second the deterrent coating on the outside of the grains may be rubbed off and will be absent from any fractured edges which will cause the powder to burn more quickly raising pressures.

Tests run some years ago by a commercial entity did indicate that potentially dangerous changes in powder charge burning characteristics do take place after PROLONGED periods in either a vibratory or a tumbling cleaner.

The key word here is prolonged. Many manufacturers of ammunition do a final cleaning of their product either by tumbling or a vibratory process before boxing it for shipment. In no case is this allowed to exceed more than just a couple of minutes. The intent is not so much to “polish” but to remove any traces of contaminants which might in time leave marks on the finished product. There seems to be a consensus among the ammunition manufacturing engineers that a minute or two of vibratory cleaning has no discernable effect on burning rates, especially for loads that are compressed, or nearly so. However, all have emphasized the need for EXTREME CAUTION not to overdo the process.

They also pointed out that there is a considerable difference in effect on the powder charge depending on whether the process is by “tumbling” or “vibrating.” It would appear that tumbling has less effect on the powder than vibrating, though this is mostly a matter of degree. The admonition to use EXTREME CAUTION to insure that the process never exceeds a couple of minutes applies equally to either process.