Hello, and thanks:
Without a tight bullet fit in the case, OAL can shorten during cycling.
Bullets should not be twistable in the case.
If so, check for a sized bullet OD that is not the same as case mouth's wall ID.
Often case mouths are expanded too much, and too deeply. Then even with a slight amount of crimp, the bullet still spins and sets rearward during cycling. Measuring a previously cycled, loose bullet, uncrimped round often presents a shortened OAL. Softer alloy bullets distort easily, and are more likely to be loose.
If the sized bullet OD is more than .001" larger than the case mouth ID, seating the over size bullet in the tight case mouth often "resizes" the sized OD below optimum.
Years ago, seasoned Spencer skirmishers said a SLIGHT amount of crimp helps feeding, but too much crimp can raise a doughnut that reduces bullet grip. Tremendous trifles.
Smiles,
El Supremo/Kevin Tinny