Throat begins where?

[Doctor Sam]

I have read many times that we should either slug or do a chamber cast to determine the size of the throat and size our bullets to that diameter.  I did a chamber cast on my MN 91/30 that included about a 1/2 inch of the bore.  After a suitable period of time I tapped it out and measured it after an hour.
From the mouth of the case to the lands and grooves is a smooth radius curve without a break of any kind.
Just exactly where should my measurments be made?  The radius curve has no breaks until the bore is engaged.
The bore measured 0.314".  The area next to the case mouth measured 0.329".
Someone tell me what size bullet I need.
Many thanks.
Dr. Sam

[Dusty Miller]

Tell me more about this "chamber cast" thing!

[Forest T]

Dr. Sam I to would like to know more about this chamber casting. what did you use to make the cast??   Forest T

[Larry Gibson]

Doctor Sam

Most Soviet style chambers for thier cartridges do not have the distinct step at the end of the chamber neck that US and European chambers have. Some of the Finn MGs are an exception. Take a fired case and mesure the inside diameterof the neck. You want to have your bullet sized as large as possible and still allow some expansion (minimum of about .001") of the case neck. I have found .312" to be about as large a size bullet in most Finns. Some Russians will take .314 dimeter bullets. With softer alloys you will still get some riveting in the throat area past the case mouth on firing which is harmfull to accuracy.

Larry Gibson

[Doctor Sam]

I used Cerrosafe Chamber Casting Alloy from MidwayUSA.  Very low temp melting mixture made for this application.  Melted it in a very small old cast iron pan on the wife's electric range.  
Plug the bore with a patch about 1/2" above the throat and pour the metal into the chamber.  Be careful not to overfill: only part of the chamber needs filling as you are interested in the upper end of the case, throat, and bore.  
After a few minutes to cool it can be tapped out.  As I understand the procedure, wait one hour then measure.
Dr. Sam

[Dusty Miller]

OH! We're talking RIFLES here, no?  Also, keep in mind that this product contains cadmium which is very toxic.  Use just enough heat to melt the alloy and do so in a well ventilated area.

[Doctor Sam]

Good advise.  Our vent-a-hood moves 440 cfm directly to the exterior roof.  
Very little heat/time is required as it melts at 140*-190*.  Less than boiling water.
Dr.Sam

[DPRinks]

Using Sulphur is also easy, just do not get it too hot, of course, you do not want to get cerrocast too hot either.
Put a plug in the barrel beyond the chamber, pour the melted Sulphur in the chamber, after it cools, insert a wood dowel fom the muzzle and push the Sulphur out.
Unlike cerrocast, Sulphur does not change dimensions over time, you can measure it as soon as it is at room temperature and several days later and get the same results.
Sulphur is brittle, so do not drop it or hit it.
Don