Sorry that I don't have any personal experience with the .375, as I think it should be a fine cast bullet gun
The new Lyman book list only IMR 4198 of the "IMR" powders in their info, but Davers link will likely provide others.
Now, with good alloy, lubes, proper fit of the bullet to the bore, and possibly a gas check I would not hesitate to use data from, for example the Hornady manual which lists IMR4064 and IMR4350 beginning at velocities of 2100fps.
The Nosler book list IMR4831.
The Hornady and Nosler info is for jacketed bullets, but I'd not fear using it for cast.
Now, do you need that velocity, not likely and in fact I found a Wide Flat Nose 355gr bullet at 2300fps from a 45/70 to be overly destructive.
BUT!!! with the smaller meplat of the .375, that damage is not so likely.
I fired that 355gr cast at 2300 - 2500fps in the 45/70 with minimal signs of leading, so would not be afraid to shoot cast in that velocity range in the 375. Not at all.
With the 45/70 and it's larger meplat, I find the 465gr at 1650fps to be much more to my liking. Great put down on deer and elk and much less meat loss.
Don't cast your bullets overly hard, as that can lead to failed bullets. Wheel Weights or even WW/lead at a 50/50 ratio is likely plenty hard. Water quenched if you want them a tad Harder.
Be sure and allow your bullet to "age" for at least 7 days (14 is better) before use.
AND!!!! make sure your bullets are not too small for your bore size.
Well I been shooting "Cast Hardened Bullets" for several years with very good success, 3 & 4 inch groups with lever guns (444, 45/70 etc) never a complaint really with open sights and a rest to shoot from.
However, I did learn a thing or two from Marshalls book about slugging the barrels and "fire-lapping" them also. It opened up an all new picture, as far as accuracy goes. Now with my .444 Marlin, the groups have shunk up to less then 2 inches at 100 yds off the bench. My younger friend was doing the shooting during the session, he is 46 years young.
We put the Leupold 2 x 7 on the lever gun and watched it wipe away those 3 & 4 inch groups, to down under 2-MOA. Most were in the 1.5 to 1.75 MOA area. Once again I was not shooting just looking through the spotting scope. So I think cast bullets can surely shoot as good as copper clad and sometimes even better.
4895 is good for reduced loads down to 60% case fill, which in the H&H should be around 45 grains. This charge behind a 270 gr lead gaschecked bullet should give about 2000 fps. Start here and work up to 50 grains, looking for your best accuracy.
If you just want to go as close as you can to factory ballistics, but avoid the use of factory jacketed bullets, you need the hardest alloy you can get, gas checks, and a die to seat them on the bullet. A possible alternative to all of this would be an ungrooved, paper patched bullet which starts out about a hundredth of an inch undersized to allow for the paper.
But if you just want a bullet for casual amusement or use on game from small deer downwards, you can miss out on some of that velocity in the interests of cheapness and convenience, and you may find bullet hardness, which you can now afford to reduce, is not desirable.
One thing you need to do is slug and measure your bore, or push a bullet into the rifling and see if it leaves any spaces for light to leak past in the lands. If this is the case, there is a good chance that a moderately soft bullet (say lead plus 10% tin) will expand to seal the bore under the first blow of the powder gases. With a very hard bullet, including antimony or type metal, the bullet may not expand. What then happens is that hot gases escape through the lands all the way to the muzzle. They blast atomised bullet metal ahead of the bullet, which irons them into the hot bore. Hard alloys may cause more bore leading than softer ones.
Another point is expansion on game. At a velocity which causes a flat nosed or hollow lead-and-tin bullet to mushroom, a very hard one is likely to have the mushrooming lobes break off, leaving a lightened bullet of no more than the original diameter. The scattering fragments have energy too, and may be more destructive on the animal, or less.
The .375 rifle has less case neck length than most rifles designed around lead bullets. Lubed grooves undovered by brass are no more than a nuisance if the throat of the rifle admits them, but you need to make sure they don't pick up dirt, especially abrasive dirt. Lube grooves exposed in the powder space are more harmful. With the lube burned or melted away, they leave exposed lead contacting the bore. I think, though, you should have no trouble getting velocities in the 188 ft./sec. class without stripping the rifling.
It is worth reflecting on why cast bullets may not fly as true as jacketed or other swaged ones. If you imagine a bullet with its centre of mass a thousandth of an inch closer to one side than the other, that centre of mass is actually travelling down the barrel in a spiral path, with pitch equal to the rifling twist, and diameter .002in. But Newton's First Law says that a body remains at rest or in a state of uniform straight-line motion unless it is acted upon by some external force. When the bullet exits, it flies off in a straight line, in the direction the end of that spiral was pointing. Now, the .375 H&H has a faster twist than most of the black powder or formerly black powder rifles which usually use cast bullets. It will be less accurate unless you can reduce the width of that bore spiral, by quality of components and careful loading to maintain bullet straightness. In particular if you can obtain .375 in. swaged lead bullets, they will probably be more accurate than cast.
There is actually no reason why the .375 H&H bullet needs a meplat at all. If you are interested enough to buy a fairly expensive mould (and you don't need to reduce your .375 jacketed consumption much to pay for one), you will find plenty in NEI Handtools, Inc .
Weighing fibrous filler material to the grain sounds like a pretty exasperating business. I don't know what is in cigarette filter tips in the US (I don't mean in cigarettes, but in tips for rolling your own) but in the UK it is accurately measured, lengthwise-oriented fibrous material which I believe to be kapok. It could be cotton, but the main thing is that it burns but doesn't melt and smear, and comes out as low-litter, self-extinguished short fibres.
I would however disagree with his comment that a wide meplat is not needed.
From my limited cast bullet hunting experience, coupled with my reading on the subject for some years, I'd say, the wide meplat is what imparts the shock and tissue disturbing forces to the tissue of the game we hunt.
Considering that expansion is a very iffy thing with cast bullets, running from none to greatly excessive depending on many factors, a wide meplat is one constant in what is basically a non-expanding bullet.
The very hard alloy that John speaks of, as he later points out, is very likely to shatter if it impacts bone. Not good! But if it doesn't hit something that causes that negative issue, it will just pencil through the tissue with little if any of the shock/wound channel/disruption of vital tissue needed to effectively put down game.
The meplat on a .375 cast bullet will be none to large as it is, and if that small meplat is a negative as per your desires of possible extended range capabilities, then it is possible that the thought of shooting .375 cast bullets for game needs to be rethought.
Saying that, my 45/70 has LESS longer range capability then a .375 should have, and it is great fun to hunt with and very effective on deer and elk, shooting a 465gr WFN at 1650fps.
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