The transition from case neck to the bore / groves diameter is traditionaly a 45 degree angle. After that throating is usualy done with a seperate reamer. 

40 Rod

Gert:

Not another one of "those" actions....

If I were me and I were chambering for a specific bullet and case combination I would first make up a dummy round with the bullet seated to the desired depth with firm neck tension. Measure the diameter of the brass neck with the seated bullet  and add .003". This is the diameter of the parallel neck section of the reamer. I would suggest a bullet that is .001" larger than the groove diameter, but this is not as critical as one might think. It is more important to match the bullet diameter to the chamber than it is to the bore.

At the end of this neck should be a 45* "case stop" transition from the neck diameter to what you refer to as the "throat". You do not want to use a 90* transition as this will shear lead off of the bullet.

The throat or parallel freebore section of the chamber is generally .0015" over bore diameter, which would be .0005" larger than bullet diameter. With a bore riding bullet you will not need much of a throat.

At the end of the throat is the leade angle, the transition from the throat to the rifled bore. This is where a BP chamber should depart from the classic smokeless powder chamber. With BP you need a good deal of initial resistance to the bullet movement in order to get a consistent burn and hence a low velocity spread.  I use a 4* per side leade angle.  This provides enough resistance for a consistent powder burn and is still gentle enough for good accuracy.

As for the length of the throat, that depends on how far out you would like to seat the bullet, but for a target rifle I like to have the case mouth end over the middle of a grease groove. The thinking here is that when the big light goes on the bullet is going to bump up and fill the neck and throat completely, including the air space at the end of the case at the transition to the throat.  In theory,  with a grease groove at this area, this empty space will fill with lube instead of lead. I actually have no way to know if this really happens, but it sounds right and makes me feel good about myself. 

So there you have it, all the news that is fit to print. I reckon you will have to translate all this into metric. * = degree and all angles are per side, not included angle. If any of this doesn't make sense or you have any questions feel free to write me here or email direct. 

Glenn Fewless

Ah come on HST. I for one KNOW for sure you are a closet martini guy! Come on, admit it, you'll feel Sooooo much better. 

A certified martini nut,
MI-shooter

hst;
Didn't I see a BSA Martini International poking it's nose from under the tarp on your pick up?

Dale53

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Only because I didn't have a trailer.

Glenn

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This, from the guy who thinks Hepburns look OK? Shame, shame, shame! Joe

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Actually, the singular is 'Peabody'.


Well, it seems that there is some sort of conspiracy going on. The rabble has been roused and they storm the castle. Not to fear, my kindred souls of aesthetic enlightenment, we shall persevere for right is on our side. Ours is as the strength of ten, for our hearts are pure.

Glenn