Manual for the BBT "P-Max" internal ballistics simulator

Reloading ammunition is not that complicated. You take a cartridge case, put a primer in the bottom, some powder in the middle and a bullet in the top. You load the cartridge into the gun, pull the trigger - bang! You need to take care to choose the right amount of the right kind of powder so you get the right kind of bang, but this is not an esoteric art. A program that simulates what goes on when you pull the trigger should not be that complicated to use either. But... there is a bunch of quite complex stuff going on 'under the hood', and you need to be aware that unless you feed the simulator the right information at the input, you are not going to get useful results at the output.

Save yourself a lot of time - read the manual below.

Powder choice
There are powders that are designed to burn very fast for pistol and small rifle cartridges. There are powders that are designed to burn real slow for big guns and cannons. In between, there are hundreds of powders with a gradation in the quickness of burning. A few of these powders will be well suited for the particular cartridge, calibre and bullet you have in mind.

Ideally what you are looking for is a powder that fills the case as much as possible so that the burning rate is consistent from cartridge to cartridge and you maximise the energy available from the powder.

The burning rate of powder should be such that it is just all-burnt before the bullet exits the barrel, with a good working maximum pressure. In any case, the powder should be all-burnt before the bullet exits the barrel otherwise powder is just being wasted.

Powders that are too fast will tend to be all-burnt well before the end of the barrel and high maximum pressures will be needed to get good muzzle velocities. Powders that are too slow will not be all-burnt by the time the bullet exits the barrel and will not achieve good working pressures, even when you have a compressed load. This simulator will help you find the powder that will give you a good balanced load.

The powder libraries are listed by manufacturer in alphabetical order. The powders for each manufacturer are listed in a drop-down box next to the powder manufacturer. Single base powders are grouped above double base powders, with the fastest powder in each group at the top.

Powder space
This can be entered in cubic centimetres or grains of water.

Many experiments over the years have shown that the actual shape of the case does not matter when it comes to the internal ballistics of chamber pressures and bullet velocities. Tall thin cases will give the same results as short, fat cases with the same case capacity.

What the simulator wants to know is the volume inside the cartridge case behind the loaded bullet when there is no powder in the case, called here the 'powder space'. This should not be confused with the case capacity, which is the volume of the case to the case mouth. You are encouraged to measure this yourself, for your particular cases, rather than rely on powder spaces from elsewhere. We have made a Youtube video, showing an easy way to measure the powder space in your cartridge. It is important to get the powder space right as the maximum chamber pressure can be very sensitive to load when the loading density is near 100%.

However, a list of approximate powder spaces is given for a number of common cartridges to get you started.

Powder weight
This can be entered as grains or grams. The simulator knows what the bulk density of the chosen powder is and it will not run if the calculated loading density is greater than 110%, which would be a highly compressed load.

Bullet weight
This can be entered as grains or grams

Bullet type
The 'shot start pressure' is important. That is the pressure required to push the bullet into the rifling and get it moving up the barrel. The final maximum chamber pressure is strongly coupled to the shot start pressure, so the type of bullet will have a significant effect on the pressure.

Conventional copper jacketed bullets or monolithic solid bronze/brass bullets generally will have the highest shot start pressure. If the bullet has been coated with molybdenum disulphide - either as a dry coating or thin smear of 'moly' grease, this will greatly reduce the shot start pressure and the barrel friction. (Beware, loads that are safe with moly'd bullets can be dangerous if you shoot the same load with an un-moly'd bullet.)

Lead bullets are easier to engrave into the rifling than copper jacketed bullets, so will have lower shot start pressures. Be aware though, that solid lead bullets should only be fired at relatively modest pressures as the bullet will distort if the base pressure is too high and it is accelerated too hard up the barrel.

'Bore riding' bullets, (as made by Nielsen and Peregrine for example), are becoming more popular these days. They are generally turned from solid brass or bronze and the main bullet diameter is actually (or near to) the bore diameter of the barrel. They have thin drive-bands which are engraved by the rifling and seal the bullet in the barrel, so the high pressure gasses behind the bullet do not leak past. The shot-start pressure and the barrel friction of these bullets is less than conventional bullets, so that a faster powder can be chosen than would be appropriate for a copper jacketed bullet of the same weight.

Monolithic solids which are full groove diameter and don't have drive band grooves on them, (as made by Barnes and Woodleigh for example), are harder to engrave and so require a higher shot-start pressure than conventional copper jacketed bullets. They will also have a higher friction force in the barrel.

This is entered from a drop down list, which includes most rifle calibres. If there is a particular calibre you want which is not on the list, contact us and if it is appropriate, we will add it on.

