COAL and pressure spikes, different powders.

[Palladan44]

Does COAL (which dictates the space between your projectile and powder) and pressure "spikes" have any favored powders or combinations over another?

Does faster burning powder (like WIN 231 in a .38 special) care less about COAL than say loading WW-296, a slower powder, in a 357 Magnum? (Where the case will be loaded with propellant to 80% capacity rather than maybe 25%)

Id like to fully understand the phenomenon.

 

[fxvr5]

Not sure what you mean by pressure 'spikes'.

All powders reach a 'peak' or maximum pressure when they burn. With fast powders the peak appears sooner (in time) than it does with slow powders, from the graphs I've seen.

 

[Palladan44]

Ive read about this a bit, and i see the term 'Pressure Spike' a lot. I just assume it means an increase in pressure. Unintended increase in pressure with a published load, but if bullet is seated not correct, whether it be too shallow, or too deep, the pressure curve changes i guess. Id like to learn more about it.
And why it happens.

 

[Ru4real]

Does COAL (which dictates the space between your projectile and powder) and pressure "spikes" have any favored powders or combinations over another?

From what I’ve read, definitely!

For example, I started using Tightgroup this year, and from what I’ve read, Tightgroup is sensitive to OAL. I’ve not tested, however.

I have tested H110, and in the 45 Colt with 255 gr cast, it is not OAL sensitive, IMO. Meaning that in my testing, variation in OAL by as much as 0.1” didn’t produce velocity deviations outside my normal velocity deviations for fixed OAL loads.

I have no way to measure pressure. A spike may be occurring with my 110 example, but if so the spike is inconsequential. I took my testing to mean that a good portion of the powder is being lit and burned with the bullet in the barrel where the major counter force is bullet/bore friction.

My assumption with 110 is that burn rate inhibitors reduce the effects of OAL variations.

 

[FROGO207]

If you put a pile of mordern smokless propellant on the ground and light it you get a pile of flaring, smoking, slow burning stuff. No explosion. Put it in a closed container and it will explode. The smaller the container the faster the burning propellant produces gasses. Your bullet is basically the cork in the container to relieve that pressure. The heavier the bullet and the smaller container the faster the pressure rises. Manufacturers take advantage of this to make propellants of varing burn/pressure producing rates. In handgun ammo the OAL/COAL is a minimum value to keep the pressure at the safely tested value. There are certain propellants that when presented with smaller volumes have pressures rise faster than others. Those are what the OP is refering to. Generally the faster the burn rate is the faster it burns under increasing pressure. The pressure rises and it burns faster in an exponental way. If you stay within the reloading guidelines and keep the bullet from setting deeper into the brass you remain safe. The reason we have to work up loads is because no two guns or componant sets are identical and might produce overpressure sooner than the setup they tested with
Also the old methods of testing with a copper crusher just measured total pressure and the new tranducer method will show fast spikes that were previously unknown. Hope this helped.

 

[243winxb]

https://apps.dtic.mil/dtic/tr/fulltext/u2/a491715.pdf

https://discover.dtic.mil/

Executive Summary:
On 3 October 2007, ORA Inc. conducted a series oftests to evaluate the velocity of
detonation (VOD) of commercially available double base smokeless powder. Testing
was conducted at the request of Special Agent Steven Beggs, Bureau of Alcohol,
Tobacco, Firearms, and Explosives (BATFE). Four separate six pound double base
powder charges (Alliant BULLSEYE ) were detonated. All charges were contained in plastic
bread bags and were unconfined. A high speed switching circuit and crushable switches
were used to measure the VOD ofthe unconfined powder charges. A high speed
computer data acquisition system was used to record the output ofthe switching circuits.
Velocity of detonation was measured between 19,000 to 25,000 feet per second, with 3 of
4 measurements at 19,000 - 20,000 feet per second. Given the possible influence of
charge/switch geometry on the VOD measurement, a conservative lower bound for VOD
could be set at 15,000 - 17,000 feet per second.
Based on these tests it is concluded that the smokeless powder tested meets the definition of a high explosive. - it detonates in an unconfined configuration using a number 8
commercial detonator with a conservative VOD between 15,000 - 17,000 feet per
second.

 

[rfwobbly]

To me these are 2 different things. Generally speaking...
• For a given load, chamber pressure increases as the bullet is seated deeper into the case.
• So called "pressure spikes" have more to do with the behavior of individual powders.

These erratic behaviors can be induced in some powders by having too little powder present, in others by having too much. In still others by either !

Most common powders are "double base". That is, they are doped with highly reactive nitroglycerine. In my mind it's the "nitro" content that contributes most to how erratic a powder may behave. If it's important for you to work with powders that display "less edgy, calmer" behaviors, then you can look for "single base" powders, which contain zero nitroglycerine. This will not eliminate all danger, but will go a long way toward more predicable outcomes.

 

[__steve__]

Does the loaded static case pressure contribute in any margin? For instance, with a large relative case volume/powder load ratio, like a 11gr titegroup in the massive 500 s&w (dreaded “light” load for the 500) loaded and crimped at 30°F, then later that year in summer the charge is fired after sitting in the sun when it’s 98°F. The 0.30” seating depth along with the increased temp might double relative volume of air inside the case. Does this play in results? It certainly would if oxygen was a catalyst, but only pressure is here.

 

[Palladan44]

To me these are 2 different things. Generally speaking...
• For a given load, chamber pressure increases as the bullet is seated deeper into the case.
• So called "pressure spikes" have more to do with the behavior of individual powders.

These erratic behaviors can be induced in some powders by having too little powder present, in others by having too much. In still others by either !

Most common powders are "double base". That is, they are doped with highly reactive nitroglycerine. In my mind it's the "nitro" content that contributes most to how erratic a powder may behave. If it's important for you to work with powders that display "less edgy, calmer" behaviors, then you can look for "single base" powders, which contain zero nitroglycerine. This will not eliminate all danger, but will go a long way toward more predicable outcomes.

Blue Dot must contain a high amount of nitroglycerin then.

 

[Palladan44]

How much pressure is rammed into a pistol case when a bullet is seated?
I first noticed this when i sealed some primer pockets, and through the still wet sealant, there were bubbles coming out of the primer/pocket space. This was hours after loading them.
If you retest handloads fairly quickly after loading, is it possible were getting different results than we might a few weeks later?