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B. target b, one shot from Group 2 C. target c, one shot from Group 3 D. target a, one shot from Group 1 Etc. until three five-shot groups
are completed, then repeat the entire process, excluding step 1, for three more five-shot groups. Cartridges would be fired in the order outlined above in order to equalize the effects of climatic changes, barrel fouling, shooter fatigue, etc. Additionally, all loaded rounds would be checked for and marked with bullet run-out and all five shots in a five- shot group would be shot with the high side in the same location in the chamber. I would then return home, clean
the rifle, prep and reload the 33 cases, and then, on a suitable day, repeat the entire shooting process – at which time I would have 12 five-shot groups for evaluation – four groups for each of the three rim thicknesses. Case selection presented a minor
problem. It was easy to select cases for Groups 2 and 3 from existing stock but, as stated earlier, all cases having rim thickness of 0.057" and less had been thrown away. Therefore, for Group 1 cases I selected some having a rim thickness of 0.058 inch and modified them by reducing their rim thickness by 0.003 inch, then recutting their primer pockets. Rim thickness reduction was done by hand using 200 grit, then 400 grit, silicon carbide sandpaper. This same procedure had previously been done with no ill effects (except to my fingers) on cases having rim thickness greater than 0.064 inch. I don’t necessar- ily recommend reducing rim thickness on rimmed cases and you are on your own if you elect to do so. In summary, I now had cases
meeting the criteria as shown in Table 1. I wondered what the CHS, par-
ticularly on the 0.055 rim cases, would be after they are fired two times. The following information is offered to help you make a determination: (1) The estimated pressure for the
accuracy load in the rifle being used is 36,000 CUP. This is based on pressure ring measurements, velocity, powder
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burn rate, case life, and overall case appearance. (2) When fired, all cases and the
rifle chamber essentially will be clean and dry. (3) As determined by placing steel
spacers between the bolt face and the case head, the bolt on the rifle being used will close, with effort, on cartridges having a rim thickness of 0.068 inch. Very slight drag can be felt with rims measuring 0.067. This means excessive headspace of
0.013 inch for the 0.055 rim cases (0.068 minus 0.055). Surely, case head separa- tion will start or occur – or will it? We all will soon find out. After all shots are fired, I would
measure the group sizes and determine the center point for each of the 12 five- shot groups. Group sizes would let me know if varying rim thickness affects accuracy and group center points would let me know if varying rim thickness changes point of impact. The resulting data may prove adequate to answer the questions or it might indicate additional data are necessary. For those interested, the accuracy
load for the 0.6 MOA rifle is as follows: 12.7 grains VihtaVuori Oy N 120
powder (1)
Remington 61/2 primers Hornady 40-gr. V-Max bullets Remington brass Bullet seated 0.060 off lands (DOL) (1) This is a compressed load
which is safe in the rifle I shoot it in and as I load it. It may not be safe in
Group 1 No. Cases
Rim Thickness Times Fired
Typical Neck Runout Length
Weight Variation
Primer Pocket Depth Case Head Separation
(Thin Rims) 11
0.055" 4
0.001 1.391" 0.8 gr. 0.120" 0.000"
your rifle(s). It is not suitable for load- ing in Winchester cases because of their smaller internal capacity as compared with Remington cases. Based on 25 chronographed shots
with this load, the velocity in my rifle averages 2,710 fps from its 22-inch bar- rel. Browning advertises this rifle as having a 20-inch barrel but it just ain’t so. I generally use a different load when shooting prairie rats in order to increase velocity to around 2,850 fps. I previously noted that I might,
on one or more occasions, deviate from the main topic to discuss other topics of general interest or importance. Consider yourself warned. Some (many?) of you may be
surprised that the accuracy load above shows a DOL of 0.060 inch. This is not the most accurate DOL for this rifle/ load but is desirable and/or necessary for several reasons including: (1) The leade in this rifle is some-
what longer than desirable. This seems to be true in 90 percent of the rifles I have purchased in the last 20 years, based on the bullets I normally use. (2) In an effort by bullet manufac-
turers to increase ballistic coefficients (BC), many modern bullets have long ogives, long boat tails, and short bear- ing surfaces. This exacerbaTes (1) above. Many bullets available today have
full diameter lengths considerably less than 1 caliber long, with some approach- ing one-half caliber. The desirable or required length of bullet in direct and
TABLE 1 Group 2
0.060" 4
0.001 1.393" 0.8 gr. 0.120" 0.000"
Group 3
(Medium Rims) (Thick Rims) 11
11
0.064" 2
0.002 1.394" 0.7 gr. 0.120" 0.000"
www.varminthunter.org Page 135
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