Does uncleaned brass affect accuracy?

Jim Scott sent the following question:

I have wondered about this for some time and am not a competitive shooter, but like to shoot the smallest groups possible.  Given that brass of any caliber is manufactured to exacting standards, the case capacity should be the same in each one, especially from the same lot.  As a case is fired and burnt powder residue is built up on the inside, I would assume that it would build up in varying amounts from case to case.  Cleaning the cases after each firing and reloading as we have always done makes them nice and clean on the outside, but does little if anything to the insides, using the traditional  tumbling methods with the different types of media such as corn cob and walnut shell medias.  Depending on our loads some cases can be reloaded many times before showing signs of fatigue and we end up tossing them away.  That being the case, wouldn’t the powder charge in each case fired, produce different amounts of pressure, in turn resulting in different velocities?  I have always been of the understanding that today’s smokeless powders burn faster as pressure is increased.  It seems logical to me that a case that has a smaller capacity ( due to more residue on the inside than the next or previous case) would build pressure faster due to the smaller capacity and therefore result in a different velocity than others that possibly don’t have as much residue build up in them.  But then maybe it doesn’t matter at all, and the changes in pressures and velocities would be so small that it would be negligible?  I thought that if anyone would know an answer to this question it would be a highly skilled bench rest shooter, as they strive for the utmost accuracy out of every round they shoot.  I would sure appreciate hearing your thoughts on this very much.  Thanks very much for reading my question, I really appreciate it and look forward to hearing from you.


Jim Scott
Baker City, Oregon

Dear Jim,

In my world I am not concerned with cleaning the inside of the case body. I, and many others, do clean the inside of the neck of the 6PPC case with a 6mm bore brush. At one time we used to polish the inside of the necks but did not find that it helped at all. I feel certain that the changes in temperature and humidity play a far bigger role than the difference in internal volume of the case caused by powder fouling.



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