Reloading .303 British : Tweaking my Varget Match Load

I’ve ruled out cartridge case weight variations. For the round robin test done on powder charge weight, I checked all the cases fired that day.  Of the 25 rounds fired, the max weight was 168.7 gr and min was 164.1 for am extreme spread of 4.6 grains. Muzzle Velocity and Elevation POI had no correlation with case weight. 

I’ve chosen 40.5 grains of Varget charge weight for the bullet seating depth test that gave the least amount of vertical spread of POI at 600 yds for a targeted mean velocity of 2410 fps. That selection came from the plot below. I’m expecting a slight velocity increase with the deeper bullet seating depths so I dropped the 40.6 grain charge by 0.1 grains to try and stay away from that elevation step up that occurs around 2430 fps.  In the plot below, the cartridge OAL was approx 3.07 inches for all shots. 

I measured the throat length on the No. 4 T, now at 1700 total rounds fired, with a Hornady Bullet Comparator and OAL Gage. The 174 gr SMK contacts the lands at 2.708 inch Cartridge Base to Ogive of the bullet. 

Here are the bullet seating depths (all dimensions in inches) I’ve chosen and approx cartridge overall length.  Note the bullet jump to the lands is quite a bit, such is the nature of the Lee Enfield chamber and throat dimensions:

Load #   CBTO   OAL    Jump

   1        2.470  3.050   0.238

   2        2.475  3.055   0.233

   3        2.480  3.060   0.228

   4        2.485  3.065   0.223

   5        2.490  3.070   0.218

I’m really not expecting to see much correlation of seating depth to muzzle velocity variation, but I want to rule this out and see if there is a seating depth that brings tighter groups at 600 yards. 

Weather permitting, I’ll run the ladder test on Friday.