Will Carbon Build-Up Inside Cases Raise Load Pressure?
As a cartridge case is reloaded multiple times, burnt powder residue and carbon builds up on the inside of the case. Unless the case interior is cleaned in some fashion, eventually you’ll see a slight reduction in case capacity. One of our Forum members from Australia wonders about the effects of reduced case capacity: “If the capacity of the case decreases as the crud builds up, then it effectively reduces the size of the cartridge (inside). Wouldn’t that change the pressure produced from that of an equivalent clean case?”
Interesting Test of Case Capacity Changes
Forum member Fred Bohl has actual test results that can help answer the above question. Fred proved that, over a 20-reload cycle, the case capacity of uncleaned cases did decline a small amount. However, surprisingly, this did not seem to affect the actual chronographed velocity of the load. Extreme Spread (ES) did increase, but Fred believes the higher ES was due to changes in case-neck tension, rather than due to the slight reduction in case capacity. Fred reports:
“Back when beginning to use ultrasonic case cleaning, part of the motivation was to get the inside clean based on the assumption that allowing burnt residue to build up inside cases would affect capacity, and, ultimately, performance. An experiment was done to test this hypothesis. The load used, 30.5 grains of RL15 behind 107gr SMKs in a 6mmBR, was selected for best group and lowest ES in prior load development. It turned out to be 92% of initial case capacity and neither “full” or compressed. (I would suspect that different powders, load weight, and total case capacity might produce very different results.)
We took 30 cases of identical initial capacity and tracked three lots of 10 each:
LOT 1: No Internal cleaning
LOT 2: Cleaned with media in tumbler
LOT 3: Cleaned with Ultrasound machine
Each case (in each lot) was shot and reloaded 20 times. The simplified results after 20 reloads of each lot were as follows:
Lot 1 (not cleaned) – 0.3 to 0.4 gr. loss of capacity, 5 to 8 fps greater ES.
Lot 2 (tumble cleaned) – 0.1 to 0.3 gr. loss of capacity, 4 to 6 fps greater ES.
Lot 3 (ultrasonic cleaned) – no loss of capacity, no detectable change in ES.
FINDINGS
There was no detectable correlation of velocity change to the lots. An oddity was that on very hot days Lot 1 velocities were, occasionally, slightly higher. From results of another ongoing test, I believe the above differences in ES are probably due more to variance in bullet grip tension than case capacity. The ultrasound cleaned cases (LOT 3) did maintain the lowest ES, but we are not 100% sure of the reasons why. More consistent bullet seating might be the reason.”
Editor’s NOTE: Fred’s results do suggest that carbon build-up inside the uncleaned cases might cause a slight increase in pressure that shows up on hot days. Fred has posted that: “A local shooter reported doing the 20 reload, no-clean test on a .308 that gave a loss of capacity of 2.0 grains, doubled ES and significant velocity changes. However, I don’t have any details on his load weight or powder.” Obviously a lot of carbon can build up with 20 reloads. Many shooters retire their brass before then.
Ultrasonic Cleaning and Neck Lube
Some time ago, Jason Baney did a lengthy test on ultrasonic cleaning. Jason found that with his ultrasonically-cleaned cases, the inside of the necks got so “squeaky clean” that he needed to use dry lube in the necks. Jason uses the $10.95 dry lube kit from Neconos.com. This applies ultra-fine Moly powder to the neck using small carbon steel balls.
Similar Posts:
- Can Carbon Build-Up Inside Cases Alter Pressure?
- Ultrasonic Case Cleaning — Does It Really Help Maintain Load Consistency?
- TECH TIP: Dry-Lube for Ultrasonically-Cleaned Case Necks
- Ultrasonic Case Cleaning–Tech Tips
- Two Tips for Better Ultrasonic Cleaning
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Tags: Brass Tumbling, Carbon, Case Tumbling, Pressure, Pressure Increase, PSI, Reloading, Ultrasonic
Let’s think this through. The chief reason carbon residue inside a case would increase pressure would be that there is sufficient carbon to reduce the internal volume of the case. Here’s a relatively easy check. Weigh each case with the presumably evil carbon inside it. Run the case through some process that removes the carbon – either ultrasonic or with stainless pins. Weigh the cases again. The difference is the mass of the carbon inside the case. We can then use the ideal gas law to compute the pressure differentials attributable to reduced internal volume. My best guess – without going through the exercise – is that the calculations will show that the effect on velocity and pressure owing to carbon buildup will be approximately zero.
An important side issue is this – how many loadings are most of us going to get from our brass -with or without cleaning? Most of my brass is going to be pretty well cooked before there’s any chance of carbon buildup being an issue.
At least for me, carbon inside the case neck allows for a more easier and uniform seating of the bullet. I have had cases that were cleaned with SS media and upon seating the bullet….the inside case neck galled the copper jackets.
Editor: Yes, we have discussed this in a couple articles. Ultrasonic cleaning can also leave the neck interiors in need of lubrication. Some guys dip the neck in a dry lube. Others may run a lubed nylon brush inside the necks.
Was annealing every used in this study?