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I normally shoot my 7.62x39 with jacketed bullets in Lapau Brass. I neck size them using a LeWilson die. I want to shoot some reduced cast loads indoors at 25 yards. I have done this before with this rifle but I normally full length resize and flare the case mouth with a Lyman M die.
I was wondering what if I just open the fired case neck with the Lyman M die and seat the bullet with out resizing the case. Any thoughts any one.

How much smaller is the inside diameter of the fired case than the diameter of the boolit?

I would be surprised if it was smaller, most boolits/bullets will just drop into a fired case so usually need sizing.

You on the whisky again Dave?

I am out of Whisky at the moment Adam. Anyway I wonder if it's because these cases have been used with cast loads before but no the boolit does not drop in the case they seat to the first lube band and I can not push them in by hand hence my question.

I'd still neck size Dave so that you have some decent neck tension.

In these larger cases with small amounts of fast powder you want to ensure that the primer flash finds the powder before the boolit starts moving so as to ensure a good consistent burn.

In smaller case like the 310 cadet with better case fill this isn't an issue and immediate powder ignition is more certain so you can get away with hand seating. Not something I'd do with the larger cases/small amounts of fast powder as it could lead to erratic velocities.

I will neck size them then expand the neck to be on the safe side.