OK, here we go with the 21.5" barrel and the usual 155 gr. bevel base load. Here's how the bullet looked when sized to fit this throat:
155 gr. bevel base
16.2 gr. Lil Gun
Win mag primers
1.913" COL (0.009" jump to rifling)
The first group was shot with bullets that were several weeks old, meaning they were prolly about 30 BHN. The .359 bullets were resized in a .357" push-thru die but because they were hard they sprang back to 0.3575". There was some moderate mirage when I shot the first group so I decided to wait until evening to shoot more groups.
The 2nd and 3rd groups were shot with bullets that were heat treated this morning, meaning they were prolly only about 20 BHN because they haven't had time to reach peak hardness. They were sized before hardening so they actually measured 0.357". There was only a tiny bit of mirage during group #2. By the time I got to group #3 the mirage was gone.
As you can see, group #1 was respectable, group #2 opened up, and group #3 opened up even more. Also the vertical point of impact kept moving up as more shots were fired.
Group #1, mean radius = 0.50", velocity = 1982 fps, standard deviation = 0.8%
Group #2, mean radius = 0.78", velocity = 1986 fps, standard deviation = 1.13%
Group #3, mean radius = 0.93", velocity = 1984 fps, standard deviation = 1.22%
After group #1, there was slight fouling about 4" ahead of the breach (the 25" barrel had also fouled about 4" ahead of the breach). The fouled spot was only about an inch long. The rest of the barrel was squeaky clean. I did not clean the barrel between groups.
After group #3, the fouled spot was slightly more fouled. It was not severe fouling, most people would not worry about it, but nonetheless it was there, and a tight patch showed significant specs of lead. Observations, Questions, and Conclusions
-- was the first group merely luck, or was it because of the clean barrel and/or harder bullets ?
-- did groups #2 and #3 open up because the bullets were softer, or because of fouling ?
-- getting rid of the "tight spot" did not eliminate fouling. It so happened that the tight spot was about where the fouling occurred, 4" past the breach, but that is also near where peak pressure occurs. Apparently the fouling at that spot is due to peak pressure?
-- Are 30 BHN bullets more accurate than 20 BHN bullets ?
-- I don't know if this 21.5" barrel is an improvement over the 25" barrel because there are too many variables. I need to retest with 30 BHN bullets to eliminate the hardness variable.
-- could it be Lil Gun that it causing the fouling? Recall that I have alternated between WW296 (or 297 or H110, same thing) and Lil Gun. Both are capable of similar velocities, but ..... what if Lil Gun burns hotter than WW296 and causes more cast bullet fouling? You might not notice it right away, it might take 20 or 30 shots before fouling built up enough to cause a problem.
-- what caused the point of impact to drift? Fouling? Barrel heating up? Things To Try Next Time and Down the Road
-- heat treat at higher temperature. I have been using the kitchen oven at 435 degrees, but have not verified the accuracy of the oven thermostat. I use 435 because from experience that's a safe temperature even allowing for the inaccuracy common to kitchen ovens. But .... my heat treat oven has a PID controller so it ought to be able to safely heat treat at 450 or so. I just need to make a pan that fits in the heat treat oven.
-- let the heat treated bullets age at least 2 weeks. That should bring them up to 30 BHN or so.
-- comparison test between Lil Gun and WW296 with a focus on fouling.
-- possibly revisit coatings to eliminate fouling.
-- possibly go back to a gas check bullet to eliminate fouling. I've been using the bevel base because it seems to have a slight edge in accuracy, but the catch is that accuracy seems to deteriorate as more shots are fired and fouling builds up.
It's going to take some time to refine my heat treating process and to let the bullets age-harden several weeks, so this barrel may be put on hold for a month or so.
In the meantime I feel comfortable that the hardware is good -- no more "tight spots" or "bad chambers" to worry about, so I can focus on the load rather than the gun. That's the way it should be.