Pressure vs foops

Reloading equipment, methods, load data, powder and projectile information.

Pressure vs foops

Post by Blr243 » 22 Feb 2022, 1:35 pm

For yonks looking at adi reloading data and when choosing which powder to use I notice the highest chamber pressure don’t necessarily give the highest muzzle velocity .....I would have thought higher pressure gives higher FPS It’s about time I knew why is it so , or rather why it is not so ?
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Re: Pressure vs foops

Post by bladeracer » 22 Feb 2022, 1:55 pm

Blr243 wrote:For yonks looking at adi reloading data and when choosing which powder to use I notice the highest chamber pressure don’t necessarily give the highest muzzle velocity .....I would have thought higher pressure gives higher FPS It’s about time I knew why is it so , or rather why it is not so ?


I don't know the science, but different powders burn at different rates. If they burn very quickly they run out of steam sooner, if they burn more slowly the pressure lasts longer. If you have a particularly short barrel you'll want a fast powder, for a really long barrel a slower powder so it's still making pressure right up to the bullet exiting the muzzle. .22LR uses very fast powder as it has to work in pistols as well as rifles. By the time it's poking along eighteen inches of barrel all the powder is gone and pressure is dropping, so velocity is being taken by bore friction. Even after all the powder is burned, the bullet still needs about 250fps to actually overcome the bore friction and fall out the muzzle. I guess the actual number can be calculated, but the lowest I've measured a bullet that actually exited is 275fps, anything lower stopped in the muzzle. That was in a .243.
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Re: Pressure vs foops

Post by bladeracer » 22 Feb 2022, 1:57 pm

I used to look at velocity vs pressure when choosing a powder, now I just grab AR2206H then try other powders if I'm not getting results.
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Re: Pressure vs foops

Post by rc42 » 22 Feb 2022, 2:59 pm

Actually an interesting subject and most of load development is finding the ideal powder type and amount for any particular projectile type and weight which is being fired from a specific length of barrel. Even within optimum ranges for these variables, some will produce better groups than others in any particular barrel so our search is never ending. Lack of powder options also keeps sending us back to the beginning.

Fast powder is needed for short barrels and light projectiles, it does create a very high initial pressure spike but that's needed as it doesn't push the projectile for very long as it accelerates and is out of the barrel so quickly. Care must be take not to go over pressure as the gun can be damaged and accelerating the projectile too fast will cause rifling to strip the outside rather than gripping it so you get under rotation and tumbling, this also sets limits on usable FPS for a given barrel length.

Slow powder is needed for long barrels and heavy projectiles as they take longer to get up to speed spend much longer in the barrel. Powder volumes are higher which also means any weighing errors are less significant so generally it's harder to go over pressure. Slower powders generally need more case volume too, hence having much more space for powder in rifle cases.
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Re: Pressure vs foops

Post by Oldbloke » 22 Feb 2022, 8:06 pm

bladeracer wrote:I used to look at velocity vs pressure when choosing a powder, now I just grab AR2206H then try other powders if I'm not getting results.


Smart boy. I treat AR2208 the same.

I think its fair to say that generally speaking there isn't a lot to be gained stuffing around with other powders. There are exceptions of course.
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Re: Pressure vs foops

Post by on_one_wheel » 22 Feb 2022, 8:32 pm

I'll leave playing with mutable powder types up to the F Class boys. I can only imagine the amount of fussing around to find that perfect pressure ramp that matches the node of the resonant frequency and projectile exit timing of that particular barrel, at it's current state of wear, with their shoelaces fitted and tensioned to precisely the same point that they were when they had that perfect score.

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Re: Pressure vs foops

Post by Blr243 » 22 Feb 2022, 9:08 pm

Thanks for the replies .....I understand now the initial high pressure from a fast burn , tapering of and giving An end result of a speed less or the same as other powders whose highest peak pressure is less than the former but at a different rate/ duration/ timing
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Re: Pressure vs foops

Post by on_one_wheel » 22 Feb 2022, 9:19 pm

Just as long as the noisy end goes BANG rather than the action, Apparently that's the most important bit to get right.

I should write a reloading book, I'd only need a page or two :lol:
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Re: Pressure vs foops

Post by bladeracer » 22 Feb 2022, 9:53 pm

on_one_wheel wrote:I'll leave playing with mutable powder types up to the F Class boys. I can only imagine the amount of fussing around to find that perfect pressure ramp that matches the node of the resonant frequency and projectile exit timing of that particular barrel, at it's current state of wear, with their shoelaces fitted and tensioned to precisely the same point that they were when they had that perfect score.

I've got 3 powders to cover everything, 2208, 2209 and black


It's become more of an issue recently with the lack of the faster rifle powders and no pistol/shotgun powders being produced. It requires experimentation with powders not usually listed for particular cartridges.

I have all ADI powders except the APS powders and their 50BMG powder. But I mainly use AR2206H and Trailboss.
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Re: Pressure vs foops

Post by Blr243 » 22 Feb 2022, 10:42 pm

Stoked with my kg of 2206h I scored today. I’ll probably grab a very big bucket of 2208 next time I see one , then I can ignore and care nothing about shortages for quite a while
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Re: Pressure vs foops

Post by boingk » 22 Feb 2022, 10:54 pm

Apart from the burn rate and pressure you're also looking at gas velocity.

Gas velocity is increased in part by bottleneck cartridges. Fat column of powder ignites and then has to squeeze through a smaller neck into the barrel, thus velocity goes up. Look at the WSSM's for the extreme version of this concept.

This is why you generally use a faster burning powder in straightwall, large bore cartridges, as they do not have this effect and cannot fully utilise the slower burning powders. Also the projectile moving down the bore also leaves a relatively larger void behind it which must be filled with pressurised gas, compared to the volume of the case in the chamber.

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Re: Pressure vs foops

Post by on_one_wheel » 23 Feb 2022, 12:33 pm

I wonder what the red phosphorus stuff on match heads goes like, :huh:
I'll add that one in my " OWW's Howdy Don't Do Reloading Manual" :thumbsup:
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Re: Pressure vs foops

Post by Don_Stevenson » 24 Feb 2022, 7:37 am

on_one_wheel wrote:I wonder what the red phosphorus stuff on match heads goes like, :huh:
I'll add that one in my " OWW's Howdy Don't Do Reloading Manual" :thumbsup:


Technically the red bit of a safety match doesn't contain any Phosphorous these days. The match head is mostly potassium chlorate and there is a small amount of red phosphorous embedded in the striker.

The potassium chlorate is still energetic enough that loading it in a case would probably be a bad idea although its mostly an oxidiser and would perform "better" with a fuel.
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