BP metallic cartridge question

ok so I’m new to the the reloading of black powder metallic cartridge thing and I understand it is quite a bit different than smokeless reloading.
I just bought a 1871 Henry Martini Mark II in 577-450. Searched the inter webs and did quite a bit of research.
My understanding is the original military load is a 480 gr bullet over 85 or so grains of 1F powder.

The load I’d like to try is
Brass formed from 24ga Shotshell
Large pistol primer
70 gr of 1F (I would like to use Pyrodex but don’t know if that’s a good idea. I have a ton of it so any advice in this area would be awesome)
Cotton filler, paper wad, wax cookie, paper wad or two again
.459 405 grain bullet papered up to .464 or .468
This should make for a mild load that is easy on the gun I would think.

Now aside from the Pyrodex question here’s the next and biggest question. A ton of folks are suggesting to slug the barrel to get an accurate bore measurement. I understand the concept of wanting to have an accurate bore measurement but:
1. HM barrels are cone shaped. They are larger at the bore the constrict halfway through then constrict again right before the muzzle.
2. They were standardized when manufactured.
So with this what is the purpose of slugging the barrel and if necessary what measurement would I go off of; the largest, intermediate, or smallest, or average.
Any help or advice would be appreciated thanks

This feels like @AZDAVE territory, hopefully Dave can help you Erik.

@Gert Odendaal would you happen to have any insight on this one?

@gizmo @PHOENIX PHIL Erik Loading black and or nitro for black will take a loong phone call. or several medium length ones. I have a Martini carbine and can walk you through several options or black, substitutes, and nitro for black options. Will PM my contact #

gizmo,
there is one thing to take into account when comparing black to smokeless.
when you fire a black powder cartridge with an alloy bullet, the bullet bumps up to fill any space that it is within, before it starts to move.
for best accuracy, if the bullet already fills that space before firing, the less it has to bump up.
the dilemma is that if you fill the space (in the throat area) fouling will make loading the next round difficult.
being a military rifle, the martini needed to be fired multiple times in a dirty condition.
hot dry weather will make this condition worse than cool humid weather.
the tapered throat was meant to give clearance on the bullet in a clean barrel and less clearance in a dirty barrel, but still some.
tapering the throat meant that the bullet more quickly was in full contact with the barrel as it was bumping up, creating a gas seal.
martini bullets had a hollow base, but this was not meant to expand and seal the bore like the minie, but more as a place to store the folded tail.
you will notice in pictures of original spec 577/450 rounds that they had a wad(s) under the bullet, a beeswax wad(s) under that, and a further wad under the beeswax.
this not only obturated and sealed the gas, but helped with fouling.
for no military use, some tallow added to the beeswax could be an advantage in all but the hottest conditions.
the martini henry was never a target cartridge compared to say the 461 gibbs, as requirements for the two were opposing.
rate of fire compared to extreme accuracy.
however the martini is quite adequate for its trajectory when hunting, and being able to reload is more of an advantage than extreme accuracy.
black powder should be droptubed into the case to pack it consistently.
best performance in bottleneck cases seems not to include compression, but filled to the bottom of wad mentioned above.
in proper cases this is a fair charge, and in shotgun cases reformed an even bigger one, which some find to have recoil.
experimenting with fillers is worth doing to reduce recoil through reduced charges.
with bottle neck cases, no bullet or wad should go below the neck/shoulder junction.
fillers should be incapable of packing so tight under pressure that they cannot enter the shoulder and move through it freely for obvious reasons.
fillers/wads/bullet need to push firmly on the powder when the bullet is seated for consistent ignition.
in the day, manufacturing tolerances were not as today, and bore diameters etc varied somewhat.
that is another reason for the tapered throat.
it just allows for more variance.
in the day, the brits had a powder called curtiss and Harvey,
they had very tight specs for powder and this powder filled those specs.
it burned cleaner than anything we can get today, even swiss, but we have to use what we can get.
you could consider making a blowtube to breathe moist air into the barrel before reloading by softening the fouling.
this is where tallow is your friend, as in the presence of fouling and moisture it becomes soap.
bruce.

Y’all are awesome thanks!

So would Pyrodex work? Original loads were 1F, my understanding is Pyrodex RS is closer to 2F which is a touch faster burning. If it would be an acceptable substitute would I need to reduce the load a bit.
I decided to go with the 24ga brass as the new brass made for it is hard to get and reaaaaaalllly expensive.
I bought this as a hobby gun to get enjoyment from the whole process so making the cases I think will be part of the fun.
The HM has actually been a bucket list gun of mine, particularly the 1871 Mrk II as it was the rifle used In The Zulu wars.

gizmo,
take into account that in the Zulu wars, many reliable repeat shots were the order of the day!
number one priority for the military.
while we remember Rourke's drift, there was another battle that the Zulus won.
what gave them the advantage was that the brits fired so many rounds in a no wind condition that they could not see the Zulus coming, and suddenly the stabbing spear was the weapon of advantage.
Rourke's drift was managed by engineers, who understood how to concentrate the enemy in order to make sustained fire more effective,
just as is done today with machine guns.
definitely go with 1f.
bruce.

Isandlana is the battle you are speaking of which happened earlier in the day. Very fascinating, in addition to the smoke, there was a 13 minute lunar eclipse that happened as the chest (center of the Zulu line) approached the British line. It was something like 10,000 Zulu against 1850 Brits and conscripts. This shouldn’t have been a problem but a recent study suggests that the British line was actually about 300 yards ahead of where it is traditionally thought to have been. This left about 3 yards between each soldier. There is a theory that as the battle came into full sway, many soldiers had their MH’s jam as a result fowling build up. We are talking about a small percentage but...... say that 5 percent of the soldiers experienced this it left large holes in the lines which allowed Zulu to penetrate the line. With what we know of how the Zulus would attack, once the chest broke through the line the Zulu left and right horn slammed into the lines from the flanks. It was really amazing in that it was the perfect storm that developed and literally destroyed the British 1rst of the 24th.
The unit at Rorke’s Drift was the 2nd of the 24th. This is an interesting point too as at the time when units were designated in that manner while the 1rst was deployed the 2nd would be at home yet in this instance, for whatever reason both were deployed at the same time.

Another cool deal with it is that yes they had throwing spears (ipapa) but the main weapons were the knobkerri (a stick with a ball on the end) and the Iklwa (stabbing spear). The Zulu attack was always in three prongs. The chest, which was the center, followed by the horns. The horns were the right and left flanks which would go out and around and slam into the enemy from the sides in a sort of pincer movement.
After Rorke’s Drift the British defeated this by making a square. They would fortify with wagons and trenching in a square pattern. When the horns would try and flank they would simply run into another front line. In two battles specifically the brits had Gatling Guns on the 4 corners.
In one instance they made two independent squares with artillery in the middle covering both.
While, IMO, Isandlana came down to the Brits being cocky and over confident it was a mistake that wasn’t repeated there after. The Brits tactics evolved as the war continued on and the Zulus did not.