Chamber and throat sizing?

[Canuck Bob]

I'm sizing so the bullet fits the throat. If the throat is say .311 then the bullet should be .311 or a hair under. Sometimes folks talk about sizing a rifle so that bullet fills the chamber neck with a little room of say .002 for the brass to avoid a pressure spike. This suggests the sized portion of the bullet ends at the chamber face because in my slugged guns the throat is smaller if this were done. It also suggests some swaging action in the throat area as soon as the bullet moves. To me this would a positive for a sealed throat against the early pressure.

I am sticking with throat sizing for my rifles but was hoping to understand better this whole bullet sizing science. thanks.

[blackthorn]

Ideal bullet--0.002 under maximum allowable chamber size.
Ideal throat size--slightly under ideal bullet size--transitioning to slghtly smaller
Ideal bore (read maximum inside barrel measurement) size.

Start with a chamber cast or impact slug and work from that.

[Char-Gar]

Bob..You are spot on in your approach to sizing for the throat. I am lazy and don't feel the need to make a chamber cast or throat slug.

I have a series of 30 caliber sizing dies. I make up some dummy rounds with the bullet in question sized. 309, 310, 311 and .312. I try them one at a time and select the one that shows some light contact marks with the throat without removing any metal.

If that dummy round shows contact marks with the chamber neck (which is very seldom) I will try another make or brass or neck turn the brass a couple of thousands to give a little room for the case neck to expand and turn loose of the bullet.

My way is not the best, or better, just the way I do things and it seems to work for me. I trust a real bullet and a real case in the real rifle better than I trust digits on a scale.

A little Dy-Kem (machinist lay out fluid) on the case neck, shoulder and bullet will help in seeing what is happenings. The dummy round will pick up some marks as it chambers, but as the bolt turns into battery the marks will be around the circumference of the case neck or bullet. That is what you are looking for.

[BABore]

The idea boolit fit.

- A slip fit into the case neck of a once fired case with crimp removed. Brass springs back approximately 0.001" per side when pressure drops off. This fit can be ran tighter so total clearance is 0.0005" per side but only advisable in very controlled situations. Not for hunting and general plinking. Fit here can be adjusted depending on throat form. In some cases it is necessary to use reformed brass or brass with heavier neck wall to reduce the tolerance.

- The boolit should engrave lightly into the throat about its full length. It should be light enough for loaded round extraction for general use. Any clearance is detrimental to accuracy.

- Nose should engrave lightly into the beginning of the rifling. How hard is an accuracy variable. Some gun/powder combinations like more or less nose engraving.

- The case needs to be centered in the chamber. It does little good to have perfect boolit fit if the case is laying in the bottom of an oversized chamber. Fireformed cases and partial sizing is in order. This includes straight walled rifle and pistol cases.

The goal is to give the cast boolit no other place to go other than straight into the bore. If the perfect fit can be obtained then the next major hurdle is how you drive it as in how hard and where the pressure peaks. The boolit base needs to be protected from excessively high pressure, especially before it's fully into the rifling. Gas checks are a big help. It is sometimes necessary to go even beyond this in high velocity/pressure applications. Buffering compounds are a help here.

[44man]

Babore is right.
I have never understood the .002" thing. The neck does not "pop" open when the powder lights. The boolit is pushed out before pressure forces the case neck to the chamber wall. The boolit is gone before the neck expands. why would a perfect fit at the neck cause more pressure then one with .002" clearance?
Explain how initial ignition makes the neck expand and leave the boolit "free floating".
If you force the case neck into the boolit when you chamber, then, yes, more tension, but does the brass really need to open .002" to "let the boolit go?"
How about some pressure tests with a perfect fit compared to .002" clearance? Same diameter boolits but different brass thickness, don't skew the test.

[BABore]

I may be, but your not. Brass does expand and then spring back. If this was not the case, then you could not remove or rechamber the fired empty. What? You never saw a smoked neck from not having enough pressure to expand and seal completely? If the brass did not expand to seal the case neck area of the chamber, before the boolit was fully out of the case, gas would always be forced past the case.