How fast is too fast?

[ArchAngelCD]

I have shot coated bullets from the MBC in a Marlin .357 Magnum with no leading. I do however shoot 158gr bullets, not 125gr bullets. IMO such a light bullet in a Carbine is a waste. If you change over to a heavier bullet you will mitigate your overspend worries.

 

[fxvr5]

It is my belief: Velocity is a direct result of Pressure. Pressure builds up to cause a bullet to depart. Higher pressure yields higher velocity.

Just my $0.02
dg

While we're at it, higher pressure does not necessarily produce higher velocity.

A fast burning powder can produce higher peak chamber pressure than a slow burning powder, but produce lower velocity than the slower burning powder.

This is a well known fact, proven a zillion times.

Many people believe what you've said, and what you're said makes intuitive sense. Unfortunately, it's not true. Ballistics is not necessarily intuitive - it's a bit complicated. When you look at the details, intuition falls apart and physics takes over. Oh well, we can learn how it really works, because there are lots of sources available to us on the web to help us make sense of it all.

 

[dgod]

Great Info fxvr5, 5" Barrel 1581fps, same load in 18" rifle, 2097fps a difference of 516fps. Proves my belief, same charge, same bullet longer barrel means higher velocity, surprisingly the same pressure from both.

Thanks, fxvr5
dg

 

[Walkalong]

The load I was talking about was 125 gr lead over IMR-4227. I know that's a fairly slow powder for pistols, but should perform much better in a rifle.

If close to or at max, definitely. One thing to remember, the powder that gives the fastest velocity in a long barrel also gives it in a short barrel, assuming the same pressure ceiling. (SAMMI max for the caliber)

 

[Walkalong]

QuickLOAD does not differentiate between different types of firearms. No need to. Varminterror notes revolvers might has lower peak pressure because of their design with the cylinder/barrel gap. Maybe, maybe not, given that pressure peaks so quickly, 0.28 inches of barrel travel (see below), which means the bullet might not be out of the cylinder yet.

In any case, barrel length does not affect peak pressure because pressure peaks after less than 1" of movement, often long before that (at least in handgun cartridges). Pressure declines after that but the bullet is still accelerating. See this graph: http://www.6mmbr.citymaker.com/i/Tech Diagrams/QuickLoad_Chartx400.gif The blue line is bullet speed, the red line is pressure.

QuickLOAD parameters: 125 grain XTP. Overall length 1.590, IMR 4227, 20.0 grains (max charge).

5" barrel; pressure 45,456 psi, velocity 1581 fps
18" barrel; pressure 45,456 psi, velocity 2097 fps

Bullet travel at peak pressure: 0.28 inches.

Excellent post and explanation.

 

[Toprudder]

I agree with everything @fxvr5 said. I will add that it is not peak pressure, it is the "area under the curve", in this case, the pressure curve. A slower burning powder may peak at the same, or even less, pressure than a faster burning powder, but it maintains a higher pressure for a longer period of time than the faster burning powder. (Maybe someone can post a couple of graphs from Quickload to demonstrate this). This is analogous to two drag racers, one is quicker off the line but the other maintains more torque to the rear wheels for a longer period and is fastest at the finish (higher "muzzle velocity").

As for what works well in my 16" 357mag carbine, 158 XTP/FP bullets and 18.0gn of 300-MP powder. That is less than max charge. I am getting 1800fps from my carbine, and more than accurate enough for me (1.5"@50yds with iron sights). I have since found Zero 158gn JSP bullets that are $0.11 each plus shipping, so I will load some of those up to see how they compare.

 

[WestKentucky]

The only difference in peak pressure will be due to BC gap. The distances of bullet travel referenced by fxvr5 above would put peak pressure in a revolver right at the BC gap meaning that the revolver would be leaking gas at the worst possible time. That seems very logical since we have all seen photos showing gas escape in a glorious fireball. That also helps to explain gas cutting on top straps as the heat and pressure are immense when the leak starts.

 

[Varminterror]

Varminterror notes revolvers might has lower peak pressure because of their design with the cylinder/barrel gap. Maybe, maybe not, given that pressure peaks so quickly,

That’s a good point - my paradigm/perspective is skewed because I don’t run QL on enough loads which peak on the cylinder side of the gap - if it peaks on the cylinder side, Max pressure is the same in that revolver vs. a sealed breech firearm, although net pressure impulse and net total power (area under curve) by the time it leaves the muzzle are reduced. If the peak is on the barrel side of the gap, the max would be lower in the revolver than a sealed bore/breech firearm AND the net lower. Either way, it’s never higher because of barrel length.

Some of my big bore rounds are barrel side of the gap - but it’s pretty intuitive certain cartridges, especially like 45acp or 9mm revolvers would ALWAYS be hitting their peak in the cylinder.

 

[Toprudder]

Besides the cylinder gap, I also think there will be less pressure in a revolver due to the larger jump the bullet has to make before it ever forms a gas seal in the barrel. In other words, a lot of gas blows around the bullet while it is traveling out of the cylinder and into the forcing cone.

 

[Offfhand]

Gas check lead bullets are my favorite in the .357mag. With the bullet base covered by the metal cup there's no possibility of the base melting and depositing lead in the forcing cone or bore.

I love the phrase "no possibility". now where have I heard that before?