03 February 2019
So finally made a chamber cast of the little 310, interesting in the fact that maybe I should be thinking about a different diam projectile. Although the gun shoots, boy is it dirty at the end of a session. Liberal amounts of Ed's Red and patches before the bore comes out clean. I can imagine that after close to 100 years of use with varying loads/projectiles that maybe it isn't as pretty in there as it could be. These projectiles are PC'd so this is what I think I am cleaning out of the bore, rather than just a dirty bore.
So what I have found out from measurements:
It has a .320 bore (across the grooves) and .337 at the case mouth. The leade is .323/.324 and in front of the case mouth is a little pitted and out of round but can be expected with a rifle this old, I'll live with it. The rifling starts at 1.315"
My loaded round, unsized case and projectile hand pushed into case mouth, at the case mouth is .336/.3365 OD, projectile diam is .324. My loaded rounds have the ogive end at 1.310.
The last couple of thous' is a tight fit by hand and once in the projectile stays there (leade diam), found this out the hard way when I decided to change my mind and ejected a chambered round only to have nearly 5.0 gns of WST casually falling into the action.
BUT the bright side of that was I then learned how to disassemble the action.
I am using Win SPP at present but am finding they are starting to flow at the 1300fps+ mark
My first thought was headspace but the primers aren't backing out, then maybe an excessively large firing pin hole, seems a nice fit so I can only put it down to soft primers. I shall invest in some Small rifle primers and see what happens.
Considering it was a little overcast and 93f with 80%+ humidity as it had been raining that morning, first time in 3 months. My eyes aren't what they used to be especially with such fine little sights I thought the groups were acceptable considering, all 10 shot groups at 50 yds. I was happy with 4.4gns of AS30 (clays). A little sloppy with the 4.8gns of AP70 but still acceptable with a projectile I now know is oversized for this bore.
310 cadet + 1 1272.0 4.8 WST 1264.0 1253.0 1255.0 1252.0 Average 1259.2 Standard Deviation 8.6 Mean Average Deviation 7.0
310 cadet + 1 1280.0 4.8 AP70 1273.0 1274.0 1276.0 1270.0 Average 1274.6 Standard Deviation 3.7 Mean Average Deviation 2.7
This was also with once fired brass so the lay in the chamber would be different and also the neck tension so I believe this is more of an indicative velocity than previous tests.
So now I have a good reason to look at moulds and get one that fits the bore.