Just out of pure curiosity I have tested the 5 rifle cartridges I have in my gun locker: 243Win, 6.5x55, 30.06, 308Win and 9.3x62 from manufacturers Viking Arms Sweden (30.06), Tikka (6.5x55 & 308Win) & Schultz & Larsen (243Win & 9.3x62).
I have selected ONE load for each cartridge and made between 10 and 20 rounds for each rifle based on cases with the widest weight spread I could find in my inventory, up to 22grs difference but generally less. Case brands are Norma, Lapua, Remington, RWS and Sellier & Bellot
I weighed all the cases, FL sized cases, loaded the rounds, fired/chronographed same and finally measured the fired case volumes. The resulting Case Weight vs Case Volumes are plotted in Figure 1.
All the plotted trends for each cartridge are close to parallel, with roughly similar slopes ranging from 7.3 to 8.3. This coefficient's unit is grains of brass per grains water and if we convert to metric specific gravity, SG unit = grams per cubic centimeter, we end up with the same number we started out with, so the slopes or coefficients in this plot point towards the indicated SG for the case brass used. That's why all trends turn out close to parallel.
Figure 1. Case Weight vs Case Volume.
With respect to how to assess the case volume measurements I am unsure how to treat the fact of elastic springback of the brass. The fired cases are not perfect casts of the rifle chambers; That is difficult to achieve. I guess one should neck size and fire continuously until sticky bolt lift and then measure case volume. I didn't bother doing that since I do not have NS-dies for other than 308Win and 6.5x55. However a rough estimate would dictate springback to be of similar value for most if not all cases (?).
I think (!) it's most likely that all the cases are made of quite similar brass alloys with comparable specific gravity and the variance in indicated brass SGs are most likely due to small sample numbers for each cartridge.
I believe that case weight is a very good indicator of case volume during firing since the case will be blown to cast or match the chamber inner dimensions more or less, but it will also jump back to various degrees. This is more or less supported by the good correlation between velocity and case weight show in Figure 2. The slope in this diagram is meters per second per grain brass, and yields the numeric consequence of changing the case weight by 1 grain in meters per sec. The higher, steeper the slope (=higher coefficient) the more sensitive to case weight variations. We could call this slope 'case weight sensitivity' of the cartridge (USING THE PARTICULAR TEST LOADS SHOWN IN TABLE 1).
Figure 2. Velocity vs Case Weight. 243Win was chronographed with Prochrono Digital and the rest using a Magnetospeed V3 device.
In sequence from most to least sensitive to case weight variations is 243Win -> 6.5x55 -> 30.06 -> 308Win -> 9.3x62. This sequence makes me wonder about overbore index, since 243Win is more of a barrel burner compared to 308Win and 9.3x62.
The case weight sensitivity vs Overbore Index for each cartridge is shown in Figure 3. This limited dataset could suggest that a higher Overbore Index is associated with higher sensitivity to case weight variations.
I am left with the feeling that this could be why the Overbore Index is in some cases a good indicator of inherently accurate cartridges, because the lower the Overbore Index also correlates with a lower sensitivity to e.g. case weight variations and therefore less vertical spread on longer distances as a penalty for not sorting by case weight. Just a thought….
Figure 3. Case Weight Sensitivity vs the Overbore Index for the cartridges investigated.
I just wanted to share my results here
Regards,
Pemo
I have selected ONE load for each cartridge and made between 10 and 20 rounds for each rifle based on cases with the widest weight spread I could find in my inventory, up to 22grs difference but generally less. Case brands are Norma, Lapua, Remington, RWS and Sellier & Bellot
I weighed all the cases, FL sized cases, loaded the rounds, fired/chronographed same and finally measured the fired case volumes. The resulting Case Weight vs Case Volumes are plotted in Figure 1.
All the plotted trends for each cartridge are close to parallel, with roughly similar slopes ranging from 7.3 to 8.3. This coefficient's unit is grains of brass per grains water and if we convert to metric specific gravity, SG unit = grams per cubic centimeter, we end up with the same number we started out with, so the slopes or coefficients in this plot point towards the indicated SG for the case brass used. That's why all trends turn out close to parallel.
Figure 1. Case Weight vs Case Volume.
With respect to how to assess the case volume measurements I am unsure how to treat the fact of elastic springback of the brass. The fired cases are not perfect casts of the rifle chambers; That is difficult to achieve. I guess one should neck size and fire continuously until sticky bolt lift and then measure case volume. I didn't bother doing that since I do not have NS-dies for other than 308Win and 6.5x55. However a rough estimate would dictate springback to be of similar value for most if not all cases (?).
I think (!) it's most likely that all the cases are made of quite similar brass alloys with comparable specific gravity and the variance in indicated brass SGs are most likely due to small sample numbers for each cartridge.
I believe that case weight is a very good indicator of case volume during firing since the case will be blown to cast or match the chamber inner dimensions more or less, but it will also jump back to various degrees. This is more or less supported by the good correlation between velocity and case weight show in Figure 2. The slope in this diagram is meters per second per grain brass, and yields the numeric consequence of changing the case weight by 1 grain in meters per sec. The higher, steeper the slope (=higher coefficient) the more sensitive to case weight variations. We could call this slope 'case weight sensitivity' of the cartridge (USING THE PARTICULAR TEST LOADS SHOWN IN TABLE 1).
Figure 2. Velocity vs Case Weight. 243Win was chronographed with Prochrono Digital and the rest using a Magnetospeed V3 device.
In sequence from most to least sensitive to case weight variations is 243Win -> 6.5x55 -> 30.06 -> 308Win -> 9.3x62. This sequence makes me wonder about overbore index, since 243Win is more of a barrel burner compared to 308Win and 9.3x62.
The case weight sensitivity vs Overbore Index for each cartridge is shown in Figure 3. This limited dataset could suggest that a higher Overbore Index is associated with higher sensitivity to case weight variations.
I am left with the feeling that this could be why the Overbore Index is in some cases a good indicator of inherently accurate cartridges, because the lower the Overbore Index also correlates with a lower sensitivity to e.g. case weight variations and therefore less vertical spread on longer distances as a penalty for not sorting by case weight. Just a thought….
Figure 3. Case Weight Sensitivity vs the Overbore Index for the cartridges investigated.
I just wanted to share my results here
Regards,
Pemo
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