Myth-Busting Powder Burn Rate vs. Barrel Length

[Andy]

After seeing several statements suggesting that you "need" a long barrel to burn slow powders and for you to "get the most" from magnum Cartridges, that short barrels "need" fast powders and the like, I thought that it might be useful to share a couple of points.

The point where Peak Pressure is reached (A, B and C below) coincides with the point in which all the powder that will burn has been burnt and it varies from about 1-4" from the boltface. Pressure drops thereafter and remains high enough to accelerate the bullet. That's the theory and tests to determine the effect of progressively shorter barrels on loads with fast, medium and slow powders have been done several times and bear it out.

Essentially the powder that will produce the highest MV will do so regardless of barrel length. All powders regardless of the amount or the cartridge in which it's loaded will produce higher MV with progressively longer barrels - to a point that typically exceeds 30".

 

[Eagleye]

My personal experience over the past 50+ years of stuffing cases with various powders
agrees with this.
There are still a few who believe that faster powders work better in shorter barrels, but
the facts are quite persuasive.
For many years I have loaded the 30-06 with a 180 Partition and Norma MRP. I have consistently
been able to reach higher velocities with this combo in barrels from 18½ to 26", beating out stalwarts like IMR4350,
regardless of barrel length.
Velocity does diminish as barrels get shorter, but the highest velocity is still
attainable at safe pressures with the MRP.
Regards, Dave.

 

[Suputin]

Then why does 4350 produce a massive muzzle flash in my short barrelled rifle when Varget does not?

 

[jethunter]

Then why does 4350 produce a massive muzzle flash in my short barrelled rifle when Varget does not?

Incomplete powder combustion due to the pressure being too low, maybe.

 

[tootall]

Then what is the point of having well over 100 different powders to choose from?
Why not just the very fastest pistol powder?


(NOT meant to be reloading advice! Newbies, DO NOT load your magnum rifle with 80 gr of pistol powder!))

 

[medvedqc]

Then why does 4350 produce a massive muzzle flash in my short barrelled rifle when Varget does not?

Incomplete powder combustion due to the pressure being too low, maybe.

i have another example with rl15 and varget.

varget seems good with short barrel not that much rl15.

 

[Grawfr]

Then why does 4350 produce a massive muzzle flash in my short barrelled rifle when Varget does not?

AFAIK, the bright light at the muzzle when the bullet exits isn't fire from burning powder. It's hot gases and particles (soot) that glow because of their high temperature, just like a bar of iron glows when you heat it. So the hotter (and higher volume) the gases are, the more of a muzzle flash you get.

 

[Suputin]

I realize that muzzle flash isn't due to still burning powder. However there has to be something to the powder burn rate that produces a huge flash with slower powders and hardly any flash with faster powders.

Not to mention that slower powders produce higher velocities in longer barrels while faster powders produce higher velocities in shorter barrels.

Incomplete powder combustion due to the pressure being too low, maybe.

I was not using a light load. And on top of that any light loads I have used do not produce a massive fireball. Thus low pressure does not cause a huge fireball.

 

[kawicrash]

Then what is the point of having well over 100 different powders to choose from?
Why not just the very fastest pistol powder?


(NOT meant to be reloading advice! Newbies, DO NOT load your magnum rifle with 80 gr of pistol powder!))

Awww... I was just headin' for the R-1.

 

[Ganderite]

Powders use an additive to reduce muzzle flash. Military powders have it. Some others don't. Flash does not relate to the speed of powder or the unburnt powder. It relates to the powder chemistry.

The amount of anti muzzle flash additive can vary a bit from lot to lot.

If a lot of military powder is found to not have enough, it gets rejected and sold as surplus or to a commercial loader, who does not care much about muzzle flash.

 

[tootall]

I recall firing some Chinese 5.56 from a Mini-14 at dusk.
Glorious big basketball size ball of fire at the muzzle!

 

[Andy]

......Not to mention that slower powders produce higher velocities in longer barrels while faster powders produce higher velocities in shorter barrels.......

This shows that you missed the entire point of this thread. I realize that this myth will never die, but I keep trying.

 

[Eagleye]

Not to mention that slower powders produce higher velocities in longer barrels while faster powders produce higher velocities in shorter barrels.

