Effect of Primer Selection on Muzzle Velocity

Load:

PPU Case, neck sized with two reloads

WLR Primer

40.5 grains Varget (ADI AR2208) all charges weighed

174 gr Sierra MatchKing

Cartridge Overall Length from 3.050 to 3.070 in .005” increments

6 shots of each increment in COAL.

There was virtually no correlation of MV to COAL:

COAL     MV    ES    # Shots

3.050   2407    48       6

3.055   2406    56       6

3.060   2393  102       6

3.065   2392    52       6

3.070   2420  100       6

These are some high extreme spreads in velocity, particularly 3.060 and 3.070 COAL.  3.060 had the slowest shot at 2350 fps and the 3.070 COAL had the max velocity at 2490 fps.  Excluding these two shots gives the MV and ES as follows:

COAL    MV    ES   # Shots

3.050   2407  48      6

3.055   2406  56      6

3.060   2402  75      5

3.065   2392  52      6

3.070   2405  24      5

I had larger vertical POI spreads on this test than on previous tests at the same muzzle velocity. I don’t know why. Perhaps variable bullet jump at the same velocity is different than variable charge weight at the same velocity.  But, the trend of increasing elevation with increasing muzzle velocity is still there, as are large muzzle velocity Extreme Spreads despite weighing every charge.  These are some of the largest ES values I have seen.  

Hard to make any firm conclusions here, but the 3.05” OAL showed the lowest elevation spread. This happens to have been my standard .303 match load for many years.  3 of 6 shots were within the height of the X ring with two more just outside the X ring height (open triangles in the plot below). 

I’m beginning to suspect the barrel is the cause of the large muzzle velocity extreme spreads. Although it has less than 2000 rounds through it, there is significant pitting in the grooves.  I’m tempted to screw on a new CBI barrel and see how it does. 

No, never shown before to be an advantage. Not to me anyway. 

Yes, I have done that on my FL dies. These happened to be neck sized cases in a Lee Collet die.  I’ve experimented with different sized mandrels and expanders as well (-0.001, -0.002).  Once I started annealing case necks, I went back to a standard size expander and mandrel. 

Something suddenly changed today, had trouble with lateral spreads too even though there was little to no wind. Lateral zero was off by 3 minutes. I wonder if the scope graticule has come loose, sure hope not.  I check the mount thumb screws during the shoot, they remained tight. Pads not loose either.  Might remove the scope and shoot with aperture sight. 

This gave me the opportunity to check parallax, and it is definitely parallax free at that distance. I can detect a slight amount of parallax error at 100 yds, perhaps 1 MOA at most at the extreme edge of the view thru the eyepiece. 

I’m quite relieved to see this, worried the scope was nackered.

My Dad made that vice in High School, about 1947. He made the sand casting molds too and had a local foundry pour them. Wished he had stamped his name and date into the base. 

I’m beginning to suspect I’m “over cleaning” this barrel. The grooves are somewhat pitted, and perhaps it needs some carbon build up to smooth it out and settle the barrel vibrations down.  This might reduce bullet velocity variations too.   For years, my routine has been Hoppes 9, but I recently changed to C4 which is taking the bore down to bare steel every time. Looking back thru the target plots, groups opened up after the first time cleaning down to bare steel.  Although such a cleaning routine works on new match grade barrels, perhaps not so on old pitted service grade barrels. 

This cleaning is with just Hoppes, and I’ll see how it shoots next time out. 

I just borescoped the barrel after soaking in Hoppes 9 since Saturday afternoon, it is quite dark with carbon. But, it feels smoother pushing patches thru than it does after cleaning to bare steel with C4. I’m going to shoot the rifle a few more times letting the carbon harden up.  If it doesn’t improve, I’ll replace the barrel. 

One other thing I’ve noticed. High velocity shots go high, but they also go right. Low velocity shots go low-left. It’s quite noticeable.  Either the bolt locking lugs don’t have even contact or the scope affects the way the action bends under load. The scope mount is quite stiff and screwed to the left side of the action body, so there could be a tendency for the action to bend towards the right (pointing the muzzle to the right).  The amount of deflection would be proportional to the thrust load on the bolt.

Winds were 1 to 3 mph (variable direction), even at 600 yards, not much effect on horizontal spreads, less than 1 minute. 

I believe the scope and mount affects the muzzle “jump”. The mount is quite stiff.  Maj Reynolds wrote about a trial carried out by the Small Arms Inspectorate at Enfield in 1945 on 24 No. 4 T rifles to ascertain the affect the telescope and mount had on the angle of jump:

Without Scope Fitted   4.5 to 12.5 Minutes

With Scope Fitted       -2.0 to 4.75 Minutes 

Clearly, the scope does have an affect.  I don’t know how much jump would be necessary for the rifle to show positive compensation, but it would have to be a positive (muzzle moving upward) value.  The test shows some rifles had a negative angle of jump (muzzle moving down as the bullet exits the barrel). 

None of this should affect muzzle velocity though. I still don’t have a good explanation to offer for why I’m getting large extreme spreads.  But I am hoping that leaving some carbon fouling in the grooves will settle it back down to where this rifle was shooting before. 2 MOA at 600 yds May be as good as it gets with this barrel. 

Ask a hunter

They'll tell you they use a fouling shot or sight for a "cold, clean Bore" because the first round is the money shot!