joeb33050
posted this
24 April 2014
6.3 PRIMER TESTS
C. Dell
"I have been told many times down through the years that if a gun has a light hammer fall the groups fired with that gun will string up and down. I had always assumed that this was true as the persons reporting this effect were very respected men who had done a lot of really serious shooting.
Back before the primer shortage developed I had made a test rig to test primers all by themselves (a stand alone test of primers). Basically I was able to hit primers with a very repeatable force and was determining how much force it took to reliably fire them as well as the velocity that they would impart to a specially made pellet. Results were determined by measuring the velocity of the pellets about fifteen feet from the muzzle of the test rig.
This testing was discontinued when the supply of the desired primers for testing dried up.
These tests have not at this time been resumed. Two observations were, however, made from the data collected at that time. One: the observed average velocities were not affected by how hard the primer was hit. Second: about one primer in five regardless of make or grade was to a noticeable amount deviant from the group average. This took the form of either being significantly higher OR lower in velocity than the average.
These properties were discussed with various shooters during the time that supplies of the various primers were not available. I was often asked if the same results would be observed if I were actually shooting cartridges loaded with powder and bullet. There was at that time no practical way that I could test this idea. As a result the question was left hanging with no real answer.
Recently a method was conceived that would readily permit study of this as well as other questions. I had a spare Douglas 32-40 barrel blank with a 1-12 inch twist that could be devoted to a test program. A breech system was devised that would permit various firing pin impacts on the primer to be studied. It is not a fast system but permits one shot to be fired about every two minutes. Bullets are weighed, lubricated and swaged so that they are as uniform as can practically be made. All of the shots in the test were fired from my rail gun at a range of 100 yards. This test was designed to equate obtained accuracy with primer impact.
It was determined that six five shot groups under three different striker impact forces would be studied. The cartridge case was my 32-357 Magnum which was developed back in about 1971 but not really used until about 1993 when I received a barrel blank from Ken Bresein that was cut with Pope style rifling. It is the cartridge that my current competition gun is chambered for. The bullet is a 200-grain basic bullet from a Colorado Shooter's Supply mold. the bullet is subsequently lubricated and swaged before shooting. The swage is one of my own design and build. The powder charge for this test is 8.1 grains of Accurate Arms #7. Primers for this test were the Remington 7 1/2. One cartridge case was used for all shooting in this test. Powder charges were cast from a Redding M 3 BR powder measure and not individually weighed. Groups were evaluated by two methods, extreme spread of the two widest bullet holes and by the mean radius concept which studies all of the shots in the group.
In studying the velocity of all the shots fired on the record groups it was interesting to observe that about one shot in five was deviant from the group average by more than five feet per second. As most groups did not have an extreme velocity spread of more than ten feet per second this was considered significant. The deviance occurred both ways from the average velocity of the group. Some deviant shots measured a low velocity and others a high velocity. When more than one deviant shot occurred within a given group of five shots it would raise or lower the group average. Because of this, all comparisons were to the overall average velocity as all shots were fired under basically the same conditions of loading, temperature and load.
I will report the average velocities of each of the 30 shot study groups and the final
average velocity for the total 90 shots of the test.
In a similar manner I will report the average velocity spread within each set, the average
standard deviation, the average extreme spread and the average mean radius. All averages taken on six five round groups.
Phase I
Phase II
Phase III
Light
Moderate
Heavy
Average
Impact
Impact
Impact
Overall
Average Vel. (fps)
1227
1227
1228
1227
Avg. Velocity Spread (fps)
8
9
11
9
Avg. Vel. Std. Dev. (fps)
4
3
4
4
Avg. Extreme Group Spread (")
0.695
0.700
0.750
0.720
Avg. Mean Radius (")
0.281
0.252
0.276
0.270
This I believe shows beyond any reasonable doubt that accuracy is not affected one way or another by the force of the firing blow. If the primer goes off it goes off and the amount of energy transmitted to the powder is not affected by the impact energy of the firing pin. Even those primers that had to be hit twice in order to initiate ignition showed no difference from those that went off on initial impact.
While I believe that this eliminates one source of alibis for groups that are strung out up and down, it is one less thing that we as shooters of various old and new guns have to be concerned about. After completing this series of tests I can say without hesitation that accuracy is in no way affected by the force of the firing pin impact on the primer."
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