Expander mandrel scoring

[Ned Ludd]

It's difficult to tell from the picture how deep those grooves actually are. They appear to be pretty deep, but that may be misleading, because the [darker] oxidation layer might be only a few molecules thick. Mine don't look all that different from yours. You can't fit a finger in there to try and "feel" how deep they are, but you might try bending a short right angle with needle nose pliers at the end of a straightened paperclip (or similar) that you can gently run around the inside to determine whether or not the scratches are really deep enough to feel. If so, as suggested by others you can use very fine steel wool to smooth out any really rough or uneven spots on the mandrel, but I wouldn't polish it to a mirror smooth surface because a very slight unevenness in the surface helps retain lube on mandrel. Also, the fact that this shows up as lines/grooves inside the neck suggests the inside surface of the neck may uneven, as opposed to the mandrel itself, which has been turned to diameter. The other alternative, also mentioned previously, would be to try a carbide mandrel.

 

[Slim13]

Were those brand new unfired cases?

Since new case mouths will be irregular and out of round (on regular bulk brass), you might try partially expanding them first, just enough to round out the mouths sufficient to then chamfer the inside of the mouths, minimally, using a Wilson or similar tool. Then spin the mouths against some 0000 steel wool, to smooth them out. Now when you expand them in earnest, there will less chance of galling, and the mandrel will run in much more smoothly.
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Yes, those are (were? lol) brand spanking new. The necks were crazy tight so I ran the mandrel before loading. Will try the brush/steel wool/drill. Thanks!

 

[Killick]

Hmm...I didn't see this mentioned so in the interest of completeness I'll offer my method. I don't bother lubing the necks but I will lube the mandrel with Imperial Sizing Wax every now and then if it starts to feel stiff and squeaks (never had brass build up). Then it's "Turn and Burn" (turn and anneal), prime, charge and cap...Bob's yer uncle.