OK, last time there appeared to be a error in the powder charge, so I loaded the 190 grainers with 15.4 and 15.6 gr. of Lil Gun and gave it another chance to show us what it's got.
Since last time, the 2.5X Leopold has been replaced by a Williams 5D. The front sight is still in limbo while I experiment with different ramps and blades to get the elevation right, so for today the front sight assembly was attached to the barrel with only masking tape. It worked better than it sounds.
When the scope was mounted, I would run the pressure trace cable back along the top of the barrel, but today I ran the cable under the barrel, to avoid interfering with the line of sight.
Also since last time, I hand lapped the barrel for the third time, because there was some suspicion that the first two inches on the breech end were still a little tighter than the rest of the barrel. I was never sure whether the tight spot was for real, but I figured that if the tight spot was real, lapping would help, and if the tight spot was not real, lapping would not hurt.
Also today's loads were assembled with a lighter than normal crimp. Let's say it was just enough crimp to turn the case mouth inward, but not a heavy crimp like I would use on a wheelgun.
Sorry to bore you with all these details, but it's leading up to something.
I shot three 5-shot groups with 15.4 gr. Lil Gun and three 5-shot groups with 15.6 gr. -- 100 yards, Fed #200 primers, felix lube, 0.359" heat treated mystery alloy, 190 gr.it may be a few months before the new forum software can digest the following table
[mrow] [col] 15.4 Lil Gun [col] 15.6 Lil Gun
[row] Avg. Velocity [col:] 1719 [col] 1737
[row] std. dev [col] 1.1% [col] 1.1%
[row] Avg 5 shot group @ 100 yds. [col]4.4" [col:] 3.3"
The best group of the day was 1.9" with the 15.6 gr. charge. Now if it would just do that every time ..........
Here's a PT for the 15.4 gr. load, showing only the nicer looking traces.
Here's some of the funny looking traces for 15.4 gr.
Here's the first two strings for the 15.6 gr. load, with only the bogus traces deleted (this particular strain gage installation is very noisy and always produces a few bogus traces that have to be deleted).
Now here's the last four shots for the 15.6 gr. load. This is the string that shot the nice 1.9" group -- is it a coincidence that this string also has the most consistent traces?
So what the &*!@# is causing the second primary peak? I dunno, especially since this particular setup has always had a lot of electrical noise and always produced a few bogus traces. However, today's 2nd primary peak happened enough times that it cannot be immediately dismissed as electrical noise.
So what have I learned? Well, about 15.5 gr. Lil Gun is max for this 190 gr. bullet, and it should produce about 1725 fps.
The effect of light crimp vs. heavy crimp definitely needs to be explored.
I'm not totally sold on Lil Gun. It gives significant secondary spikes that could be hurting accuracy. Lil Gun gives nice velocities, and I've got an 8 pound keg of Lil Gun that I don't have any other use for, but I have more confidence in tried and true WW296.
Next time I'll do the crimp experiment with 15.5 gr. Lil Gun, and I'll also try 296 with the 190 cast.
Another thought is to try trimming 357 Maxi cases to about 1.32", which should chamber in this gun's goofy "toilet bowl" throat. That would put the case mouth right up against the beginning of the rifling, so the bullet would not have to leap across a 0.380" diameter throat like it does now. I'd have to cut a mold with a nose designed for the longer cases, but I think it's worth a try.