Reloading .303 British : Effect of Primer Selection on Muzzle Velocity

For the test, the basic load data is:

PPU Case

WLR Primer

Varget (ADI AR2208) all charges weighed

174 gr Sierra MatchKing

Five rounds each were loaded with the following charge weights:

40.3, 40.6, 40.9, 41.2, 41.5 grains. All weighed on RCBS Range Master 750 Digital Scale checked for accuracy with calibration weights

I fired one shot of each charge in sequence at the same target, recording muzzle velocity and plotted the point of impact carefully.  Then repeated this in a round robin style test for a total of five shots of each charge weight. 

Overall, for the 25 shots, the velocity statistics were:

MV  2417 fps

ES   118.6 fps

SD   29.4 fps

That’s not actually too bad for 1.2 grain variation in charge weight. 

Here are all 25 shots:

I then plotted the individual shot elevation (MOA) from a reference line at the bottom of the target 6 ring against muzzle velocity.  

Here was the smallest 5 shot group, 40.6 grains of Varget, under 1 MOA center to center.  Muzzle Velocity 2389 to 2427 fps.  This was shots #2, 7, 12, 17 and 22 of the Round Robin 25 shot test. I made one plot sheet per individual charge weight load. 

Perhaps not unexpected, there is a strong correlation of elevation point of impact on the target with muzzle velocity. 

But, oddly, most charge weights exhibited significant muzzle velocity variation. Here are the individual charge weights mean velocity and extreme spread: 

Charge   MV   ES

40.3     2379  53

40.6     2408  38

40.9     2414  55

41.2     2439  53

41.5     2446  32

The plot below shows all 25 shot elevations with the corresponding muzzle velocity:

The upper dotted line is a linear trend line fit thru the data. The lower dashed line represents the lower variation, but for some reason, the upper dashed line error bound is not showing. Will have to try and fix that. It’s been a long time since I’ve done anything in Excel. 

It’s quite clear to me that my trouble with 600 yard groups holding the 10 ring are because of excessive velocity variation, even when charges are all weighed. Normalized to a constant muzzle velocity, the rifle is shooting just over 1 MOA in elevation spread, about the height of the X ring on the MR Target in the range of 2385 to 2420 fps (appears to be a fairly flat spot where elevation is not changing with muzzle velocity).  Vertical spreads seem to blow up above 2460 fps, but I’d need more testing to prove that.  This might be due to the barrel and rifle moving out of the accuracy node. 

Now to find and correct the causes of muzzle velocity variation.  I need to get the Extreme Spread of muzzle velocity down to less than 30 fps based on this data and target a mean velocity of 2400 fps to get on that flat spot. 

I have all the fired cases numbered in order as shot, so I will see if there is any correlation of case weight with muzzle velocity. Next shoot I will do an another Round Robin test all loaded to 40.6 grains but vary bullet seating depth.