Educate me please

[Haywire Haywood]

http://i11.photobucket.com/albums/a193/imh_98/375win.jpg

I cropped this 375 Win chamber drawing from what Leverguns had and I'm hoping to get some education on what the different areas are called and how they relate to fitting a cast boolit to a gun. I added the red letters.

I was under the impression that the leade was the area between 'A' and 'B'. The freebore was the area between 'B' and 'D' and the second angle before the rifling starts simply didn't exist. As you can see, my impressions were way off kilter. From looking at this, I deduct that the 2 deg angle is in fact the leade. What is the other angle called? What area would be call the 'throat'.

Fitting a boolit to this chamber: As I understand it, ideally I would nudge the ogive past all the angles and into the rifling, and if that isn't possible a bore riding nose would need to be as close to the diameter of the area between 'B' and 'C' as possible to get the boolit started as close to center of the leade as possible.

How would I measure a chamber impression to deturmine the proper nose length? As you increase nose length given a set boolit weight, either the bearing length would need to shorten or the nose would get skinnier. I'm not sure what to do there.

Maybe there is a good, easy to read explaination on the net somewhere that someone would link to. I realize I've asked an indepth question.

thanks,
Ian

[Buckshot]

..................The area A to B is basicly the end of the chamber. Some chambers have a sharp ledge there. I don’t know what this area is called, or if it even has a name particular to it. B to C is the throat. Sometimes a straight cylindrical area and sometimes with a very shallow taper.

Regardless, it’s smooth and has no lands The C to D is the leade. Some call the entire surface from B to D the leade but that is incorrect for the 2 distinct surfaces. The leade begins where the lands (in effect) raise from the wall of the throat, until a point where they gain their full height.

http://www.fototime.com/CFE4FB6484BDAE6/standard.jpg
This gives you an idea of the throat and leade areas of an 1879 Argentine contract Remington rolling block in .43 Spanish. The throat in the rifle has a very shallow angle from the casemouth to the beginning of the leade.

The length of the inverted slug is the length of the throat. These were designed for paper patched boolits and such throats were common.

http://www.fototime.com/4B9EBE131ADC269/standard.jpg
This photo is of a 530gr 45 cal rifle slug that I sized to .440" which is .001" under throat dia. An aluminum rod was turned and then cut to the length of the cartridge case and then slipped inside it. The lead slug was dropped in the chamber, and then the cartridge case placed in behind it.

A piece of wooden dowel was used as a punch. It and a hammer drove the case and boolit in, to the point that the breechblock would close. Then the slug was punched out.

The upper pink line shows the angle of a rather abrupt angled leade at the start of each land, at the end of the throat. It's like 18* and only as wide as those 2 parallel pink lines. The lands are almost full height from that point. So the whole lube grooved area of that slug, is a smooth sided throat designed to guide a paper patched boolit into the lands.

A government 45-70 chamber is like that if you subtract out all the throat area and begin the leade right at the casemouth :D

http://www.fototime.com/14622A982A0BAF6/standard.jpg
This is what Dan came up with, as a design for a grease groove FNPB slug to streatch from the case, across the throat and butt into the leade.

................Buckshot

[Haywire Haywood]

Thanks Buckshot.

Ian