Barrel length
This can be entered in millimetres or inches and should be the length from the bolt face to the muzzle. When thinking about a new rifle, this simulator can be useful in deciding how long the barrel should be. Ideally, you want the powder to be just all-burnt inside the barrel for the cartridge/bullet combination you are considering. In the summary of results, the simulator tells you the distance the bullet had travelled down the barrel at the point it was all burnt - or how much powder was burnt when the bullet reached the muzzle - whichever is appropriate.

Case length
The length of the case to the case mouth is needed so the simulator knows what distance the bullet actually travels inside the barrel. This can be entered in inches or millimetres

You can choose if you would like the estimated output pressures to be in bar or psi (pounds per square inch). One bar is the pressure of one atmosphere, which is 14.5 psi. Note that psi pressures are 'actual', as would be measured by a piezo transducer in a modern pressure gun, rather than as measured by a copper crusher as was common on pressure guns before the 1980s. To avoid confusion, pressures measured using a copper crusher are usually quoted these days in CUP units (Copper Units of Pressure). 'CUP' pressures are about 17% lower than 'piezo' pressures.

Velocity outputs can be chosen to be either metres per second, or feet per second

When the simulator is run, a 'Summary of Results' is given, which includes the load density, the maximum chamber pressure (P-Max) and the distance the bullet had travelled to P-Max. Also given is the muzzle velocity, the muzzle pressure, the muzzle energy of the bullet which is given in Joules or ft-lbs., and the barrel time from 10% P-Max. There is also an estimate of how far up the barrel the bullet had travelled when the powder was all burnt, or how much powder had been burnt when the bullet reached the muzzle, whichever is appropriate.

The estimated maximum pressure will have a coloured background. Chamber pressures in modern firearms should be safe if they are under 3500 bar (50,750 psi) and pressures below this will have a green background. Pressures over 4500 bar (65,250 psi) are generally higher than is considered the safe maximum working pressures for most cartridges, so pressures over this will have a red background. Pressures in between these limits will have an orange background. These are arbitrary limits, however. You should consult the relevant CIP or SAAMI standards for the maximum safe working pressure of your particular cartridge when working up a load.

If, during the simulation run, the pressure rises above 6800 bar (100,000 psi), this is definitely over the top and there is no point in continuing. The simulator will stop and ask you to reduce the load or chose a slower powder.

Graphs are given of the chamber pressure -vs- time, chamber pressure -vs- distance up the barrel the back of the bullet had travelled, and bullet velocity -vs- distance up the barrel the back of the bullet had travelled. It is assumed that that back of the bullet is seated a distance of one calibre into the case.

The chamber pressure -vs- distance graph is of benefit when thinking about the profile of the barrel. Say the bullet has travelled 2 inches (51mm) up the barrel when P-Max is reached, then that part of the barrel needs to be thick enough to withstand the full maximum chamber pressure. The barrel taper or swamp should not start before this point.

Velocity predictions will normally be within 2-3%, or about 100 ft/sec. (30 m/sec).

Predictions of maximum pressures are less accurate due to a number of unknowable unknowns when trying to create a generalised internal ballistics model of this sort. For example, the way powders burn during the first 10% of their burning cycle is critial in determining the maximum pressure, which generally occurs when about 30% of the powder charge has burnt. But it is usually too difficult for ballistics laboratories to accurately determine the powder burn rates for a powder this early in the burning cycle, so the rates for the first 10% (and sometimes the first 20%) of the burning cycle are simply not reported. Then there is the exact nature of the throat in any given chamber. Aside from the fact that European cartridges generally have very long throats resulting in modest shot start pressures, and American cartridges have quite short throats and so higher shot start pressures, thoats erode and wear such that a load which would have been "brisk" early on in a barrel's life, becomes positively passé as barrel wear advances. In consequence, a claim that a P-Max predicted maximum pressure would be within 20% of that actually occuring in any given rifle would probably be optimistic.

Because of this, pressure outputs in P-Max are colour coded. Loads with predicted pressures highlighted in "green" should be safe. Loads with pressures highlighted in "red" should not be explored! You should proceed with care using "orange" loads. Maximum loads given in reloading manuals are generally "orange" when simulated in P-Max, whereas minimum loads are generally "green", but beware that this is a generalisation - see the comments at the end of this manual.

Pistols and revolvers
Chamber pressures can be very sensitive to load densities, particularly for fast powders, and in pistol cartridges the load density can be very sensitive to seating depth, which is why programs like this tend to have a poor reputation for modeling loads for pistol cartridges. This model is definitely not appropriate for revolvers, which are in a class of 'leaky' guns for which different modeling methods are required. In consequence, fast pistol powders are not included in the powder libraries.

