Black Powder Cartridge Neck Tension Follow Up

Previously, I ran a series of tests with four different neck tensions that ranged from 0.001 to 0.004 inches under the bullet diameter. The findings were that the lightest neck tension with my 38-50 CPA Stevens 44 ½ rifle resulted in the best accuracy with 99 percent confidence. This was reported in Black Powder Cartridge News (BPCN) Issue #118, "A Black Powder Cartridge Neck Tension Test." I continued to think about the test and did not try anything beyond neck tension 0.001 inches under the bullet diameter. Perhaps no neck tension would result in better accuracy. I tried the no-neck tension many years ago but decided to give it another test to ensure there wasn’t an optimal neck tension I was missing.

INTRODUCTION

For clarity, I should share what terms I use when describing neck tension. First, there is “interference fit”. We have a cartridge case neck smaller than the bullet diameter, which requires force to seat the bullet into the case. Once the bullet is seated, the cartridge case and bullet act as one unit. The bullet cannot be pulled from the case with the fingers, and it is unlikely that the bullet will spin. Secondly, there is a “clearance fit”. The inside diameter of the cartridge case neck is equal to or larger than the bullet. There is no force required to seat the bullet outside of a bit of finger pressure. The cartridge case and the bullet can move relative to each other and do not act as one unit. One can usually remove the bullet from the cartridge case by pulling it out with one’s fingers. Some reloaders describe the clearance fit as “slip fit” since the bullet slips in and out of the case.

Meticulously, I selected three specific neck tensions for the current experiment: a minimum interference fit, a sized slip fit, and an as-fired slip fit. These choices were not arbitrary; instead, they were made to ensure a comprehensive understanding of the impact of different neck tensions on accuracy. By selecting these specific tensions, I aimed to eliminate any potential ambiguity in the results, providing a clear and precise analysis of the effects of neck tension on accuracy.

My method for my match ammo involves a minimum interference fit. This is achieved using an expander of 0.377 inches, a modified gage pin, resulting in an inside case neck diameter of 0.376 for a 0.376 bullet. My gage pins limit me to 0.001 increments; all the expanded cases accept 0.375 gage pins, and most accept a 0.376 gage pin. This results in a 0.001 neck tension. There is a slight variation in the final inside diameter due to the variation in the brass neck thickness and, perhaps, brass hardness.

The sized slip fit cartridge cases were generated by sizing clean, annealed cases, which were expanded with a 0.378 expander, resulting in a neck inside diameter of 0.377, creating a clearance fit of 0.001. These cases were not belled since the expansion is just enough to allow the bullet to enter the case with light finger pressure and minimal run-out. I know of two national champion BPCR shooters who have used this method. When questioned, they called it a “sized slip fit” since it required sizing and expanding to get just enough clearance for the bullet. Obtaining the detailed information was difficult, but I endeavored to persevere. They also used fairly high velocities, greater than 1,300 fps.

Some BPCR reloaders prefer to reload un-sized cases as fired, resulting in an as-fired slip fit. These are the same for our purposes: fired cases are cleaned with no sizing, and the cases are primed and filled with an appropriate amount of powder, a wad and then compressed. The bullet is then easily slid into the case. There is no neck tension; my fired cases would accept a 0.378-gauge pin and sometimes a 0.379 pin. This is .002 to 0.003 of clearance between the case neck and the bullet. I will reference these as “fired slip fit”.

As part of the loading process, I measured the bullet run-out for the three loading methods. The results are in Table 1.

The fired slip fit cases were difficult to conventionally measure for run out. It was necessary to tip the bullet from one extreme to the next to measure its movement within the case. It’s not a great fit; a “wobble fit” would be a better description. I thought these might self-align when loaded into the rifle’s throat and help mitigate the wobble. This is, of course, dependent upon the cartridge’s overall loaded length, bullet seating depth, dimensional match between the bullet and bore, bullet hardness, and the degree and hardness of any fouling that might be present. Likewise, the sized slip fit and the minimum interference fit should also self-align since we don’t have the condition as shown in Figure 1.

It should be kept in mind that when measuring bullet run-out, we are not just measuring bullet alignment with the case. We are also measuring variations in cartridge neck thickness, the cylindricity of that portion of the bullet within the cartridge neck, and the mechanical accuracy of our dies used in sizing and seating. Proper brass cleaning, quality dies, and lathe-turned bullet moulds can help achieve high-quality “straight” match ammo.

