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I have used lead and cerrosafe several times in the past, but these days I use car body filler---available in small quantities in DIY stores as Plastic Padding. Grease the bore lightly with a patch, stuff some rag into it and then fill up with the Plastic Padding mixture. It should set in less than an hour at room temperature and then can be knocked out with a cleaning rod. The big advantage is that it does not shrink as it cures. Always do the chamber end of the barrel if possible and make sure you get the throat---the first inch or so of the rifling----it is this diameter that you want the bullet to be.
Fred

In the interest of (dromia) Adams blood pressure I thought it was time to stop messing about and do it properly.
This is a chamber throat impact cast of my AIA 7.62x39. I have measured it up and it makes sense but I'm going to do three more and take a average.
If any one is interested I can post pictures and describe how it was done. Before I started I read a description of how to do it on the NOE bullet makers forum as well as the one in the library.
I used this home made .310 mould to cast plug and four fired unsized annealed cases. They were previously used to shoot cast boolits.

The mould worked Ok it was found that trying to drill a hole that was exactly .310 did not really work. So the mould was bored to size using a boring tool cutting the flat bottom was the awkward bit. What I found when casting was the mould dropped boolits that were oversize using pure lead. I ended up turning the lead castings to the right size on the lathe. I have recorded the settings I used so if I want to do it again I would just turn the lead down on the lathe rather than trying to cast it. Much easier.
Working out the right length of lead to use so it does not get stuck in the bore needs to be considered as well. All in all it did not turn out to be a big deal after all. I would happily do it again for any of my rifles. Only difference is I would turn the lead slug to size rather than cast it. Hmm so whats next. Oh yeah barrel spider.

why are you trying to do it to an exact size? isnt the point of thumping it, to make it compress into the space? so the tighter it is, the more likely it is to lock up in the barrel when you start thumping, and not the chamber as is the idea?

also, why does the mould need a flat bottom?

Just for those of you that want a simple way, there is absolutely no need to faff about making moulds and turning lead slugs down on lathes.

A cast boolit will do fine so long as it is cast from pure lead also as the previous poster has said diameter and shape isn't too critical either, so long as you have enough lead to fill the space when pounded down.

So what were the measurements from the impact cast?

The previous posters are absolutally correct a boolit mould and a boolit cast of soft lead will work fine. Providing you have a boolit mould of the correct size. I did not I know my bore is .312 so I Made a mould in .310 unfortunately they came out over size and would not go into the bore so I stuck them in the lathe easy easy and made them the correct size. The mould does not have to have a flat bottom.
Anyway job done.

So what were the measurements from the impact cast?

This is the first time I have done this so its a bit of a grey area to me. I would like to do a couple more and compare the data.
But my first stab at it using a micrometer the bit in front of the case mouth measures .333 the centre of the cast measures .315 the bit were it engages the rifling and there are rifling grooves on the slug measures .313/.314. Don't know if that's typical or not.

I'd be looking at a .315" on the driving bands boolit for that rifle then long enough for the driving section to have enough in the case mouth (say a calibe .30"-.315") and enough to touch the freebore at the .315" point.