Do lead bullets really seal the bore with BP?

[HenryC460]

I'm wondering about a statement I have seen written in a couple of places. It goes like this:

Bullets for black powder cartridge are sized bore diameter (or somewhat below groove diameter at least), because the fouling from the first shot or two would keep a groove diameter sized bullet from chambering. (The argument continues that) this is alright, because the pressure curve for black powder is something special. Pressure builds, (it is argued), in a way that bumps the bullet up to groove diameter and this makes a seal against hot gases leaking past.

Hand-in-hand with that argument about black powder is the following argument about smokeless:

But you can't get away with bore-size bullets in a smokeless cartridge, because smokeless doesn't bump bullets up to groove diameter, and the gases leak past like a cutting torch and cause problems. You must start with groove diameter or even 0.001" larger (says the argument's partner).

That may be the case. Some day, they're gonna sell crystal clear fused quartz barrels, and some guy with a high speed video camera is going to settle this once and for all. I'm just wondering.

Now, I don't want to argue about relative pressure curves. If someone else wants to argue about them, that's great. I don't, because I'm ignorant. These pressure curves are probably known. I've got me a Pressure Trace device, and if that dude on eBay hadn't sold me a non-working laptop, I'd be able to hook it up and measure the pressure curves. Anybody who's got the data or has seen it, I would be just tickled to hear about it. If that solves everything, I have one less thing I need to worry about in this world.

Here's the part of the problem that keeps me scratching my head. Why do we have to compress the black powder before we seat the bullet?

I'm thinking that maybe black powder burns differently than smokeless. Maybe it burns like a very rapidly smoked cigar, from one end to the other. Or like a solid rocket engine. Maybe the powder nearest the primer gets lit and the rest gets pushed like a plug down the bore, burning like a solid rocket behind the bullet, and maybe the powder that's waiting to be burned is what provides the seal against all that hot smoke getting past the bullet and causing problems. You can blow gas through a small orifice (like the space between the bore sized bullet and the groove-sized rifling groove), but it's pretty near impossible to ram a powdered solid through.

But if recovered bullets have rifling marks on them, does that mean I just wasted 15 minutes of my life typing this? (I type slow).

I'm not sure. It's one thing for a bullet to go from zero to 1100 f.p.s in the time it takes your rifle to say "bang." That at least provides time (4 or 5 milliseconds anyway) for the lead to squish out and conform. It's another thing for a seated solid lead bullet at rest to received such an impulse from the hot gases behind it to deform so fast that it fills the grooves before any of the gas can get past.

Does any of that make sense? What if smokeless powder burns like a hot, churning, flaming dust storm in the chamber, requiring a groove-sized bullet to keep it from leaking past, and what if black powder burns like a traveling cigar, providing the push and (initially at least) the seal?

[felix]

It appears you have answered your own question, and accurately enough to explain your observations sufficiently. Your last sentence sums it all up. ... felix

[Bullshop]

Black powder is compressed to make it burn more completely so as to leave less fouling.
Smokeless powder can be used with boolit of bore diameter or under groove diameter but the burn rate must be sufficiently fast.
For instance I shoot a Springfield Trapdoor 45/70 with a .4611" groove and a .454" bore and can get excellent accuracy from boolits down to .457" diameter.
It takes a bit of balancing between boolit hardness and powder burn rate. I was shooting this rifle yesterday with boolits that drop about .458" in a BHN 8 alloy using Reloader #7 for fuel and was able to make three consecutive hits on a 2'x2' steel plate at 500 yards. I would say that it was working.
The Re#7 is about the slowest powder that seems to work well with boolits under groove diameter. I have found that even it can be touchy as to providing enough pressure fast enough to work well. Again with Re#7 I find another balancing act between pressure and alloy strength. There was a narrow window between not enough pressure fast enough to get the seal and too much pressure for the soft alloy to deliver good accuracy.
Burn rate at about that of 2400 or not slower that 4227 are easier to work with but not the only game in town. I first got dismal results with RE#7 until I found the sweet spot that provided just enough pressure fast enough to seal the bore but still gentle enough for the necessarily soft alloy to hold the grooves.

[montana_charlie]

Why do we have to compress the black powder before we seat the bullet?
We don't have to compress it to make it work. We compress it to make it work better.

