Smokeless powder burn rate is not an inherent, fixed property; it changes dynamically based on pressure, confinement, and volume. A fast-burning powder like 700-X behaves completely differently in a massive 12-gauge shotgun hull versus a tiny 9mm handgun cartridge due to interior ballistics variables. Case Capacity and Free Space (Ullage)
12-Gauge Trap Load: 16.2 grains of 700-X sits at the bottom of a large plastic hull with significant empty space above it before meeting the plastic wad and 1 oz of shot. Upon ignition, the gas has immediate room to expand, keeping the initial pressure curve low (9,000–10,000 PSI).
9mm Handgun Load: ~4 grains of 700-X fills a high percentage of a tiny 9mm case. The powder is tightly confined with virtually zero free space. Ignition creates an immediate, dense wall of high-pressure gas with nowhere to expand instantly, driving pressures past 30,000 PSI.
Payload Weight and Bore Resistance
12-Gauge (1 oz = 437.5 grains): While heavy, the shot column rests in a plastic wad inside a wide (0.729-inch) smoothbore barrel. It offers relatively low initial resistance to forward movement. The barrel volume expands rapidly as the payload moves down the tube, dropping the pressure curve safely.
9mm (115–124 grains): Though the bullet is lighter than a 1oz shot payload, it is forced directly into a tight, rifled .355-inch steel bore. The engraving resistance of the bullet biting into the rifling creates an instant bottleneck, holding the gases back and forcing a sharp pressure spike.
How Pressure Dictates Burn Rate
Smokeless powder burns faster under high pressure (described mathematically by Vieille’s Law: $r = aP^n$, where burn rate $r$ increases with pressure $P$).
In the 9mm, strict confinement causes a rapid pressure spike, which forces 700-X to burn at an accelerated rate, magnifying the pressure further.
In the 12-gauge, the large expansion ratio prevents pressure from spiking, meaning 700-X burns at a much more leisurely, controlled rate, yielding high volume at low PSI.
Load safety depends entirely on balancing charge weight, payload resistance, and available volume. Changing one variable radically alters how a powder’s “burn rate” translates into actual chamber pressure.
The Atmospheric Illusion
Most standard charts rank powders based on unconstrained or low-pressure testing—often observed in open air or low-pressure reference vessels. At standard atmospheric pressure ($14.7$ PSI), a powder burns at a stable, predictable baseline rate without generating usable ballistic force. Pressure-Dependent Divergence
Because different powders use varying chemical deterrents, grain geometries, and compositions, they have unique pressure exponents ($n$ in Vieille’s Law, $r = aP^n$). This means they do not all respond to confinement and rising pressure at the same rate.
Two powders that sit right next to each other on an open-air burn rate chart can diverge completely when subjected to tight confinement and high pressure.
One powder’s burn rate might accelerate exponentially as pressure climbs, while another’s increases at a much flatter curve.
Relying on a chart to predict how a powder will behave inside a sealed, high-pressure metal chamber is dangerous because unconstrained burn behavior bears little resemblance to closed-system interior ballistics.
This topic comes up pretty frequently on forums. I have posted charts myself, but never to be used as data for working up loads or using the next powder on the list as a substitute. Hopefully it puts into perspective what the charts can be used for, and should not be used for.
Slower powders are typically extruded, may be double base (nitrocellulose and nitroglycerine) and have additives like retarders. they do drive up the cost a touch. Fast flake powder, like 700x is relatively cheap to produce.
All powders are getting expensive,
Difference in price is minor. I just start reloading and come to the conclusion that reloading is not a savings money, its a trap…
It depends what u r reloading and when u bought your supplies. I am reloading trap for a fraction of LGS prices, but I bought the supplies long ago.
Before I got into reloading I read, watched, and listened to the folks who had been dong it a long time. I wrote the below over 12 years ago and reposed it here with updates based on my actual experience reloading
You’re definitely right that component prices, especially primers and powder, have made it way harder to justify reloading standard plinking rounds like 9mm or bulk .223.
The ‘trap’ usually happens when people try to beat factory bulk prices. Where reloading actually pays off is matching or beating $2.00+/round factory match ammo for around $0.85/round, or keeping niche calibers affordable. You don’t really save money to put back in your wallet, you either get custom precision you can’t buy off a shelf, or you get to shoot twice as much for the same budget.
Just bought reloading equipment (press, die set, wet tumbler, case trimmer + ++ still buying small stuff ) that’s a one time investment but powder and primer sucks……..
I don’t currently reload, because I lack a suitable place away from my main house (a huge 1894 Victorian with open cavity plaster walls, no way I’m going to have a large quantity of powder in there But if I did reload it would be at least as much for ammo supply insurance (in the event of political upheaval) as for cost reduction.
I reload everything I shoot. Timing on purchasing components is definitely key. Find someone getting out of it and your per unit cost drops, alot.
At this point, it’s an enjoyable and useful hobby. I enjoy spending a quiet hour or so now and then (instead of watching TV or playing on my phone) on a miserable weather day or any other day. The instant gratification crowd will probably not take to reloading easily. Not a ‘scam’ by any means.
This post reminded me how long I’ve reloaded and how things have changed. I started reloading for my 16 ga. using DuPont Hi-Skor 700X in the 60’s. It’s still one of my favorites for light loads in 16 ga. & 12 ga. shotshells. I even have a couple of those old unopened DuPont 8 oz. cans of 700X left. Price then was less than $2 for an 8 oz. can.
correct
But if I did reload it would be at least as much for ammo supply insurance (in the event of political upheaval) as for cost reduction.