Lead alloy behavior when cooling

[handyrandyrc]

I was wondering if you fellas could help out. I've spent an hour searching topics to find what i want, but can't seem to dig it up...

When wheel weight cools and ages, I understand it hardens somewhat, over a period of days/weeks/etc., due to antimony content. My BIG question is this: Does it increase slightly in diameter during that time? Stay the same? Decrease in diameter?

I am going to check it myself to see what it does, but I wanted to see what result I should expect. I have some rifle bullets I just cast, and quenched half and air-cooled half. The quenched half measure about a thousandth larger in diameter than the air-cooled half. That measurement is only 5 ten-thousandths larger than bore size for my rifle. I understand 1-2 thousandths larger is what a person is trying to get for a proper "fit" in the rifle.

Now I'm wondering if I let these sit, whether the diameter will change at all over time.

[leftiye]

IF you've got a freebore area in your rifle's chamber, then bullets can/should fit that as tight as possible (without being pressed in, and/or sticking). Don't force them into the rifling either.

Alternatively, if no freebore -then the boolits 1/2 thou over groove dia. will probably work well UNSIZED if they are hard enough that your pressures don't obturate them (reads bulge out in the middle) before pushing them into the rifling. Lower pressures for softer boolits if not supported.

One might go (err) on the safe side of this pressure/ hardness tradeoff as a principle as long as obturation isn't needed to acquire a gas seal of the bullet in the bore.

[handyrandyrc]

And freebore would be defined as the area between the case mouth and where the rifling begins? I have probably .055" of space in the barrel before there is good rifling. I assume this is the freebore?

This is a Mosin-Nagant 91/30 rifle. I did a cerro-cast of the bore, and my measurements in one hour were .3135" grooves and .303" lands. My quenched bullets came out of the mould at .314" -- the air-cooled ones measured .313".

[44man]

Depending on your alloy and also the temperature cast at, yes, boolits will increase in diameter. Just let then age after casting. I gain .002" on my .475 boolits but this will change with the size of the boolits.

[Dale53]

44man;
As I understand it, most of the hardening AND size change takes place within two weeks or so (bullets get harder and bigger).

Dale53

[leftiye]

Handy, Yes, I think I understand your description of freebore correctly, and it is the same as I use. Freebore is the area of the rifle bore behind the rearward end of the rifling, and in front of the case mouth.

If you only have about .050" of freebore then most of the boolit becomes unsupported immediately when ignition occurs. The gasses (and maybe even some powder) are pushed between the boolit and the case neck leaving the boolit by itself in mid air. In most chambers this isn't a big deal because there are only a couple of thousandths of an inch clearance between the cartridge and the chamber neck prior to firing, and the case neck can't really go anywhere, and the boolit stays straight on with the bore whilst being pushed into the rifling. You need to calculate the clearance between your ctg neck, and the chamber neck.

You can see what a loose chamber neck will cause in this scenario. That's probably why boolits with bore riding nose sections are sometimes more accurate- they align the boolit with the bore for engraving regardless of these considerations. Also, if your boolits collapse due to high pressures you have no guarantee what their resulting shape nor balance will be. They will have to be hard enough that the part of the boolit behind the leade can't deform. Otherwise accuracy is an even more chancy thing.

As a final parting shot, do whatever you can to insure the coaxial alignment of the boolit in the leade when the cartridge is chambered.