I would be very interested in hearing your thoughts on what you consider to be the ideal breechseat chamber/throat and more importantly, why. Actual experience or theory…….interested in hearing both. 

I just got done rebarreling  a #8 Premier Tip-Up rifle and started playing around with the 3” barrel slug after cutoff. Both this 25/20ss rifle and another in 28cal. will utilize the Mos tapered breechseat bullet. The (2) red lines on the paper represent the “normal” 1/16” ahead of the case mouth. The red marks on the lower cerrosafe cast show a cylindrical throat @ base band diameter that satisfies the 1/16” parameter with the base band engraved halfway thru. Some my best groups have been with the bullet base fully engraved and better than .300” ahead of the case. I have the ability to experiment on this slug and want to take advantage of the same if something “new” appeals to me. Thanx for your help!

Jeff_Schultz wrote on Nov 10^th, 2023 at 7:04pm:

So long as you don't distort the bullet when seating it.

A breech seater is fitted to perfectly fit in the chamber and centers the bullet which is injected fully into the rifling.

So the chamber does not matter.

Does not matter if the leade angle is 1/2 a degree or 2 degrees.

What matters is that the breechseater fits perfectly in the chamber and the bullet is perfectly centered in the seater in order for it to be injected straight into the rifling.

The terminology used to describe the area beginning at the "ball stop" to the beginning of the lands is used loosely as the leade or the throat. Based on my readings/experience, if the lands are present immediately at the ball stop, no leade exists. Nor is there any throat. If the partial height of the lands is present at the ball stop (or step) no leade exists. Since we try to achieve a seal by having the bullet engraved to or on or just past the base band, then an almost full height of the lands would be satisfying. Minimal bullet/case gap with minimal insertion of bullet. 

I made up these drawings, many years ago, to try to clarify the the different terms and how they are used in chamber drawings by SAAMI. "B", or the middle drawing, is the type of throat that the 32/40 & 30/30 have. "A" is a 308 or Remington type chamber and "C" is what 30-06 & 8mm Mauser have.

First, the throat is very thing after the end of the chamber case neck (especially for a cast bullet rifle) to the end of the leade angle at the start of the bore.

In the case of the freebore chamber, the leade angle starts at the end of the freebore, even if the freebore is larger than the groove.

Does that help?

If you mean that the lands would 90° to the chamber, I wouldn't like that. I think it would shear the lead from the bullet.

Many chambers will work, the key is to get the bullet seated w/o distortion. If it takes to much pressure to seat the bullet, the base will deform, maybe even the lube groove and/or the bands and grooves forward of that.

So, it's the bullet fit to the throat, that makes a good BSing chamber. You can do it with the throat or the bullet shape, or both. You want to see some sort of engraving on the bullet, from the base to the ogive, if possible. 

The chamber that I use, for 30, 32 & 33 cal, has a .100 freebore, .001, over groove size and a 1° leade. With that, I can BS both tapered and cylindrical bullets.

A note of caution, with a freebore, it is harder to pick the point that the bullet engraves in it, unless the bullet base is about .003 larger than the freebore.

The ends of the lands have a slight angle making the top of the cliff safe from damaging bullets. A tapered bullet would not touch the ogive and would probably slightly engrave the first driving band.