Hornady 75 BTHP ladder part 2

[Stats Shooter]

So after finding an accuracy node somewhere around 24.2 grains of Varget, I did a .1 grain ladder beginning at 23.9 and ending at 24.4 since the groups opened up a bit at 24.5 in my initial ladder. I also tested 25.1 grains just to see if I could induce pressure signs since I found little evidence at 24.8 grains.

In the 2nd ladder I found the best groups at 24.3 grains. 24.2 had a 1" group and a .8" group which was a bit worse than the first 4 shot ladder. 24.3 grouped well at .68 and .51. 24.4 also grouped well at .81 and .56. There was a somewhat gusty wind during testing which I blame for the groups being a bit worse than the initial test and I intentionally kept the barrel warm to ensure this load will perform reasonably with a warmish barrel. (Previous experiences with alliant and non-extreme hodgdon powders makes this a necessary test method for me).

BTW the 25.1 grain load did not have any MAJOR pressure signs however there were some more pronounced extraction marks and the primers showed some ridges within the dimple and experience has taught me that in CCI 450's that is a precursor to a pierced primer....also 25.1 grouped terribly. I only tested to find the upper safe range and I found it. I believe 25.4 would result in loose primer pockets or piercing of primers.

Here are the specs
COAL 2.270 avg
Base to ogive 1.88
Trim 1.750
Varget powder
CCI 450 primers
Lake City fireformed annealed brass

I believe my next step is 100 rounds, 33% at 24.2, 33% at 24.3 and 34% at 24.4, all while testing velocity.

 

[Stats Shooter]

Here is my initial test results:


Well I began the ladder test today. COAL avg 2.270, base to ogive 1.88. After 56 rounds all feeding flawless I will stick to that jump.

Now, here is where things have gotten interesting. I began the 75 gr BTHP ladder at 23 gr of Varget moving up in .3 gr increments. I shot 2, 4 shot groups per load. 23, 23.3 and 26.6 grains were mostly unremarkable around or above MOA. However, 23.9, 24.2, and 24.5 all posted sub moa groups with the best "worst" group of 23.9 being .76" and 24.2 being .67". The best 'best' group were also from those charges at .27 for the 24.2 grain load. 24.5 had one group just above moa and one below.
The 24.8 grain load had both groups below moa also but not as tight as 24.2.

At 24.5 and 24.8 primers begin to flatten and at 24.8 there are mild extraction marks on some cases. I'm debating on trying 25.1 which is above ANY published load I have looked at including Lyman which is the highest to find out if in my gun 24.8 is the definitive upper bound of safety given flat primers and slight extraction marks but beyond that I guess there is no point as it is probably not a charge I will use .

My next step will be 2x5 shot groups beginning at 23.9 to 24.3 in .1 grain increments to find what appears to be the true sweet spot for this bullet.

Oh btw
LC once fired brass (fire formed in this chamber from factory ammo)
CCI 450 primers
Trimmed to 1.75
Gun Ruger SR 556 Varmint 1:8 twist 20" stainless bull barrel.

I must say however that I have never had a ladder test were all charges shot so well. Normally things are wide open then tightening up , then opening back up. This bullet/powder combo never truly opens up. It can make ladder testing difficult because any slight mistake shooting can give erroneous results when things are this fine.....hence my second ladder to truly discern the sweet spot.