I think I have this right. If so then...

[SPRINGFIELDM141972]

I think I understand that the ideal chamber throat should be the exact diameter as the groove diameter of the bore. Correct?

1. If so why would we want to oversize the boolit driving bands by .001 or .002"? Does'nt this just increase the chamber pressures needlessly.

2. If we should seat the boolet so that the first driving band is engraved by the rifling doesn't this in effect do the same as #1.

I don't intend to start any arguments, I just would like some elementary education on the subject.

Kind Regards,
Everett

[felix]

Everett, you are indeed correct. Problem: The barrel is not made at the same time (person) as the cartridge (projectile especially). Therefore, we must compensate for the dimensional differences between components. We never want the projectile to be smaller than the barrel, and so we must allow the projectile to be larger to take up any possible slack. Thus we have an angled leade-in. That leade-in, plus any freebore, must be commensurate with the cartridge case holding the projectile. Ideally, there should be no cartridge case, and that is why the BR crowd does their best to eliminate that variable. Witness breech seating, naval cannons without a case, and a compromise where the cartridge neck is made as thin as is practical. ... felix

[44man]

Rarely does the throat come out the same size as the groove to groove since it is done last. It is mostly larger.
A cast boolit a little larger will not raise pressures much at all, if any. Nothing to worry about.
You can only engrave so much and if the back of the boolit is in a larger throat, there is nothing to keep it on center.
Felix is right and most brass lays in the bottom of the chamber, not floating in the center. You can even see this with fired brass that is expanded more on one side then the other. Even some ejectors on bolt guns can hold a case off center with spring pressure.
The reason I get such good accuracy from my revolvers is the boolit fit in the throats and by neck sizing so the back of the brass is a tight fit in the chamber. That holds everything on center.
Everything you do to keep the whole mess in perfect alignment with the bore will make the gun more accurate.

[Pilgrim]

Cast boolits engrave so much easier than jacketed bullets that there is no comnparison (appropriate comparison anyway) between them. You won't even see a blip in pressure due to the over sized boolit, and I doubt you could measure it even if you wanted to, unless you have on hand some very spendy high speed transducers, recorders, etc. Seating the cast boolit into the lands also doesn't really increase pressures for the same reason. It might actually reduce pressures as compared to a boolit started off center that then has to be swaged to fit the throat/barrel. Seating the boolit into the lands gives you the best chance of starting the boolit in axial agreement with the bore, which then gives you the best chance at obtaining good accuracy out the rifle/case/boolit combination. Pilgrim