TESTING METHODOLOGY

All ammo was loaded at the identical overall length with the same Paul Jones Money bullet cast from my 20:1 lead alloy. A total of 57.8 grains of 2Fg Swiss powder with a 0.090 LDPE wad and a CCI Large Pistol primer were used. Each powder charge was compressed, 0.200 inches, to the same depth below the case mouth, 0.350 inches, before seating the bullet by one of the three described methods. All bullets were sorted by weight to ensure they were all within a +/- 0.5 grains maximum range. Ninety cartridges, plus foulers and sighters were loaded for the test.

Thirty shots were fired for each of the three neck tensions to ensure we had a large enough sample size to be statistically valid. Shooting was completed over two days. The firing consisted of one five-shot group from each of the three tests. This was done to avoid shooting one neck tension test under a better condition than the other tests. The three tests were shot with a certain amount of randomness to prevent bias. This means three, five-shot groups were shot sequentially: neck tension #1, neck tension #2, and neck tension #3. For the following sequence, the order was switched to #3, #1, #2, # and then #2, #3, and #1. All targets were carefully labeled before stapling to the target holders. I could read the target labels using a ground spotting scope to ensure I was on the correct target. All testing was completed from the prone position using cross-sticks. Before starting a testing string, a couple of fouling shots were fired. After each shot, two microfiber patches saturated with 90 percentage water and 10 percentage water-soluble oil were used to wipe the barrel. A mop was used to dry the chamber before loading another cartridge. Being very selective, I shot only on relatively calm mornings to avoid being a victim of the conditions.

ANALYSIS

After the firing, we had six, five-shot groups for each of the three neck tensions tested. If we were to measure the groups, this would only give us information about the two widest shots in each group and ignore the size or “quality” of the other three shots. I prefer using a modified string measurement method. This technique has been described in prior Black Powder Cartridge News articles. The center for each group is determined by summing the distance from the lowest shot to all the different shots in the vertical direction. Averaging these gives us the group center in the vertical direction. The process is completed in the horizontal direction by measuring the distance from the furthest left shot to all the different shots in the horizontal direction. Summing these measurements and averaging gives us the horizontal center. Measuring the distance from each shot to the calculated group center is simple. The sum of these measurements provides us with the string measurement. This process is not as complicated as it might sound and can be completed with a No. 2 wooden pencil and a ruler.

The average group sizes are presented in Table 2, and it can be seen that the inside neck diameter of 0.376 and minimal interference fit indicate the best results. Since group sizes are based on the two widest shots, I do not feel they give the best information.

It was a struggle to figure out the best way to present the mean radius data from the completed testing; I arrived at Graph 1, which gives an excellent visual representation of the results. Along the x-axis are listed the inside neck diameters, and all the dots stacked in the vertical direction represent each shot. For each inside neck dimension, there are 30 dots representing 30 shots. Counting them all is difficult, as some are superimposed on each other.

A careful review of Graph 1 shows that out of 30 shots for each of the three tests, two shots were two inches or more from the group center for the minimum interference fit, seven shots were two inches or more from the group center, and the fired slip fit resulted in eight shots that were two inches or more from the group center. Again, all shots may not be seen on the graph as some are the same distance from the center and will plot on top of each other.

The average distance from the group center was 1.02, 1.5, and 1.5 inches for the minimum interference fit, sized slip fit, and as fired slip fit, respectively. Comparing the sized slip fit to the as-fired slip fit, the average distance from the group center was the same. The as-fired slip fit did have a few more wide shots, as seen by the number of shots in Graph 1, which are greater than 2.5 inches from the center.

As shooters we can read and adjust for the wind. We cannot adjust for random vertical dispersion. I constantly review my data to determine if any test might exhibit more vertical dispersion than another. A one-way ANOVA table can analyze three or more tests for vertical dispersion. This was completed, and I can confidently state with 92.5 percent confidence that the minimum interference fit has less vertical dispersion than the other tested methods. This does not rise to the gold standard of 95 percent confidence, so the reader must draw his conclusions. The sample groups presented for the minimum interference fit, sized slip fit, and as-fired slip fit indicated the rounder groups from the minimum interference fit. In contrast, the sized slip fit has some vertical. Contrast these with the as fired slip fit; the vertical tendency is apparent.

CONCLUSIONS

I do not plan on doing any more neck tension tests anytime soon. After coming full circle, I will stick with an inside case diameter of 0.376 inch for a 0.376-inch bullet. As an item of interest, my 45-90 Creedmoor rifle uses a 0.459-inch Paul Jones bullet. I have found that a 0.460-inch expander, which expands the case to 0.459 inch, gives the best results. S