Why .001" over barrel diameter for LSWC bullet?

Why does a LSWC bullet have to be at least .001" over the barrel diameter, especially when a JHP bullet seemingly performs well .001" - .002" less than the barrel diameter?

I'm not sure that's true...

Typically a 45ACP barrel is a diameter of .450,

Typically a jacked 45 bullet is a diameter of .451,

I size my cast 45 bullets at .452 using the advice I was given by an old bullseye shooter who taught me how to cast bullets.

that's more like it.

Hard cast is tough. Still not as tough as copper. Extra size gives better accuracy since 45 barrel's have very shallow LANs.

An undersize lead bullet will let powder gas blow by and strip lead off the bullet.  A lot of leading issues are the result of too small bullets.  Going .001 over bore ensures this doesn't happen.  The pressure on firing will general bump a bullet up in diameter enough to fix a lot of this, but the harder the bullet the more bump it need if it has to expand much. Hard cast bullets in light target loads are less forgiving of this than softer bullets, but it is an issue with all bullets.

Thanks all.  This is still very strange.  It makes sense that a lead bullet should be sized a little larger than the barrel diameter but with .45 ACP (at least my 1911s) the cartridge still chambers with the case mouth up against the inside lip of the barrel where it's supposed to stop.  That's not the situation with my Pardini .32 ACP.  The shoulder or forward lip of the LSWC bullet base diameter stops the cartridge from chambering in the same way.  Adding to the peculiarity and inconsistency, the Hornady XTP .32 ACP bullet delivers the best performance (at least for me) yet it is less in diameter than the Pardini barrel.

Oal to the shoulder of the bullet?

jmdavis wrote:Oal to the shoulder of the bullet?

I don't understand the question.

Could the shoulder of the bullet be preventing the round from chambering completely?

jmdavis wrote:Could the shoulder of the bullet be preventing the round from chambering completely?

Yes, exactly.

So a bit more seating depth might resolve it?

jmdavis wrote:So a bit more seating depth might resolve it?

Yes but the shoulder of the bullet would be at the mouth of the case.  And then the COAL would be very short.  Or a smaller diameter bullet like the Hornady XTP would resolve it.

Could it be that you are crimping the lead bullet to much and extruding the shoulder to larger than it should be?

If the crimp is to tight the lead will move out of the way. The path of least resistance would be the shoulder area above the case mouth.

I'm sure you seat and crimp in separate stages but this is just a thought. I have had friends do this very thing with the 45 ACP and had the slide hang up about 1/16 of an inch from going into battery.

Or the barrel has a very short leade or no leade which will not allow the shoulder to enter because the rifling is right at the end of the chamber.


Virgil

The problem with the smaller diameter though might be the accuracy. Do you have Oldcaster's data for his COL?

Virgil,
I went back and checked the crimp again and just removing the flair the cartridge still bottoms out on the lead bullet shoulder.  Thanks for the idea though.

It isn't something exclusive to .45 or L-SWCs--for me, it is universal.
Mike Venturino (sp?) gets OK results with cast bullets at groove diameter using type metal. He certainly knows a LOT more than I do about casting, but I can't get near his results with groove diameter lead bullets.
My bullets have to be at least 0.001" larger than the actual groove diameter.
In general, to prevent leading and get decent accuracy, modern commercial cast bullets that run 18-22 BHN need to be at least 0.002" over actual groove diameter.
SAAMI specifies a 0.4520" nominal jacketed bullet and a 0.4530" lead bullet for .45 Match ammo.
Even the swaged bullets I have tried are now using a 92-6-2 alloy and are harder than they should be.
Why? Just the way life has been for almost 100 years now.
COL: I load so the shoulder of the L-SWC (whether 9x19, 9x21, .38 Super, .40S&W, 10mm Auto, .45 Auto, or .45 Win Mag) just touches the lede/rifling or is no more than 0.002" off. I also like a seating stem that ONLY touches the shoulder of the bullet.