Accurate 1680 - large fireball in 7.62x39

[Unclenick]

Complicating matters further, bore friction is not constant throughout. It peaks when the pressure peaks because that is when acceleration peaks and the g forces try to squash the bullet, bulging it slightly outward outward against the bore surface. . This is why, absent a narrowing taper or extra roughness further down it's length, a bore's heaviest metal fouling occurs where the bottom of the bullet's bearing surface is in the bore when the pressure peaks.

An interesting example is the 22 Long Rifle. Depending on the load, the powder burns out when the bullet is in the range of about 16 to 19 inches into its tour of the tube. At that point velocity ceases to change much. Friction and the dropping pressure as it goes down a longer tube seem to roughly neutralize one another. This has an interesting consequence. Because the expansion ratio is so high, variations in chamber size don't tend to have a significant effect on 22 RF velocities. Coupled with the relative stability of the velocity after 19" of travel, most any 22 RF rifle with a barrel that long or longer will shoot match ammunition to Irving 50 fps of the velocity published on the box. This provides the shooter with a convenient way to confirm his chronograph is working as it should.

 

[rc]

Yeh I think you have unburned powder because of low pressures. 4198 might be a better alternative to low speed loads in that caliber. Since it's a bolt gun the pressure curve and gas pressure is not as important. Don't overlook the .312 100 grain hornady XTP for 2000fps loads in that ranch rifle and you may be able to use something like blue dot with careful work up.

 

[44 AMP]

1) I am not sure that the phase change (e.g., water into steam) observation is relevant here. As a general matter, jumping a phase change requires significantly greater energy than simply temperature changes within a given phase.

What is relevant in my example is the general principle of pressure affecting the rate of reaction. Yes, water (and many other substances) reach the temp for phase change, and then require additional energy (heat) in order to make the phase change, (i forget the proper name for it) but that energy does not raise the material temperature, it changes the phase. And additional energy does not raise the temp until after the phase change happens. Then, the addition of heat energy increases the temp, again.

I have seen water frozen by boiling it, at room temperature.

The similarity with modern progressive powder is that the powders are carefully formulated and produced to maintain the makers desired burn rate as the gas pressure on the burning powder changes.

2) If (and I am not sure it is so) that this post assumes that combustion chamber increases as the bullet travels down the bore, I do not believe that is accurate for most cartridges. Combustion ends when peak pressure occurs, and I think that modern piezo equipment has shown that, depending on cartridge, peak chamber pressure occurs somewhere between zero bullet movement from the case to maybe an inch or so. After that, combustion chamber will decay as the bullet travels down the bore, simply because combustion chamber volume is increasing. (Remember "Deflategate" and the Ideal Gas Law?) However, so long as post combustion chamber pressure exceeds bore friction, the velocity of the bullet will increase. Up to the point where the bullet exits the muzzle and combustion chamber vents to the atmosphere.

I think you are confusing a few things, here.

First, combustion does NOT end when peak pressure is reached. Combustion continues until all the powder is burnt, or the venting of the system (bullet exiting the muzzle) allows powder not yet burning to be physically separated from the ignition source (the other granules of burning powder) enough to prevent ignition. Certain loads and barrel length combinations will expel still burning and unburnt powder with the powder gas when the bullet exits the bore.

Yes, the area that gas occupies (combustion chamber) does increase as the bullet moves down the barrel. This is referred to as "expansion ratio" and is something taken into account when formulating powders. There is a "balancing act" involved. Remember that as long as there is powder burning and the system is closed (bullet still in the barrel) there is gas pressure being added. This is balanced against the rate of expansion of the pressure vessel area (chamber and bore as the bullet moves down the barrel).

The reason the OP is seeing a large fireball is that the combination of the powder he is using (its burn rate) and the area it has to burn in (barrel length) -which also includes the time it has to burn- results in enough powder still burning (and not burnt up) when it is blown out of the muzzle to create a large flash.

Shoot a load from a short barrel (pistol or carbine) and get a large fireball at the muzzle, shoot the same ammo from a barrel several inches longer and the fireball will be significantly less. Longer barrel =more time to burn, and so despite the added increase in the volume of the system, from the longer barrel, more of the powder burns and the percentage of powder still burning when it exits the barrel is less.

Generally. Exact amount of difference depends on how the different factors involved combine and interact with each other.