‘Final (book) report’ on my 5 month humidity-seeps-into-loaded-ammo experiment: Some surprising results. Maybe a failed experiment has some benefit for others, so worth reporting. [Apologies for the length of the post.]
History: Some 5 months ago, i loaded 40 rounds of 6.5 Creedmoor ammo (H4350, Lapua brass, 140 ELDM, weight sorted CCI450 primers), all via the FX120 - so weight is accurate to within 0.02 gn. [This is a repeat of an earlier experiment where 4 out of 40 rounds changed weight by up to 0.14 grains after loading the (unsealed) ammo and letting it sit outside for a week (humid summer conditions, US Gulf Coast).]
Objective of the experiment: Wanted to see if sealant helped or not, and to see what neck tension works best to avoid air leaks into or from the cases that could potentially change water content of the powder, thus powder weight and powder burn rate, and therefore muzzle velocity and group size/vertical at long range.
Loaded 40 cases, using mandrels/pin gauges mounted in the Porter Precision die, varying from 0.2615” (2.5 of nominal “neck tension”/interference fit if you ignore brass springback) to 0.2645” (very little actual grip, only springback of the brass provides about 0.2-0.4 thou of interference fit), in steps of 1.0 thou.
Re-weighed all 40 rounds at periodic intervals, and finally shot them over a crony (Labradar) some 5 months later:
Only two out of 40 loaded rounds changed weight by more than the measurement error (0.02 gn). Both rounds had extremely low neck tension (0.3 to 0.5 thou), both were unsealed cases, and both bullets could be rotated by hand. Could even pull the bullet out, and leave it like that to see how much weight change occurred over a 24 hour period due to humidity differences from day to day.
Clearly, this is an impractically low amount of neck tension, and the loaded ammo is not robust enough for road travel over potholes, or air travel with large pressure changes in the luggage compartment of a plane.
One round changed by 0.06 gn (maybe 6-7 fps effect) and the other by 0.08 gn (around 8-9 fps). [This change was about half of what i saw in the first experiment.] These two bad rounds would have taken an ES of 20 (good enough) to around 30 fps (not so great). SD would have moved maybe 0.5 fps (very little). In competition, this could cause two possible misses at 800 plus, depending on whether you are middle of node or edge of node.
All 20 of the sealed rounds remained within 0.02 (measurement error) in weight, even the rounds where the rather impractical 0.2645 mandrel was used and only springback was holding the bullet in place, so no air leaks. It seems the sealant helps to glue the bullet in place and prevent air exchange. This is obviously a good thing.
Then reseated all the bullets 30 thou deeper, to avoid bullet weld coloring the experiment, and measured seating force via the K&M force pack:
Unsealed rounds expanded with the 0.2645 mandrel required 7-10 lbs of force to reseat, which is extremely low, while the 0.2640 expanded cases needed 20-28 lbs and several made a audible “thunk” (sudden change in COAL when the bullet weld broke loose). The variability in force needed to reseat the bullet was 2x higher than when the bullets were originally seated. The 0.2635 and 2630 expanded necks made a louder “thunk” sound (when the bullet is breaking loose, so probably more pronounced “bullet weld”). None of the unsealed rounds reseated smoothly or gradually, like they did when originally loaded.
Sealed rounds: 0.2645 expanded necks needed 14-16 lbs to reseat, no thunk. The 0.2640 expanded necks needed 20-28 lbs, most smoothly glided in, but a few made an audible thunk sound (bullet weld). So bullet weld was less prevalent in the sealed ammo, but the problem was still there. Most reseated smoothly. The 0.2630 and 0.2635 expanded cases needed 35 to 48 lbs, which is 2-3x more variability than i am used to. The loud ones must have excessive bullet weld. So bullet weld is less prevalent in sealed cases, but not absent.
Then shot all 8 batches/40 rounds: Each batch of 5 rounds had progressively less neck tension (by 1 thou). In total i fired 20 unsealed and 20 sealed rounds:
Sealed rounds, low neck tension (0.2645 mandrel): These rounds were the most variable in terms of reseating force required (very disappointing!), and gave the highest ES (37 fps). So sealing cured the air leaks (same water content for the powder kernels), and it could reduce but not cure bullet weld, BUT it made the neck tension variability worse. So a net loss. Disappointing!
Some of the unsealed bullets seated with very low neck tension could be rotated by hand, some not. [Sealed bullets could not be rotated, glued in place with the sealant.] Speed was about 10 fps slower for the two super low neck tension batches compared to the rest, but this difference is likely not statistically significant due to the tiny sample sizes of 10 vs 30.
No significant difference in average speed between the other batches (2710-2716 fps).
Possible conclusions from this rather messy (failed?) experiment:
What we already know: Low neck tension is impractical. UNLESS you seal the neck - maybe. The glue/sealant helps to turn it into more robust ammo, and this also avoided humidity exchange with the ambient and prevented weight changes, and possibly maybe also avoided some potential speed changes. However, seating force variability went up. So still a bad idea to use very low neck tension, even if you seal the rounds.
Also as a group of 20, the sealed ammo had the worst ES. That was surprising and rather disappointing. In future, i would not bother to seal target ammo, simply leave it in a sealed ziplock bag with the opened humidor pack inside. But i would load long and reseat all the bullets to the correct COAL before shooting them. [I would still seal hunting ammo that might be exposed to rain or get dunked in a stream when you slip and fall.]
“Bullet weld” develops more frequently in unsealed cases, but both batches had this problem. Not enough benefit to justify the sealing effort. [Ammo was intentionally stored under adverse / extreme conditions for 5 months.]
It is a good idea to control powder humidity in your reloading room via a large capacity electrical dehumidifier AND a suitable piezo electric humidifier to add humidity when needed. Yes more kit required.
Sealing the rounds at the bullet / neck junction and at the primer helps with humidity changes of loaded ammo, but does not cure bullet weld, which has a more significant effect on speed. So load the bullets long and reseat the morning before the competition to the correct value. Pack an arbor press and a Wilson seater for reseating at the range. I got an SD of 11-13 fps for most of the batches, while my freshly loaded ammo usually gives 7-9 fps.
Excessive neck tension has negative effects: The 0.2165 mandrel was too small and bullet seating force was very high: Effectively the soft lead core bullet was being used to resize the neck. Pull force was no different (using a Forster collet bullet puller die and a baggage scale) between rounds expanded via a 0.2615 and a 0.2625 mandrel. Even with the correct seating stem, a variable depth indent was left on the bullet nose - which is not good for BC variability.
Tentative recommendations:
I would guess that the cigar humidty pack left inside the powder canister should help. Have “installed” these in two of my 8 lbs canisters, and will see if my ES comes down in future. Too early to tell.
Before getting on a flight to go to a competition, i can see some benefit to weighing every case and writing the weight on the case body. Then weigh again after the flight. Trouble is you need a milligram scale and nobody in their right mind is going to fly a $650 force restoration lab scale in their luggage twice a month. The cheap small battery operated milligram scales using a standard load cell may or may not be accurate enough. But worth a try.
Reseating the day of the competition will likely help if ammo was loaded a week or a month prior.
Run adequate neck tension: 1.5 to 2.0 if cases are annealed. Can go to 2.5 to 3 thou if cases are not annealed, but watch out for damage inflicted by the seating stem.
It is probably not worth sealing your target ammo. [But try it if you like and let us know.]
chronoplotter.com
another thing to worry about in the reloading process…why do I do this again?
