Powder Position in Handgun Cartridges => Big Pressure Changes

[SandRoad]

In another thread I was taken to task over my comments about light loads which don't fill cartridge cases. I remember seeing references and now I found them.

If you note, the EXACT same 38 spl load varied in peak pressure and velocity, depending on where the powder was sitting in the cartridge case.

• Pressure from 14,200 psi to 27,900 psi (38 spl pressure is 17,000 psi, and 27,500 is max proof pressure)
• Velocity from 1,100 to 1,430 fps

Remember this is the EXACT SAME LOAD in all tests. So those people who doubt that a KABOOM can't happen with some powders or light loads should consider these results.

These tests were performed by KEN OEHLER (yes, the same one)

==> The PDF file can be downloaded here http://www.mediafire.com/file/mmjoomd1zzi/File0001.PDF (look for the "Click Here to Start Download" button in yellow box on righ of page)


Below are low res images

Here are SAAMI spec for various cartridges

 

[trevj]

Yep.

That'd be why there are minimum loads listed for most powders, and that some powders have a rep. for being miserable anywhere near such low end loads.

Cheers
Trev

 

[shootist22-250]

Most definitely true.
I experimented with blue dot powder in a 223.
From 8.0 gn behind a 55gn bullet to 14.2gn.
At 14.2gn with the powder mainly spread around the bottom of the cartridge (a bit of a guess here...since I just shoved the cartridge into the chamber while the rifle was in a horizontal rest) chronyed velocity was 2840 fps.
Second test methodology was to stand the rifle vertically, muzzle up, close the bolt on the cartridge and being specifically very careful not to jar any powder forward towards the neck. Chronyed velocity was 2660 averaged over 3 rounds.
Variances were found in all loads tested at 0.2gn intervals upwards from 8.0gn to 14.2gn between where the powder was placed.

 

[ben hunchak]

I'm sure that data is good....the problem gets worse as the cases get larger and the powder % is lower, the best way around this is for light loads, to use powders that are tested and meant for that purpose. IMR 4759 and AA 5744 come to mind, they are meant for that usage. Folks that shoot Cast Bullet Benchrest,(I try) probably have the best handle on this, as they have to wring the best accuracy out of loads usually using small amounts of powder. The loads used by them are published and loads used by winners should have the above problem solved.. A good example is the loads used by folks in the .308 Win....20 grs of 4759 and 30grs. of Varget are common winning combos.

 

[Cerdan]

I've noticed this phenomena myself, when loading .38spl with Green Dot. My chrony was telling my first shot was consistently 200fps slower than the following shots. After a couple of hours of reflexion I realized it was because the recoil knocked the powder in the 5 remaining cartridges nearer to the primer, whereas on the first shot, the powder was probably farther away from the primer.

I now tilt my revolver up before emptying my cylinder when I shoot these loads.

...that said, I may be missing something here, but if this is about your debate with H4831 in that other thread with regards to the risk of detonation when downloading certain types of powders, this is another topic entirely. I don't remember anyone denying that powder position could affect pressure.

 

[SandRoad]

...that said, I may be missing something here, but if this is about your debate with H4831 in that other thread with regards to the risk of detonation when downloading certain types of powders, this is another topic entirely. I don't remember anyone denying that powder position could affect pressure.

My point is that there is a lot we don't know about how powders react. It is only recently that strain gauges have been used on barrels to determine pressures and allow for a lot of experimentation, such as in this article.

Some of those "detonations" may have actually just been overpressure caused by powder position. They used Unique in the above experiments and it resulted in proof load pressures under specific circumstances. Unique has historically been considered a very "safe" powder.

My original point is that it is best to use powders that tend to fill the case, or use a filler. This should also apply to handgun cartridges when used with slower handgun speed powders.

 

[Boomer]

The burning rate of the powder and the design of the specific cartridge case is critical when loading small powder charges. I don't use a filler, yet I load cast bullets as low as 1200 fps in the .375 Ultra with Unique. I would not choose to do this with a reduced load of rifle propellant as there is enough evidence to acknowledge that the practice is unsafe. Having said that, a straight wall case like the .458 can be safely down loaded with rifle propellants to a level that could prove dangerous with a bottle neck cartridge of similar capacity, and then the smaller the bore size the greater the chance for problems.

A volume of powder can only produce a given volume of gas, hence pressure. Because powders deflagrate rather than detonate, there is no shock-wave to tear apart our rifles, regardless how we load them. The problem arises if something interferes with how much volume there is for that gas to fill when it reaches its peak pressure. This might be the result of too large a charge of fast burning propellant which reaches its maximum pressure prior to the bullet moving down the bore and effectively increasing the chamber volume, or it can occur when a light charge of slow burning rifle propellant is loaded in a bottleneck case.

