Excuse my failing to look this up in some worthy tome, but do I need to expand the mouths of my .45-70 cases if loading cast bullets with gas checks?

I expand my 45-70 brass when loading cast bullets checked or not.

Speaking from experience, more than likely and probably.

It is a definite yes; if you don't you will shave lead off the bullet, gas check or not. Good thing is that all straight wall die sets come with an expander that expands, and bells the mouth for this purpose.

I have got so I expand the case mouth for my jacketed stuff too.

Lets make this clear for a new guy who might read this; all dies "expand" the necks of brass. What we are talking about is further expanding the neck for a very short distance, to just over bullet diameter, and/or belling the mouth at the same time. All straight wall case dies are set up for this procedure. It prevents shaving lead, and promotes straight seating/starting. And the Lyman M die will do it for bottlenecks. Doesn't hurt and probably helps to use one on bottle necks unless you only use boat tails.

I use the LEE die, it will work with all calibers. I also bell some case mouths when using certain bullets which have very flat bases with 90 degree sides like the 110 gr. Speer spitzers which I sometimes load in my bolt action Savage .30-30. It prevents crushing case necks.

Hip

Thanks guys, I've got the die but not tried it yet or previously used cast bullets. A kind AR member has given me various gas-checked bullets to try for OAL/throat clearance.

PS: I notice the Lee die instructions say to pour the powder through a funnel and this expander die. What is the point of doing this, considering the acknowledged danger of powder sticking to lubricant in the die?

Also, with new, unfired cases, is lubricant needed just to expand the mouths, considering this powder-sticking issue? If so, and the powder is put straight in, does this mean some lubricant may get pushed inside by the bullet?

The amount of flaring required is only small as you can see on the case mouth second in on the image below. The flare is just enough to hold the cast bullet in place as it is inserted into the shell holder to seat the bullet, second case image from left. Then the bullet seating die smooths the flare out, no crimp needed and no lead or lube shaving.
Cartridges chamber perfectly.

Thanks Eagle, so, with so little flaring needed, is it OK not to lube the mouth?

I've never had to lube the case mouth.

The reason Lee allows you to charge powder in the expanding die is that they expect you to use their case lube, outside and inside, and Lee lube is not petroleum based and therefore will not affect your powder.
Do you need to lube your cases JUST to flare the mouth? No.
Do you need to lube your case neck insides to pull it over an expander button? Yes, usually. Not so much with thin walled straight walled brass. Depends.

First of all make sure your cases are the same length - or you will flare some more than others.

Then set your expander die where it just open the case mouth very slightly.

Try one of your bullets.

If you can seat it, that is it.

No need to go overboard with opening the case mouth.

I have seen some opened so much they actually refuse to go into the seating due!

A Lyman type "M" die expander helps a great deal with minimum flaring and seating your bullets straight.

I normally only expand the case to step B, this allows the bullet to start straight. The .223 expander is a good example of the diameters. And how it can help reduce bullet tilt and reduce runout.

Thanks everyone. Being a bit impatient and having the press out to use, I carefully imposed enough flaring to let half the gas-check into the neck, sans lubricant. Brass is a greasy metal, anyway, so I thought it might lubricate itself that far.