My recently acquired no name hammer drilling was listed as a 9.3 x72 at so yesterday good friend Tom Hall and I tried to slip a new Sellier and Belloit cartridge in and it stopped with about 3/8" to go. I put some black marker on the cartridge, pushed it in until it stopped and then rotated the cartridge in the chamber and it showed a ring about 1-3/4 up from the rim, this is where it's binding.
I slugged the bore and it's .366. The S&B bullets are also .366 but COTW show the 9.3x72R as using .376 bullets.
I then made a chamber cast and the chamber is .430 at the base diameter, the S&B ammo is .424, COTW shows .427 and another dimensional drawing I found shows .422.
The neck diam. of the chamber casting is .386 (this was kind of hard to see on the casting, what I did was take the case length from COTW, 2.84, and measured up from the bottom of the rim, marked it and then measured). COTW says it should be .385, the S&B cartridge is .382 and the dimensional drawing says .390.
So all my chamber dimensions are larger than the S&B cartridge except at that tight spot where the S&B is .495 but the chamber is .492.
Tom tells me he remembers reading something about older guns not being chambered with perfectly straight walls but rather had a slight convex curve to them. The base and neck diameters are the same but are not connected by a straight line.
I held a straight edge up against the chamber casting from the top of the rim up about 1-1/2"(I used a straight edge razor blade with the edge blacked out) and sure enough the wall is curved, I can see light through the gap. The straight edge touches at both ends but is a few .001's shy in the center.
So I have a few possibilities.
- I have a 9.3x72R with a proprietary chambering from years ago that has curved walls, will running a finishing reamer clean this up?
- I thought there was a possibility it's a 9.3x70R but that's hard to determine as the only difference between the two is case length (.009) which is very hard to tell on the casting, and neck diameter (.386) which is right between the two COTW standards, .385 for the 72R and .387 for the .387R.
All thought and opinions welcome as I would like to get this hammer gun shooting.
