Shooting and Rocket Fuels

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All About Pressure

On each page of this web site there is a small symbol in the upper left. Those symbols are all in some way related to the main topic of this page. That topic is pressure. Pressure is the main concern for any smokeless shooting rifle. There must be adequate pressure to move the bullet to speed but maximum pressure always has to be kept below the safe containment level of the rifle. So a memory aid might be: enough to go but not to blow.

How much is a safe but efficient pressure? How does that compare to a black powder or substitute shooting rifle? What pressure might be made by a given load? How does powder speed relate to maximum pressure? Those questions and more are the topic of this section.

Pressure is required in all forms of shooting without it shooting is impossible. But how much is too much? The answer to that question depends on circumstances and varies greatly. If you are shooting a classic shot gun pressures are very restricted. Any level above about 11,500 pounds per square inch (psi) might be  unsafe.

 

On the other hand a modern bolt action rifle might be very safe at pressures over 60,000psi. Other action types shoot between these two extremes. A modern lever action rifle might reach 44,000psi where an older rolling block would be best held to 28.000psi or less.

 

A modern smokeless muzzle loader should be able to take an ample amount of pressure. As much possibly as a bolt action. That does not mean it should be shot at that pressure. For one thing muzzle loaders employ a vent liner. This check valve device allows primer fire to reach the powder while restricting the amount of chamber gas that can be blown back to the primer. Higher pressures eat vent liners.

 

Another consideration is most smokeless shooting is done with sabots. Sabots are pressure sensitive. If chamber pressures go past 40,000psi the sabot will disrupt either blowing totally apart or affecting accuracy greatly. So an inherent limit could be set at about 40,000psi. Is that really a high pressure?

For a muzzle loader shooting black powder or a substitute 40,000psi is fairly high pressure. Shooting a heavy bullet and a magnum charge modern in lines might reach pressures of 30,000psi. Still that’s closer than you thought it would be isn’t it? So the smokeless rifle has to take more pressure. But it is built to.

 

As long as loads are restricted to a reasonable level safety is not an issue. Those who argue that point at this late date don’t want to face the facts. It’s already been mentioned that smokeless powder can be adjusted to fit the need. Loads for rifles of the 19th century are readily available at very restricted pressure. In fact a load could be designed using smokeless that shoots to a lower pressure than black powder or a substitute.

 

There is considerable print devoted to safety. We have 100% agreement with the often heard statement: never attempt to shoot smokeless powder in a BP rifle. However, there is still a minority who will not accept smokeless from any rifle. Those who insist any smokeless powder load is too much for even a well designed rifle are either unaware or acting on an ulterior motive. 

How do you tell how much pressure a given load might shoot? There are two basic ways. Pressure can be borrowed from data made for a similar caliber or it can be measured. Pressures used here are qualified both ways. If a load was borrowed it was also measured to ensure it worked as planned. If it was developed it was checked against other’s measurements as well as our own

 

The first loads measured were provided by the Savage factory suggestions. The image directly to the right of this text is a graph of one such load. It is a pressure graph taken by the RSI pressure trace of a 300 grain bullet in a sabot shot over 44 grains of  VV N110.

 

As you can see the pressure is about 40,000psi. On some warm days it might be slightly higher but if sabots are to survive this is about the limit. Those who try to shoot much more than 44 grains of N110 already know that for a fact.

 

 

When you think how this works in a muzzle loader it’s surprising. What is the amount of powder the case will hold?  There is no case, but what that really means is it will hold a lot of powder. Then consider the bore size. Do you know anyone with a cased round 50 caliber rifle? Even a 45 caliber would be considered overkill for hunting applications in North America. What do you use a 50 caliber smokeless shooting rifle for? Not many people use them to hunt deer. If you buy that then it should follow the loads for a 50 caliber smokeless muzzle loader are unusual. 

 

Making thirty to forty thousand pounds of pressure can be done in several ways. The current Savage manual only lists three ways. Other custom rifle makers offer more but load data is not growing on trees. This type of shooting hasn’t a great following compared to cased cartridges or other muzzle loaders for that matter. So information is limited to few sources.

 

Direct information is needed. To get it consider how a smokeless rifle works. It seems easy to see that the larger the bore the more powerful the rifle would be. You don’t think of 17 and 22 caliber rifles as real bruisers: do you? From the other prospective a 375 (any of them) is seldom thought as a small game round. So the average person may think: “larger calibers shoot higher pressure”. That’s totally wrong! All modern calibers shoot within the pressure limits we already discussed, even the mighty 50 BMG which launches a 750 grain bullet at 2850fps. has a common pressure limit.

 

Pressure is not the only thing that determines power in a rifle. Other factors include how long the pressure can be applied effectively, and what mechanical advantage the rifle can provide. Those are directly proportional to the amount of powder that the case will hold and the bore size. More powder allows for a longer time at pressure while a bigger bore supplies mechanical advantage over any small diameter.

Though they vary in size and caliber all three of these cases employ the same pressure limit

Shooting normal pressures with normal amounts of powder a 50 caliber smokeless rifle (in any style) can be a beast. This is the point of the pressure trace to the right.

 

The load is very mild in pressure well below our 40kpsi limit, still 74 grains of powder under a 400 grain bullet are being shot. Bullet speed is over 2200fps at 4300fpe. By contrast the average 300 Winchester Magnum with a 180 grain bullet falls more than 700fpe short of this level shooting the same amount of powder and nearly 30kpsi more pressure. Did I say 50 caliber rifles were powerful? I meant it.

 

You may wonder why the pressure can be lower in this case and shoot 30 grains more powder than the load  in the upper section?

