Post,
Rick answered the question in detail. A good example is a well used M1 Garand. To clean from the breech you have to do a total disassemble. The issued cleaning rod is some sort of steel. The crown and grooves are usually dinged up over the years. Any abnormality in that area acts like a speed bump to the bullet.
BTW a good rod guide for an M1 is a empty 12GA shotgun shell with the spent primer removed.
FN
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