Can Carbon Build-Up Inside Cases Alter 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 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 chamber size. Wouldn’t that change the pressure produced from that of an equivalent clean case?”
Ultrasonic Cleaning Example:
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, indeed, decline a small amount. However, surprisingly, this did not seem to affect the actual chronographed velocity of the load. 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 1 (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. [Editor’s note: That does suggest that the 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 signficant velocity changes. However, I don’t have any details on his load weight or powder.”]
NOTE: 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 comment: 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:
- Ultrasonic Case Cleaning — Does It Really Help Maintain Load Consistency?
- Will Carbon Build-Up Inside Cases Raise Load Pressure?
- TECH TIP: Dry-Lube for Ultrasonically-Cleaned Case Necks
- Ultrasonic Case Cleaning–Tech Tips
- Two Tips for Better Ultrasonic Cleaning
Tags: Cartridge, Case Capacity, Reloading
I’d be curious whether say 25 firings or so through a new case without cleaning the inside adds any measurable weight to it.
The term ‘capacity’ here was never defined. So “0.3 to 0.4 gr. loss of capacity” means what? H20 capacity?
Your capacity measure with heavily fired & sized cases was really so consistent and accurate that you could define it like this? And yet, not define it at all in reporting of results..
Seems Hokey. Like a stretch to find value in all the over cleaning, but while having to re-foul necks to get best seating -which we already have with naturally fouled cases..
When we did the test we determined the change of volumetric capacity of the cases and reported that volumetric loss as the equivalent to the grains of the RL17 powder density. So more simply we did not measure the case weights.
However, the residue left in the not cleaned case interiors is mostly carbon and about 2 to 3 times the density of the powder. Therefore I would estimate the case weights would increase by 2 to 3 times the reported loss of capacity in grains.