I started my analysis with the bipod prone position. I did not have a rear bag on hand. Once again, my rolled up nomex flight gloves (I don’t just shoot my rifle- I fly it) came to the rescue in the form of improvised rear support. The ammo throughout the test, was a Black Hills loading of the Hornady 155 grain A-max bullet with an average muzzle velocity of 2684 from my rifle. I used my FN PBR-XP with a pillar bedded McMillan A5 stock, Bartlein 20” Remington Varmint 1-10” twist barrel, and my usual SWFA SS 3-9×42 scope. A Near base and Seekins rings provide the scope to receiver interface.
My target backer turned out to be 204 yards from my shooting position. I tried for exactly 200 but had to settle for where I could put the backer and find a flat spot to shoot. It was approximately 80 degrees and the density altitude was approximately 4500. It was a calm day with a very little wind 1-2 mph coming from 5:30 to 6:00. The FDAC and the iPhone ballistic program Shooter had a disagreement on the necessary elevation correction. I usually use the iPhone and remembered it being a little off, so I went with the FDAC, and a 0.6 mil correction.
I shot all three ten shot groups in the span of approximately a half hour. I was trying to give the barrel an opportunity to cool down to a reasonable temperature that would not induce any mirage, but I would have needed to wait longer. I only had so much time to spend shooting.
I began to discover with the first ‘control’ group, that 30 seconds between shots is a long time. It’s difficult to ride the correct balance between taking too long, keeping the barrel as constant in temperature as is reasonably possible, maintaining sufficient comfort to shoot one’s best, etc., etc… It’s a decent time interval for a slow fire group.
Here is the resulting group:
The time stress group in the photo below was evidently more comfortable for me in this position, as I shot a bit better. The average split time between shots excluding loading or reloading, was 5.72 seconds, the high being 6.76 and the low 4.41. The average split time of the entire testing of all the positions was 6.53 seconds. I think that the split time is an indicator of how easy it is to acquire and maintain an acceptable sight picture in the position, as well as work the bolt. This position is easy in both regards.
With exertion added, my group degraded a bit as is expected. I did 73 jumping jacks in a minute and it took approximately 23 seconds to get my 20 good pushups in. I had a problem with the iPhone shot timer, in that it stopped recording times at shot #2. My stopwatch indicated a total time that was approximately 7 seconds slower than the time stress without exertion added.
After all the number crunching was done, pretty much all of it by computers, here are the distances I came up with. I have two graphs. As I explained in the previous article, I set one distance limit at 86% and another more stringent limit at 99%. These represent the statistical predictions of distances at which 86% and 99% of my shots will land within the 4” target, assuming that I account for wind correctly and that my rifle is perfectly zeroed (a significant assumption- and the reason you see shots outside the black at a lesser distance).
Another thing that I’m keeping track of is how much the position degrades, if at all, as stressors are added. That should give me an idea of whether it’s only useful for shooting static targets on the range when all is well, or if the position also works well in less than optimal situations. I’m using the slow fire group as a baseline, and comparing the performance of the time stress and time stress exerted groups as percentages of that performance. In this case the time stress group was better and actually added 115.74% effective distance. The time stress exertion group gave me effectively 89.95% of the effective distance in comparison to the slow fire group.
This was the only position in which the shot group improved under any of the stressors. The 10.05% degradation in group size under exertion was small in comparison to the amount that other positions suffered. This is to be expected when the ground is doing more work than the shooter in supporting the rifle. Surprisingly, this position did not yield the greatest precision. You’ll just have to wait and see which position did.