Hello ben.
Allow me some miscellaneous quotes from
http://www.shootingsoftware.com/loadens about LOAD density.
"The percent of the available powder space under the bullet, filled with (the load of) powder, is called the "load density".
"A powder charge that completely fills the space under the bullet is often more accurate."
"..filling the case completely (to the bullet base) may hold the point of peak pressure nearer the chamber for a complete burn..and better accuracy. This is why some reloaders prefer slight compression loads."
Ben, you want to fire-form cases in conjunction with running in a new barrel. Some say that a barrel does not "settle" inside 300 rounds. Many will dispute that. In any event, a barrel should be "run in" after 10 shots. Never waste a new barrel on fire-forming.
You have a new barrel because you have finally retired your old favourite "Harry". Screw in the Harry and have some fun. After all, you know it is a good, but tired barrel. Good enough for Saturday and the occasional PM. Take, say, 280 off your new barrel and you may miss the best 280 of its life.
Let's say you go with good old Harry. You can down load and blasé away at about 50 a time until your brass is formed. Believe me when I say that some shooters PAY others to do this.
Bit tired, talk later.
Regards,
John Tracey.