Some words about primers
Primers initiate burning of the powder by spraying a cloud of incandescent particles into the powder column, which is then set alight by the radiant heat from these particles. Internal ballistics programs assume that every kernel of powder in the case starts burning at the same time and burns at the same rate. Given that the rate of rise of pressure in the models is generally very similar to what is seen in pressure guns, this is a pretty good assumption. Different types of primer will vary in how efficiently they ignite the powder column of any given powder. There have been a number of experiments where for a given cartridge load of powder and bullet, (for 308 Win and 223 Remington), the changes for chamber pressure and muzzle velocity were recorded for a large sample of primers of different makes and types. The differences were surprisingly small and well within the expected accuracy of this simulator's predictions. No attempt has therefore been made in this simulator to model how different primer makes and types will affect velocities and pressures.

Some words about ambient temperature
It is well known that as the ambient temperature rises, so does the chamber pressure and the muzzle velocity. All powders are affected by ambient temperature changes to a greater or lesser extent. The assumption in this simulation is that the ambient temperature is 15 degrees Centigrade (60 degrees Fahrenheit). This simulator does not currently have ambient temperature as a variable. You should look at the appropriate reloading manual for your powder of choice - or any other reputable source of information - to determine how differing ambient temperatures might affect the velocities and pressures of your load.

Some words about bullet jump
It is assumed that when the pressure has risen to a level high enough to push the bullet into the rifling, (the 'shot-start pressure'), the bullet then starts moving up the barrel. This is essentially what happens when the bullet is seated out to touch the lands. The effective shot start pressure (and so the final maximum chamber pressure) will be slightly lower if the bullet is seated off the lands and is moving when it starts to engage the lands in the throat of the chamber, but this option is not accounted for in this simulator as there is little information on how to calculate the effective shot start pressure in these circumstances. Such information that does exit would seem to suggest that for small cartridges using very fast rifle powders, there can be large differences of 30% or more. But for medium and especially for slow rifle powders, the difference in maximum chamber pressure would not seem to be significant.

Some words about Optimum Barrel Time (OBT)
The theory of Optimum Barrel Time invented by Chris Long in 2016 seems to have become a generally accepted means of tuning a load for good accuracy and is incorporated into QuickLOAD and other simulator programs. The idea is that the impulsive pressure pulse in the chamber creates radial vibrations that propagate down the barrel. It is claimed that this causes the bore diameter at the muzzle to increase (and decrease) by up to 0.0002". It is posited that this can affect accuracy if the bullet arrives at the same time as a node in the radial wave, at which time the bore diameter is varying rapidly. It is claimed that good accuracy is obtained if the load is varied such that the bullet arrives at the muzzle when there is an antinode in the radial pressure wave and the diameter is not changing with time, preferably a negative antinode when the barrel is tight rather than loose.

I will not waste time demolishing this theory here, but sufficient to say that it is complete rubbish. "Optimum Barrel Time" will not be offered as a option in this simulator.

Some words about the simulator computer model
This simulator is known a 'lumped parameter', 'zero/one dimensional' internal ballistics numerical system. It solves the "classical" internal ballistics equations to a high degree of accuracy and without a number of approximations and simplifications that were necessitated to achieve closed solutions in analytic systems (and which are still used in many lumped parameter systems today).

Geoffrey Kolbe is writing a book on internal ballistics which is due to be published in late 2023. In the meantime, draft chapters are available for viewing and comment.

This simulator is just that, a simulator, and a simulation is not the real event. There are assumptions made in the simulation:

  • Firstly that the entered parameters are correct.

  • Secondly that the powder you chose for the simulator has the same characteristics as the powder you use in your real load. Powder properties can change from batch to batch. Powder companies may source their powder from a number of different factories around the world, and will chop and change as currencies fluctuate or as the mood takes them. Powders can age and change the way they burn.

  • And thirdly, like the barrels from which the powder companies generate their reloading data, it is assumed that your firearm is reasonably close to CIP or SAAMI specs for headspace, chamber dimensions, throat length, and barrel dimensions (groove and bore diameter, groove width).

The purpose of this simulator is to help you select the right powder for your particular cartridge/calibre/bullet combination and to give you a reasonable idea of the performance you might expect. But remember, the place to do load development is on a range with your particular firearm, not on a keyboard in front of a computer.

While we work very hard to make this simulator as accurate as possible, there can be no guarantee that a load which seems safe in this simulator will actually be safe in your particular firearm. Consult the relevant reloading manual to determine a safe starting load. Be guided by the reloading manual on what will be a safe maximum load. Follow good reloading practices at all times.