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. Too, the powder should also be all burnt before the bullet exits the barrel, otherwise powder is just being wasted. Most shooters also want to maximise muzzle velocity while keeping peak pressures reasonable. This simulator will help you find the powder that best satisfies these requirements.

Of course, the more powder choice there is available in the simulator powder libraries, the more useful it will be. Work continues to add to the powders available.

Usable case capacity
This can be entered in cubic centimeters 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 case with the same case capacity.

What the simulator wants to know is the 'usable case capacity', which is the volume inside the cartridge case behind the loaded bullet - NOT the volume of the case to the case mouth. You are encouraged to measure this yourself, for your particular cases, rather than rely on quoted case capacities from elsewhere. However, a list of approximate usable case capacities is given to get you started. It is important to get the usable case capacity right as the maximum chamber pressure can be very sensitive to load when the loading density is near 100%.

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 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 are becoming more popular these days. They are generally turned from solid brass or bronze and the main bullet diameter is actually the bore diameter of the barrel. They have a small drive-band which engraves the rifling and seals 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 such that a faster powder can be chosen than would be appropriate for a conventional copper jacketed bullet of the same weight.

This is entered from a drop down list, which is rather restrictive, but it is important that the calibre is entered in the correct format so the simulator runs correctly. 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 millimeters 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 all burnt inside the barrel. If there is still unburnt powder when the bullet is launched, then that powder is effectively wasted. You might consider choosing a smaller cartridge burning a slightly faster powder. 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 millimeters

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. The simulator outputs are in actual or '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) the distance the bullet had travelled to P-Max and its velocity at that point. Also given is the muzzle velocity, the muzzle pressure and the muzzle energy of the bullet which is given in Joules or ft-lbs. 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 quoted P-Max 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 bullet had travelled, and bullet velocity -vs- distance up the barrel the bullet had travelled.

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.

The way a powder burns in a closed bomb is not quite the way it does in the dynamic situation inside a firearm. The closed bomb results are therefore adjusted slightly so that the simulator gives the best match to the muzzle velocity data in the powder company's reloading handbook. Predictions will normally be within 100 ft/sec. (30 m/sec) for velocities.

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 claimed 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 as a 'lumped parameter' 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 are still used in many lumped parameter systems today. For those with a technical interest in internal ballistics, a detailed description of the model is available.

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 the 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.