Call me CooCoo but Im confused - Sizing Up

[Sloffie]

I have read on many occasions that people say things like:
"I have a Lee mold which casts .312 bullets and I size them then to .313..."

How is that possible? Is it the alloy that causes a bullet to come out larger than .312 or what?

Might be a very newbie question, but I am very confused. As far as I am aware you can never make something bigger, only smaller.

[Bret4207]

I think they are referring to the mould designation, a Lee 309-170 for instance, that actually drops boolits at .312. They then size to .311. Remember that just because a mould is designated 358 or 452 or 258, that doesn't mean it will drop them at that diameter. They can be under or over by 3-4 thousandths and still be within "nominal tolerance".

Part 2 of the answer is- yes, you can make them bigger by "bumping" them. Basically it's using a lubrisizer to swage them out fatter by applying pressure. It's not a very good answer to the problem, but it can work.

[btroj]

Marry your bullets, it will then put on diameter.

Bret had the right answer. A mould frequently casts a bullet larger than the stated size.

[runfiverun]

they run them through the bigger sizer just to lube them without making them smaller or larger especially if they waterdrop them.

[Sloffie]

AHA! That makes sense. I also received an email from LEE saying the same about their molds having a tolerance. Thanks. That clears it up.

[GabbyM]

Once upon a time when sizing bullets for a 30-30. Bullets were between .3090" and .3095".
ran them through a .310" die seating gas checks. when loaded the checks which were sized at .310" expanded the thin 30-30 case necks and the smaller bullets were loose in the necks. Being only held by the neck pressure on the checks. didn't like wobbly bullets so I ran them all through a .309" sizer and went with that.

[Char-Gar]

Lee has some tolerances on their molds that are real scary at times.

[MT Chambers]

The real good mold makers will consistently be slightly oversize to allow for different dia. sizing, other makers will be all over the map. For me a small mold is usually useless..