I set back the barrel and recut the chamber & throat. The shortened barrel is about 26 1/2" long. The chamber neck is still the same 0.330" - 0.331" diameter.
I took care to minimize the gap between the case neck and the entrance to the throat. The original chamber had a gap about 0.040" long, now it is only 0.010" - 0.020" (it's difficult to measure precisely because there are several variables).
I decided to use the same 1/2 degree throat reamer as last time, but with minimal freebore and minimal clearance. Last time the throat ended up with over 0.250" freebore, this time there is 0.050" freebore, if even that. Last time the entrance to the throat was 0.312", this time it is 0.3090", if even that.
For the time being I assumed that best accuracy would be with the bullets nose-sized to jam with only one band seated into the case. That is a system that has worked well for me in my high-velocity barrels though admittedly I have not done comparison testing in this mid-velocity application.
Here is how the bullets looked after nose sizing. As you can see, they look nearly the same as the bullets for the original one degree throat -- 1/3 to 1/2 of the bullet ends up being a bore rider.
I'm not convinced that bore riding section is a good thing, but we'll give it a try.
Bullets were seated at a hard jam point. There was some mid-day mirage and a 5 - 15 mph wind that was constantly changing speed and direction, but the forecast called for conditions to get worse, not better, and I was eager to try the snug 1/2 degree throat.
From left to right, starting with the 180 gr. Loverider. I was aiming for 2300 fps but velocities were a little high. Nonetheless 9 shots went into 0.9". The one flier happened when the wind completely reversed directions and I thought "well I'll just hold steady on the bullesye and see what happens." Well, the flier happened!
On the next shot I held windage and it went into the main group, so the flier may have been my fault for failing to dope the wind.
Next, I dropped the powder charge 0.3 grain down to 34.1, yet the velocity was still too high. 9 shots went into 0.8".
Then I tried the XLR bullet. This throat did not like the XLR, probably for the same reason the original 1 degree throat did not like the XLR -- not enough support.
Then I tried the 190 gr. Loverider. This throat did not like the 190. No surprise because the original 1 degree throat did not like a 189. I sure wish I could find a 190 that would shoot in this barrel because CBA competitors prefer heavy bullets for less wind drift.
I had only five 180's left so I shot them with 33.8 gr.. The 2297 velocity was what I wanted. 4 shots in less than an inch but one flier. I can't rule out wind for the flier because it kept changing direction and speed. Conclusions
-- this snug 1/2 degree throat behaves similar to the original snug 1 degree throat.
-- I still have to size noses more than I would prefer, with about 1/3 of the bullet ending up as bore rider.
-- while the jury is still out on the perfect cast bullet throat, I feel good about the gunsmithing on this chamber and throat. The dimensions turned out just the way I wanted.
-- at the moment I'm inclined to believe there isn't much practical difference between a 1/2, 3/4, or 1 degree throat. What matters is a snug fit.
-- this throat shows potential with the 180 Loverider, if I can get rid of the one flier in every group.
-- in case you are wondering, I'm deliberately sticking with 180 - 190 gr. bullets at 2200 - 2300 fps because those are weights and velocities that CBA competitors are comfortable with. I would not be the least bit surprised if this barrel gave better accuracy with a lighter bullet, and I may try that eventually, but for now I'm sticking with conventional CBA loads just because that's what people expect.Things To Try Next Time
-- try nose-sizing just enough to make the top band 0.3015", even though it will require deeper seating.
-- 33.9 gr. Leverevolution seems to be the recipe for 2300 fps.
-- as time allows, keep trying different 180 - 190 gr. bullet designs.
-- these experiments with cutting different throats have convinced me that it would be handy to have micrometer depth stops for the reamers. It may be a while before I can build them, but they've been added to my wish list.