Some powders burn 'cleaner' when compressed. That means less fouling to be managed.
Some cartridges don't have the capacity to hold as much powder as wanted. Compression puts more powder under the bullet, to attain a higher velocity.

Start with a 'soft' bullet that is seated a half-inch deep the case.
Fill the case with enough powder to reach the seating depth.
Insert a thin card wad and seat the bullet.
The card wad assures there is no airspace in the stack.

A load described this way may only weigh 55 or 60 grains when put in a 45/70 case.
But, even though it is a 'light charge' ... and even though it is not compressed ... it WILL be sufficient to make the bullet seal the bore.

The reason black powder does that is because it develops all of it's pressure almost immediately ... before the bullet begins to move.

A smokeless charge meant to generate the same maximum pressure may not reach it's peak until the bullet is well on it's way down the bore.
If it wasn't a tight fit to start with, the gasses have already done their cutting by the time peak pressure is achieved.

The point is ... It isn't 'compression' of the powder that causes the bullet to obturate the bore.

CM

[Boz330]

The long range BP muzzle loaders shoot a projectile that is paper patched and the fit in the bore is loose enough that only the weight of the ramrod will seat the bullet. They also don't compress the powder any to speak of, BUT they do clean after each shot. Recovered bullets show rifling marks from one end to the other.

Bob

[Don McDowell]

Blackpowder doesn't necessarily need to be compressed to work well.
The reason most folks compress the powder before seating the bullet is so that the bullet nose does not get deformed during seating if the desired powder charge will be compressed when the bullet is fully seated.
Like you I"ve never quite got a grasp on this stuff about smokeless not bumping up a bullet, most smokeless loads will run more pressure in a given cartridge than blackpowder. It's best to run grease groove bullets just .001-.002 over groove diameter to help prevent leading and bullets sized that way are usually more accurate than ones larger or smaller.

[NickSS]

It's all in the burn rate of the powder. Black powder has a quick burn curve where most of the powder is burned in the first few inches of bullet travel and the rest either burns as it goes down the bore or does not burn at all and is sprayed out in front. I learned this when I first got my Chronograph and was checking velocities in several 58 caliber ML I own. The loads I was chronographing were all the same a 5000 gr mimmi ball over 60 gr of FFG GOEX. Interestingly I got more velocity out of my 24 inch barrel than either the 32 inch or the 40 inch one. I ran the test five times and came out with the same answer. Smokeless on the other hand burns as it goes down the barrel. Slower burning powders produce higher velocities until the bore capacity in reached as the powder is burning all the way down the bore. This is why in a Gas operated rifle you need to pick your powder carefully as a slower powder that will give you more velocity may also over stress your gas system.

[405]

HenryC quote:
"I'm thinking that maybe black powder burns differently than smokeless. Maybe it burns like a very rapidly smoked cigar, from one end to the other. Or like a solid rocket engine. Maybe the powder nearest the primer gets lit and the rest gets pushed like a plug down the bore, burning like a solid rocket behind the bullet, and maybe the powder that's waiting to be burned is what provides the seal against all that hot smoke getting past the bullet and causing problems. You can blow gas through a small orifice (like the space between the bore sized bullet and the groove-sized rifling groove), but it's pretty near impossible to ram a powdered solid through."

HenryC,
My thinking on this kinda goes along with your selected quote above. But, I think it is well known that both smokeless and BP obturate a bullet under the right circumstances. If either powder type charge is heavy enough, the bullet soft enough and the bullet heavy enough then the bullet will obturate as a consequence of the acceleration versus the inertia of the bullet. I also think that with BP another phenomenon is in play..... the BP ignites.... a relatively large portion, in "solid" form, is rammed into the rear of the bullet.... thus some of the obturation is a result of kinetic energy acting on the base of the bullet. This is added to the obturation caused by acceleration versus inertia- a "double dose of bump up". The definition of kinetic energy is not intuitive so may not be understood by many.

[Yellowhouse]

Always heard that smokeless pushes a bullet out of the barrel and BP knocks it out or something to that effect.

[felix]

You heard correctly. It's the nature of powder's type of acceleration curve. The smokeless pressure curve could very well be steepened, but then the containment system's peak pressure would be exceeded by a WIDE margin. 15K CUP is the max pressure produced by BP at any time, especially within a typical gun application. ... felix