Bullet driving bands shrunk. Observation.

[Bird]

Thought I would relax and cast up some bullets for a couple of hours.
Loaded the pot with clip on wheel weight alloy and prepared to cast some RCBS 45-300 fn bullets. This mold has always cast beautiful, consistant bullets since I bought it.
After the first 5 or 6 casts, I started dropping them in water for added hardness. I measured the rear driving bands, and they gave me a reading of 0.4583, which is normal and good as I run through a 0.4585 sizer.
I stopped casting to have a cup of tea and decided to take a closer look at the bullets and measure a few more. The smallest rear driving bands were 0.4579, which points to an uneven casting practice. I then measured the front driving band, which were at 0.4568, no good. Every bullet had a forward driving band of approx 1 thou smaller than the rear band.
I took the mold apart and scrubbed it out, reassembled it and cast some more. Same problem again. I checked previous bullets I had cast some months ago, and both driving bands were exactly the same at a little over 0.458 each. What was going on?
The coin dropped. The good bullets were air cooled, but the bad bullets were water dropped. I cast more bullets and air cooled them, and everything returned to normal. No more reduced forward driving bands.
From my experience with this, it appears The greatest shrinkage occurs at the hottest part of the bullet when water dropped, which is the middle of the bullet which cools the slowest in the mold. Quite logical really.
I thought it worth while to post this observation, as the undersized forward band would not engage the rifling sufficiently, and may cause problems.
I searched the forum, and found no specific mention of this. If this bullet cast sufficiently oversize, then this would not be a problem when resized to fit the bore. No more water dropping for me with this mold, if I need larger or harder bullets I will have to add lino.
I hope this will be of use to others.

[Wayne Smith]

Thanks. I'm curious how big the boolit needs to be for this to happen. .30? .366? .375? I'm guessing it has more to do with boolit diameter than weight.

[44man]

Need to age them for a time, they will expand.

[runfiverun]

the amount of antimony in the alloy will determine how much.
let them sit for a couple of weeks and measure them again.

[Bird]

I just took some measurements from other bullets that I cast 5 months ago.
Same batch of coww lead. I did not take front band measurements at the time of casting. The forward band is 0.0003 smaller in diameter than the rear band. This is much better than my last freshly cast bullets. The rear bands are at 0.4587, which is also larger than the last batch.
Do bullets grow? I think so. Maybe the hottest part of the bullet grows even more, which may account for the smaller variance of the front band in the matured/aged bullets.
I will keep an eye on this, and take more measurements in the future.

[Bird]

Wayne, I think you are correct, but I think it happens to a lesser or greater degree with all bullets. It just happens with my particular mold that the front driving band is situated in the hottest part of the mold. If it were a single cavity mold maybe the problem would be less severe, but I would suspect that there would still be some variance.
Its all just an observation, and I am still learning.

[blackthorn]

Interesting--I wonder if the same thing would happen if the bullets were allowed to air cool and then treated in an oven and quenched?

[Bird]

Interesting--I wonder if the same thing would happen if the bullets were allowed to air cool and then treated in an oven and quenched?
I think that would be a better way to go, and would produce a better bullet, at least for molds that barely produce enough bullet diameter. Either that or just simply keep adding lino.

[runfiverun]

or lap the mold out a thou or so.

using the oven will for sure make a more consistent batch of boolits.

[Bird]

I may even try lapping. 1/2 a thou would probably do.

[fredj338]

Unless you are casting with identical alloy & identical casting temps & water temp when dropping, hard to compare something done his week with last month or last year.

[Lagamor]

Interesting thread...makes me think I should size and reload shortly before I shoot for better consistency.

[Bird]

Unless you are casting with identical alloy & identical casting temps & water temp when dropping, hard to compare something done his week with last month or last year.
Yes, I agree. The lead is from the same batch, and the pot temp is controlled with a pid, and the casting rate is close to the previous batches, but there are always variables.
I will make up a test batch to measure over a period of time.

[sw282]

Very interesting thread..btw, l size and lube as l load and am pleased with its benefits