Did you read Andy's original post? Your statement is simply not true.
The powder that produces the highest velocity in a long barrel will also produce the highest velocity in a shorter barrel. I have proven this numerous times. Dave.

 

[burnt_servo]

what I am reading , if the initial firing pressure of 2 different powders are the same , bullet velocity will be the greatest with the slowest powder since it will be producing more averaged pressure over the barrel length than the faster burning powder .

 

[Andy]

what I am reading , if the initial firing pressure of 2 different powders are the same , bullet velocity will be the greatest with the slowest powder since it will be producing more averaged pressure over the barrel length than the faster burning powder .

Yes - in the figure in post #1, pressure curve "A" is for the fastest burn rate and "C" is the slowest. They have similar "Peak Pressures", but the slower the powder, the more of it you can use and produce a higher average pressure (the area under the curve) without exceeding a safe Peak Pressure. More powder means more gasses produced and therefore more energy.

 

[JAK1981]

Isn't this very similar to the notion that you are better off with a car that peaks X horsepower @ 7500 RPM rather than X @ 2500 RPM? The projectile is subjected to greater force, for longer.

 

[tootall]

Yes - in the figure in post #1, pressure curve "A" is for the fastest burn rate and "C" is the slowest. They have similar "Peak Pressures", but the slower the powder, the more of it you can use and produce a higher average pressure (the area under the curve) without exceeding a safe Peak Pressure. More powder means more gasses produced and therefore more energy.

If I understand your comment correctly, you are saying that WITH SIMILAR AMOUNTS of two different rate powders, the faster burning will give higher velocity.
But,... with a slower powder you can USE A LARGER CHARGE, therefore getting a higher yet vel.
Is this correct?

 

[John Y Cannuck]

Then what is the point of having well over 100 different powders to choose from?
Why not just the very fastest pistol powder?


(NOT meant to be reloading advice! Newbies, DO NOT load your magnum rifle with 80 gr of pistol powder!))

Each powder has it's niche.
Pistol cartridges like faster powders, even if used in a rifle. Rifle cartridges like slower powders, even if used in a pistol.
That is not to say that you can't use pistol powder in a rifle, or rifle powder in a pistol cartridge, you can, but the load must be tailored for the application, and performance will suffer, sometimes dramatically.

 

[John Y Cannuck]

If I understand your comment correctly, you are saying that WITH SIMILAR AMOUNTS of two different rate powders, the faster burning will give higher velocity.
But,... with a slower powder you can USE A LARGER CHARGE, therefore getting a higher yet vel.
Is this correct?

If it's the exact same charge weight, the same case, primer, and bullet, then yes, that would be true. At least I can't think of an application where it would not be so.

 

[Aniest]

Not to mention that slower powders produce higher velocities in longer barrels while faster powders produce higher velocities in shorter barrels.

I don't believe that Andy or Suputin are wrong: just looking at it from different directions.

Notice how a smaller amount of powder A contained in a handgun casing in a shorter barrel is capable of reaching peak pressure in less barrel where the same volume of powder C in the same casing and barrel will not reach that peak as quickly: small powder amount, lighter bullet, shorter barrel needs faster powder to reach maximum safe velocities. In a long barrel using a lot of fast burning powder A in a large rifle casing would be catastrophic (we have seen threads on CGN on this failure) so to keep the pressure curve acceptable a lot of slower powder in the same large rifle casing over a longer barrel is required to reach maximum safe velocities. It is not that powder A won't always create a faster bullet over powder B, but how huge of an action will be accept to handle the fast burning pressure curves?

I believe Andy is looking at it from a "the pressure curves of 67grains of R-1 or N310 or Titewad in my magnum caliber is irrelevant if the action can handle it" and Suputin is coming from a "no commercial receiver can handle 67 grains of R-1 or N310 or Titewad in a magnum casing so only slow burning powder are acceptable to get that velocity". So yes, it is true faster powder always gets faster velocity if all other variables are identical; but it is also true that variables of case powder to space ratios and strength of commercial actions shows faster powder is better in short barrels (handgun calibers as the assumption) and slower powder in longer barrels (rifle calibers as the assumption).

Yes, when you look at rifle calibers in handguns (TC Contender in .308, for example) they are still loaded with slow powders, and likewise, handgun calibers in rifles (Marlin lever action .357 for example) are still loaded with fast powders.

That's the way I see it.