 

 

 

The answer is the powder itself. Above the 300 grain bullet was pushed to the same speed and slightly more pressure by 44 grains of VV N110. Here the 74 grains pushing the 400 grain bullet are another powder: VV N133. The ability to shoot more powder but still stay within pressure limits is because two powders burn at much different rates. Burning speed is an important concept in understanding powder and what pressure it might produce. The speed is not to be confused with energy. All common powders have near the same energy. One grain produces the same amount of gas (if load efficiency is the same) no matter the type. However how fast they change from solid to gas varies greatly.

 

If you understand that all powder does when ignited is turn from a solid to a gas you’re on your way. If you imagine a chamber that is changing to a larger size while gas is being pumped in then the following is simple logic. The pressure depends on how much gas is pumped and how fast it enters the chamber. So if you make a small amount of gas but you make it very quickly the pressure will still reach a relatively high point even though not much gas was made. Also even if you make a large amount of gas the pressure will not go so high if enough time is allowed for the chamber to expand to a larger size.

 

 

Both chambers above have the same amount of powder and the same weight bullet. The top faster powder changes to gas quicker and results in the gas being contained in a smaller space. The bottom slower powder load changes to gas slower and more space is in the chamber at maximum gas volume. Since both powders make the same amount of gas the upper condition is higher pressure and the bottom lower pressure.

In our example there are other factors. The resistance to the chamber expanding (bullet weight) is one. The starting area for the gas to expand into (the bore volume plus the powder volume) is another. But the principle holds true no matter the caliber or rifle.

In a cartridge rifle powder is limited by how much powder will fit into a case. You can change bullet weights and adjust powder burning speed for that but the maximum powder charges can never be more than the case capacity. With only two loads so far powder charge has already varied by 30 grains (41%). So it seems you can pick any charge weight and any bullet weight and make our required pressure. That’s a nice thought but seldom is it a perfect world and this is not one of those occasions.

In the real world loads began with 44 grains or less of the following powders, VV N110, Xmr-5744, and Imr-SR4759. These loads will work well in a 50 caliber rifle if 250 grain bullet speeds are limited to about 2350fps and 300 grain bullets are shot at 2225fps or less. If you decide 50 grains of powder seems more like your level you’ll have a problem. What powder will make the correct pressure for common weight bullets at 50 grains charge weight? Sorry but there is none. The above powders make too much pressure and other powders make too little.

It was mentioned that the bullet weight had an effect on pressure. At the same charge level a heavier bullet will shoot to a higher pressure with any given powder than a lighter weight bullet. Muzzle loader bullets are not just light weight they are unheard of light. Normal 50 caliber bullets start at 400 grains and go up from there past 600 grains. Does it make sense that powders that shoot these bullets won’t work on a 250 or 300 grain bullet unless you shoot a lot more powder?

As I said 50 caliber rifles are very powerful. They make this power by pushing heavy bullets to moderate speeds. So powders designed to shoot light weight bullets (for this caliber) are not abundant. In fact, many common loads are not rifle powders at all they are pistol or reduced load powders that happen to be the correct speed to work at the 40 to 45 grain charge level. Even the fastest rifle powders (Reloader #7 or H4198) won’t work with a very light weight bullet at 55 or even 60 grain charge levels. Why? Because the pressure is too low. Can the pressure be too low. Yes it can. Without proper pressure numerous things happen. The obvious is the bullet is slow. Why shoot more powder when it offers no speed or pressure advantage? You wouldn’t because it’s a waste.

Another reason to avoid very low pressure is temperature sensitivity. Very low pressures were not designed into rifle powder performance criterion. That means changes can cause problems with bullet speed and even make for misfire conditions. Inconsistency is not a good thing to have in a rifle load. So along with not over pressuring the rifle or sabot we must also ensure there is enough pressure for performance and consistency. In normal loads a minimum pressure of 32,000psi is workable. Loads with less pressure are sometimes mentioned but the best ones make some pressure.

It is possible to shoot slower powders than mentioned so far. This is seen from the pressure trace on the left. The powder is H4198 and as seen it is shooting within our upper and lower self imposed limits for a bullet with sabot.

This should be no surprise. Yet it might surprise many when they learn the load was 68 grains under a 300 grain bullet. That may sound reasonable to some. To a 50 caliber rifle owner it would even be tame. But at nearly 2400fps this load has recoil. Recoil isn’t the best coincidental result.

You don’t get something for nothing. You can have power with a 50 caliber rifle. But even ridiculously light weight bullets for this caliber aren’t really light compared to most. A fairly heavy bullet at speed produces recoil

Similar fast shooting rifle powders VV N120, Hodgdon H4198, and Alliant Reloader #7 all can be shot within the pressure limits suggested. But none of them can start with the load levels shot by the Savage factory suggestion (40 to 44 grains). N120 (don’t confuse with N110) is the fastest burning speed but still requires 58 to 60 grains under a 300 grain bullet to make the limit. It takes as much a 65 grains to do the same with a 250 grain bullet.

H4198 and RL#7 act about the same and as shown require 64 grains with a 300 grain bullet and more  with a 250 grain bullet (67 or 68) to reach minimum pressure. Between 44 and nearly 60 grains no powder shoots 250 or 300 grain bullets at the pressure requirement. So we have a condition where loads may vary widely by bullet weight and powder charge. The catch is that still leaves a big gap in the performance level shooters would prefer with very light weight bullets.

With a 50 caliber rifle you can shoot a small amount of powder efficiently or a large amount with power (and recoil) but if you want the best compromise between the two pressure does not seem to be on our side. This is only a small section about pressure. Because of it’s importance you will see much more about the subject on other pages in the site.

 

Idealized

Curve

 

Behind this page is an article on rifle powder some details are given for smokeless and BP. The only entrance to the page is if you click here.