Author: britrifles
Subject: Effect of Primer Selection on Muzzle Velocity
Posted: January 27 2025 at 2:58pm
Here are the horizontal shot spreads vs Muzzle Velocity on the bullet seating depth test I did last Saturday. The plot of vertical spread vs muzzle velocity is shown above.
Subject: Effect of Primer Selection on Muzzle Velocity
Posted: January 27 2025 at 2:58pm
Here are the horizontal shot spreads vs Muzzle Velocity on the bullet seating depth test I did last Saturday. The plot of vertical spread vs muzzle velocity is shown above.
Winds were 1 to 3 mph (variable direction), even at 600 yards, not much effect on horizontal spreads, less than 1 minute.
You will see that I did not get the rifle zeroed up before this test, the MPI is about 1 minute left. However, only 3 of 30 shots were outside of just over a 2 minute horizontal spread, so perhaps not all that bad, even for a No. 4 T.
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Load:
PPU Case, neck sized 2x
WLR Primer
40.5 gr Varget, all charges weighed
174 gr SMK
OAL from 3.05 to 3.07 in .005” increments, six shots at each increment.
What surprised me was the very large velocity Extreme Spread (140 fps). Note the trend of horizontal POI moving right as velocity increases, especially at the extreme minimum and maximum muzzle velocity. Ive seen this before with this rifle, high shots usually go right as well, which costs more points lost on the target. I suspect it is the action flexing more at higher pressures.
The smallest vertical and horizontal spread was with the 3.050 inch cartridge overall length (1.4 MOA horizontal and 1.80 MOA Vertical). But extreme spreads in all three parameters (muzzle velocity, elevation POI and horizontal POI) were large compared to past shooting results. All that I did differently was to completely remove all fouling from the bore before this shoot. The grooves in this barrel are quite pitted (photo below) and perhaps the carbon helps to smooth bullet travel, lessen velocity variations and lessen barrel vibrations. This time, I cleaned with Hoppes 9 so next shoot will start with a fair bit of fouling remaining.
A deteriorating barrel always first shows up at long range, then progressively works towards short range. This test was at 600 yards. If leaving some controlled amount of fouling in the bore does not reduce group size, a new barrel will be fitted. I don’t see how else I can reduce the velocity extreme spreads with my loading methods. And I know now that this is the cause of wide groups and poor scores with this rifle.
