And thereby hangs a tale.
Howdy
Frankly, I never try to stuff 40 grains of Black Powder into a 45 Colt. If you go way back you will see that the only 45 Colt cartridge that could actually hold 40 grains of BP were the old copper cased, folded rim, Benet primed cases. They had more case capacity than any other 45 Colt cartridges. In this photo, left and middle are an early copper cased Benet primed 45 Colt and 45 Schofield cartridge. If you look at the cutaway you can see how these cartridges worked. The priming compound was placed on the inside bottom of the case. Then an Anvil Plate was placed on top of the priming compound and crimped in place. The crimps at the base of the cartridges are what is holding the Anvil Plate in position. When a firing pin struck the center of the case, the priming compound was crushed between the back of the case and the Anvil Plate. The flame from the priming compound would then pass through the two holes and ignite the main powder charge. Even though there was no visible primer in the cartridge head, and they looked like rimfire cases, they were a very early version of centerfire cases. I have a box of 45 Colt Benet primed cartridges from the Frankford Arsenal that were made in 1874. Those large capacity copper cases were historically the only 45 Colt cartridges that could actually hold 40 grains of Black Powder.
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Benet primed cases were not manufactured for very long, they were eventually replaced by Balloon Head cases. On the left in this photo is a piece of old Remington-UMC Balloon Head 45 Colt. On the right is a modern Solid Head case. Yes, I cut them in half myself. You can see how much more powder capacity there was in the old Balloon Head cases. Powder charges of about 38 grains were most common in the old Balloon Head 45 Colt cartridges. With modern Solid Head cases, my standard Black Powder load is 2.2CC of Schuetzen FFg. That works out to about 33.3 grains. I compress this charge about 1/16" - 1/8" when I seat a 250 grain Big Lube bullet. That is plenty of oomph for me, I don't see the need to try to cram any more powder in than that.
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Anyway, back to the original question about 'over compressing' a charge of Black Powder. Technically, no, if you can stuff a bullet on top of it, you are probably not going to have a problem, even if you cram a huge amount of powder into the case.
But other things can happen.
A few years ago I bought an original Winchester Model 1873 chambered for 38-40. This one left the factory in 1887. I had never loaded 38-40 yet, so I bought some dies and ordered some cases from Starline and some 180 grain Big Lube 38-40 bullets from Whyte Leatherworks. I buy all my Big Lube bullets from him. While waiting for my bullets to show up I found some suitable 38-40 bullets at a local shop.
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Technically speaking the 38-40 cartridge case is the same as the 44-40 case, it has just been necked down further. But from the the neck back they are the same.
On the left is one of my 38-40 reloads, on the right is one of my 44-40 reloads. Both use the Big Lube style bullets, I use a 180 grain bullet in 38-40 and a 200 grain Mav-Dutchman bullet in 44-40. I use Starline brass for both. If you look carefully you can see how similar the two rounds are. In fact, 38-40 brass can be made from 44-40 brass, you just have to neck it down a bit further.
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My standard load for 44-40 is 2.2CC of Schuetzen FFg under the 200 grain bullet. The same 2.2CC (or about 33.3 grains) that I put into my 45 Colt loads. Again, seating the bullet on that amount of powder compresses it about 1/18" -1/8".
So thinking that the powder capacity for 38-40 should be the same as 44-40, I loaded a box of ammo with that charge, and the bullets I found at the local shop. I took rifle to the range to try it out with these loads and everything worked fine. Then my Big Lube bullets showed up and I loaded up a box or two of 38-40 for an upcoming match.
When I got to the firing line at the match, and worked the lever of the rifle the first time, it jammed. I could not close the lever all the way, the round would not enter the chamber. Seeing as the rifle was over 130 years old, I was not going to force things, I grounded the rifle and finished the match with one of my trusty 44-40 rifles.
When I got home I took a good look at that ammo. As it turns out, seating the very soft, pure lead Big Lube bullets on that powder charge was the problem. The resistance to compressing the powder had caused the soft bullets to deform as they were seated. They had gotten squashed down slightly in length, and expanded slightly in diameter. It was subtle, and I had not noticed it at the time. But expansion of the bullet diameters was enough that the rounds would not chamber. Looking closely at the 2.2CC of powder in the 38-40 case, and comparing it to the same amount in a 44-40 case, it was obvious that the charge in the 38-40 was sitting much higher than the charge in the 44-40 cases. High enough that with the soft lead bullets it cause the bullets to deform as they compressed that much powder.
Lesson learned, now I load my 38-40 cases with 1.9CC of Schuetzen (about 28.5 grains) Yes, it is much less than 40 grains. But experience has taught me with modern solid head brass, and dead soft lead bullets, that is the ideal amount of powder to put in my 38-40 loads.
To the OP:
Your bullet molds look really good. Here is a photo of the 200 Grain Big Lube Mav-Dutchaman bullet, both with and without bullet lube, next to a 44 Russian and 44-40 reload of mine. The lube groove on your 38-40 mold is not quite as deep as a Big Lube mold, but it should do a pretty good job with Black Powder. Be sure you lube your bullets with SPG or some other Black Powder compatible bullet lube and they will probably perform great.
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