Back in 2021 I had an interesting experience with cast bullets bonding to .30-30 case necks, resulting in substantial velocity increase. I reported this in some detail on Cast Boolits, but do so again here, having had time to reflect on my experience.
The rifle was a Marlin 336A Rippletop, Remington cases, Federal primers, #U321297HP bullets sized to .312”, 170 gns as cast, BAC lube, Benchmark powder.
Edit: Bullets were gas checked and lubed in .323" lube sizer die, then sequentially sized in Lee .314" & .311" dies, with no further lubrication. In the light of later findings, this sizing sequence is important.
Initial shooting was with ammo loaded the night before. First up 24 gns gave 1830 fps for the clean barrel fouler, then 1782 average for the next five shots. Next trip was 26 gns, missed the velocity on the clean barrel fouler, but the group was 1898 fps – up 116 fps for two grains, about as expected. The trip after that was delayed by wet weather, so the ammo was sitting for 10 days before shooting. Load was 27 gns and I was expecting another 50 fps or so. Instead, the clean barrel fouler was 2105 fps and the group 2036 fps – up 158 fps for one grain.
I took the unfired 27 gn cartridges home and pulled them (collet). The first one tried WOULD NOT BUDGE. Then I bumped them in a few thou with the bullet seater preparatory to pulling. Each gave a distinct crack as the bond broke. After that they pulled normally.
Investigating further, I shot some 26 gn loads that had been “aged” five days. This load when shot fresh had averaged 1898 fps. The same load now “aged”, and refusing to pull, gave 1939 for three shots – another 41 fps just by ageing the ammo five days.
Case neck bonding can be clearly demonstrated either by attempting to pull the bullets with a collet puller, or by applying pressure with the bullet seating die until the bullet moves. A newly seated bullet will seat further without argument. A bonded bullet will at first resist and then let go with a pronounced crack.
In subsequent tests I found that bonding was almost universal, that it could set up in just a few days, that it was not necessarily uniform, that it re-established when reseated ammo was allowed to stand further and retested, and that it happened with all of the lubes I tried except my ersatz version of Speed Green. Lyman Orange Magic appeared to be a partial exception.
The most extreme version of bonding I demonstrated was seating an unlubed bullet in a wiped clean case neck. Using the reseating test, a newly seated bullet would continue to move, while a bullet that had been seated a few days earlier would be almost impossible to move.
This latter finding leads me to speculate that the bonding is actually between brass and lead that have been wiped clean in the various sizing and bullet seating operations. Avoiding bonding would then require a lube that resists such wiping. Alternately, the lube itself may be contributing to the bonding.
As to the effect on velocity, I may have been alerted to the issue by an extreme instance. Retesting with subsonic .30-30 loads did show some effect, but it was small and probably insignificant. Nor have I been able to prove an effect with my full power .30-30 hunting loads using 748, LVR and Varget powders (though not with BAC lube).
Applying the precautionary principle however, I now ensure that all ammo to be tested for velocity, POI etc is aged at least a week before I test. I suspect that not much changes after that first week.
For serious grouping there may be an advantage in loading, or at least completing bullet seating, immediately before use. Alternately lubing with an oversize die and seating in unsized necks may confer an advantage.
You are only as good as your library.
