OK, today we tested the revised 60 gr. GC bullet. There's been so many revisions that it's hard to keep them straight, so let's just call them #1, #2, and #3.
#1 and #2 are secant ogives, while #3 is closer to a tangential ogive. The idea behind #3 was to give the ogive a little more support.
Going into this I had settled on the following load factors.
-- 60 gr. weight
-- 40 BHN alloy
-- OBPU coating (for the time being, more coating tests to come)
-- 25.1 gr. WC844 (not necessarily the most accurate powder but I had a couple of jugs that were looking for something to do
-- full monty Ardito-style tapered glove fit, seated with only the gas check in the case
-- CCI #41 primer (not necessarily the most accurate primer but I have a bunch of them
-- free floated barrel
Today I started on the bottom row of targets, and went left to right. Then the middle row, and finally the top row.
As you can see the new improved #3 bullet didn't do all that great. It's 2.02" average was about the same as previous tests with the #2 bullet. That didn't seem right because several weeks ago this barrel seemed able to shoot 1.5" without trying hard. I wondered what had changed? Well, I am always changing things but most of those changes are tested very carefully.
One big change was the bedding. It used to have the spring plungers on the forearm but then I switched to a free floating barrel. The free floated barrel did not seem to hurt accuracy but then again the spring plungers had been tweaked numerous times so there was no cut and dry comparison.
I wanted to rule out barrel vibes causing the fliers, yet it had to be something simple enough to not require time-consuming tweaking each time the barrel is swapped. So I cut a 3/8" wide strip from an old, cheap-style mouse pad. Not the hard plastic kind but a soft foam pad. The strip was pinched between the barrel and the tip of the stock. Because the mouse pad material is so thin and so soft, it exerts very little pressure on the barrel. The hope was that the soft foam rubber would provide just enough contact to dampen vibrations. Perhaps it was just coincidence, but groups did shrink a bit with the mouse pad bedding.
I ran out of the #3 bullets so I shot one last group (top left) with the #2 bullets, which had shot so poorly last time. Darned if that wasn't the best group of the day!
Then for a sanity check I finished with some of the 77 grainers that I have tested on previous occasions, which usually run 1 3/4" or so. This time I cut the powder from 26.5 gr. to 26 gr. hoping that would help accuracy, but it did not, averaging 1.98" and with more velocity variation than last week's 26.5 gr. load.
A close-up of the final 60 grain group, using the #2 design bullets for the first 9 shots and a #3 bullet for the 10th shot. Yes, it hit the magic 1 MOA though just barely, and more than a little luck was involved. Ironically, the mirage was at its worst for this group -- I was basically shooting at a dancing orange blur.
It may have been luck but I'll take it!
Trivial stats (I include them because I use these posts as a form of record keeping):
-- average velocity with 25.1 gr 844 / 60 gr. GC = 2772 fps
-- average velocity standard deviation with 60 gr. GC = 0.82%
-- average 10 shot group with free floated barrel, 60 gr. GC = 2.02" for 5 groups
-- average 10 shot group with mouse pad barrel, 60 gr. GC = 1.46" for 4 groups
-- the #3 design, with it's tangential ogive and slightly longer bearing length, did not seem to be more accurate than #2 or #1.
-- the mouse pad bedding may have been an improvement but I don't think I "proved" that. At a minimum the mouse pad bedding did not hurt accuracy so for now I'll continue using it.
-- overall I was disappointed because I haven't made any accuracy progress for several weeks -- I seem stuck at 1.5 MOA.Things To Try Next Time and Down The Road:
-- I'm itching to move on to other things so for now the mouse pad & 60 gr. GC will be the control load.
-- test the control load with different band & groove layouts.
-- test the control load with different coatings.
-- revisit coated plain base.