In this latter example, the column of powder is driven forward by the force of the primer and forms a tightly compressed plug against the bottom of the bullet and against the shoulder of the case. If the case was full, only that portion of the powder column closest to the flash hole would compress. Because the powder which forms the plug begins to burn from the end closest to the flash-hole, pressure builds against the shoulder of the case which is contained by the chamber, but due to the plug, the bullet doesn't move. Very soon the pressure builds within the confined space of the cartridge case, rather than the combined volume of the cartridge case plus the length and diameter of the bore, until such time as the pressure exceeds the strength of the action.

Any compressible material such as dacron will not prevent the movement of the powder charge from moving to the top of the cartridge case under the force of the primer. What is does do however is that it keeps the powder column positioned against the flash-hole until such time that the primer explodes. This has been quoted as improving shot to shot uniformity, but the same is true if the muzzle is raised to 90 degrees, then slowly leveled on the target prior to firing.

 

[woodchopper]

I just try to pick a powder that reasonalby fills the case when loaded.

Bigger case slower powder

 

[Boomer]

I just try to pick a powder that reasonalby fills the case when loaded.

Bigger case slower powder

That pretty much means that you can't load cast bullets in large capacity cartridges, load small game loads for your medium capacity hunting rifle cartridge, or make light loads for your big bore so the wife or kids can shoot it without injury. To me that versatility is part of the charm of handloading. We can make stuff you can't buy. There is no danger involved if you use published data from reliable sources.

 

[Cerdan]

Some of those "detonations" may have actually just been overpressure caused by powder position. They used Unique in the above experiments and it resulted in proof load pressures under specific circumstances. Unique has historically been considered a very "safe" powder.

Again, I could be wrong, but my impression of this whole question of powder position is that, unlike the theory of detonation, low powder charges won't create disproportionately high pressures, they'll just create lower pressures the farther they are from the primer. That's what your article seems to say, as well.

This has also been my experience. My .38s loaded with Green Dot only shot slower when the powder was farther away from the primer, they didn't create pressures above what was expected due to powder position... I'm not convinced any of this has anything to do with the dreaded "detonation".

 

[SandRoad]

Interesting isn't it.

A couple of comments/observations.

1) I believe that some of the "dreaded detonations" were actually caused by effects of powder location in cartridges. This article clearly shows that position can have a dramatic effect.

Powder companies have tried to duplicate "detonations" and not succeeded, so this theory may offer a better explanation.

2) The comments about light loads in bottleneck vs straightwall cartridges can also be looked at another way. Straightwall cartridges are that way because the bullet diameter is large. This larger diameter creates substantially greater volume behind it as it begins to move, lessening the effects of a pressure peak.

A bottleneck cartridge, by definition, has a small diameter projectile, which has to move MUCH further along to create volume to reduce a pressure spike. Hence, pressure spikes in bottleneck cartridges are MUCH more severe than in straight wall cases.

3) Yes, a volume of powder can only produce a certain volume of gas, but the important thing is over what time period is that gas produced. Over a shorter time period at the beginning when total chamber volume is small = higher pressure peak.

4) The "location" of the powder within the case will change the way the primer ignites the powder. I theorize that unconstrained powder against the flash hole will tend to ignite a greater quantity of it when the first part of the primer flame easily/quickly disperses it (greater surface area of powder burning) resulting in the observed pressure peaks.

This is a very common way that solid rocket motors are designed if you want them to deliver constant thrust. As they burn, the internal configuration is such that the burning surface area stays constant over the duration of the burn. (Large solid rocket motors don't burn from one end, they bun as a cylinder from the centre outward so they don't melt the containment tube)

5) A filler won't keep the powder against the primer for very long, but can help ensure "orderly" ignition of the powder instead of the powder instantly flying around the inside of the cartridge exposing a greater surface are to the primer flame in the first microseconds.

Combustion theory, cylinder shape, and flame prorogation has been extensively observed/studied in the automotive industry to lower emissions, increase power and efficiency in engines. It has been done using direct observation (cameras) and then computer modelling.

The above article sheds some light on things we didn't previously fully understand about what happens inside a cartridge. Previously the theories were more speculation and usually based on experience and trial and error. So before dismissing my above observations/comments, much of this was also very new and enlightening to Ken Oehler, not exactly a noob to ballistics.