Sizing boolits for my .260 Remington

[selmerfan]

I will soon be receiving a 130 gr. .265" GC NEI mold. My question is, what measurement from my barrel do I use to determine my sizing die for this boolit? Do I take a throat measurement and use that, or do I use the bore measurement? I've never run cast boolits through a rifle before, so if these questions seem simple, it's probably because they are. :)
Thanks for the help.
Selmerfan
If it makes a difference, the gun is a .260 Remington M700 Mountain Rifle.

[ktw]

Hard to tell what is going to work best in your rifle, but the best guess is that it will be something between groove diameter and throat diameter.

If you have a short bullet, or a bore-rider that fits your bore well, sizing to throat is likely to be a good bet. If you have a longer loverin design that needs to be seated well out into the rifling you may have to size smaller in order to get it to chamber.

I like to buy a Lyman/RCBS sizer die in the largest diameter I think I might need; something close to as cast bullet diameter, and use it for lubing and seating gas checks. I also have one or more of the Lee nose first sizers in smaller sizes for reducing the diameter of one of these lubed boolits further for a particular rifle.

-ktw

[selmerfan]

ktw, I hadn't even considered combining the two sizing methods. Great idea! I'll try to get measurements on my rifle this week and figure it out. Thanks for the help.
Selmerfan

[Alangaq]

I usually take a pure lead musket ball and pound it into the muzzle end of the barrel with a wooden mallet then knock it out with a cleaning rod and measure it with a dial caliper. I then size .001 over that measurement. Obviously a musket ball is going to be much bigger diameter than your 260 bore, but the excess lead will just smoosh out of the way when you pound on it with the mallet. I also have a 260 but have not shot cast bullets out of it. Be sure to post your results, I will be interested to hear how it does.

[Larry Gibson]

Measure the throat and the leade.

Ideally the bullet should be of throat diameter or not more than .001" less. This will allow the bullet to enter the throat without being shoved back into the case on closing the bolt. The bullet must fit the throat best or alignment into the bore will not be concentic. Throats are almost always equal to or larger than groove depth. You want the largest bullet diameter possible that will chamber so the bullet is swaged down to the bore/groove depth instead of depending on 'bumping up" to fill the groove depth of the bore. This also negates gas blow and gas cutting reducing the possibility of leading.

Larry Gibson