Some @^*%^#!#</stong> decided to put calcium carbonate into the ammunition to help absorb any moisture that might leak into the casing. This produced calcium hydroxide powder that accumulated in the rifle, fouling it severely. Calcium salts are NON-volatile. Ammonium chloride might have worked without leaving any residue.

See http://www.jouster.com/articles30m1/M16part2.html from which this quote is taken.

This "stuck case" problem was compounded by the fact that the ball powders being used by Remington (CR8136), Olin and Federal (WC846) were much dirtier burning powders than IMR-4475, and tended to "dirty" the rifle chamber area much quicker than the earlier powder. The dirt that deposited itself in the chamber and feeding areas of the rifle added to the extraction problems – dirty chambers tend to resist extraction to a much greater extent than clean chambers by increasing the coefficient of friction between the case and the chamber walls, thus making the cartridge case more reluctant to leave the chamber. Even dirty chambers can be kept clean with constant care, but unfortunately the dirty powder was aided and abetted by a calcium carbonate deterrent coating applied to the powder that addled to the fouling problem. Alas the problem grew worse.