Load data, pressure vs velocity with different powders…

[Emmett Dibble]

I can not be the first person to ask this question and my google-fu is usually pretty strong…. (which makes me think the answer is so obvious no one asks it and I am a moron, which is possible)

When comparing load data to pick out powders to try, especially when you have several to choose from that are relevant to the cartridge you want to load…

Why do some powders make more pressure and less velocity then other powders in the same application, same cartridge/bullet, apples to apples.

Is there a rule of thumb or do you just plug along till you find the powder that your cartridge likes, using knowledge gleaned from other people loading that cartridge on powders that are already known to work well in it?

I have been reloading for 10-ish years with very good results, not a test pilot, can get ED numbers in my bolt guns in the low teens regularly.

Can someone break down the answer like talking to a 5 year old?

 

[One Eye Pete]

I normally am not the first person to use a particular cartridge, so I get ideas of which powders work best and which don't from websites such as this one. I like to use a powder that fills the case well, is not temperature sensitive, and is somewhat available. In today's world the availability is the hardest criteria to meet.
As far as the answer to peak pressure vs muzzle velocity I'm by no means an expert but I'll take a stab at it. When the primer ignites the powder it starts to burn and pressure rises rapidly even before the bullet starts to move. The burn rate of the powder increases due to the pressure increase but the bullet starts to move. The bullet moving down the barrel increases the volume containing the hot gas thus having the effect of reducing the pressure. Normally somewhere in the first few inches of bullet travel the burn rate reaches its peak and the volume is the still relatively small yielding the peak pressure. As the bullet continues to travel the volume continues to grow and the powder burn continues to slow causing the pressure to continue to drop until the bullet exits the barrel and the pressure goes to 0. With a fast powder the pressure rises quicker to a higher peak and also drops quicker. With a slower powder the pressure rises slower the peak isn't as high but the pressure also drops slower as the bullet advances down the barrel. The powder that generates the highest muzzle velocity isn't necessarily the one with the highest peak it's the one with the greatest area under the curve. That is the powder that can keep the most pressure on the bullet for the longest time as it travels down the barrel. This is overly simplified but it is the best way I can explain it.
For a given cartridge I try to use the bullet / powder combination that others have found to work well. I just dont have the time or money to do that much experimenting. I waste enough time and money getting my rifle in tune with what others have proven to work.

 

[RickKr]

That is the clearest, most comprehensible explanation I've heard yet. Thanks.

 

[Metal God]

Why do some powders make more pressure and less velocity then other powders in the same application, same cartridge/bullet, apples to apples.

One eye Pete answered while I was writing so what he said and what I said below haha

Oh man in a nut shell , it's all about the burn rate of the powders . A faster burning powder creates the same pressure faster . You'd think no big deal if they all reach about the same peak pressure but there is one thing that throws a wrench into the equation . That pesky little old thing is the bullet and how quickly it can get down the barrel .

As the powder ignites and starts to build pressure the bullet also starts to move . As the bullet moves down the barrel it is creating more and more space behind itself for the expanding gases to fill . There is a very happy medium of bullet moving down the barrel and the gases expanding behind it to be optimal for a given cartridge . If the gases expand faster then the bullet can get out of the way you get a BOOM instead of a bang . If the gases expand to slowly behind the bullet and the bullet out rounds the expanding gases you get inconsistent burn and in some cases the powder just stops burning all together because it needs pressure as well as the spark to keep burning .

In short a slower burning powder builds pressure slower and keeps the pressure higher longer as the bullet travels down the barrel creating higher velocities . I don't know if all manuals do this but all mine do and that is the powders are listed from fastest to slowest in the chart and as you will see the powders at the bottom of the page tend to give you more velocity then the powders at the top of the page . You can also see when you go up in bullet weights some of the faster powders are no longer in the data and new ones appear at the bottom of the pages as the bullets get heavier for a given cartridge .

This is because the heavier the bullet the slower the powder tends to need to be to be optimal . It's harder to get a heavy item to move then it is to get a light one to move . Therefore you need a slower powder so the gases do not expand faster then the bullet can get out of the way .

Ok that's the most basic general explanation that I can give . Others can expand upon further because past that we are getting above my pay grade .

I

 

[Jager]

Most shooters want the most velocity they can get from a given bullet, while also achieving a high degree of accuracy.

As a general rule of thumb, those two goals are most often met when choosing a powder that approaches SAAMI maximum pressure at the same time the cartridge is approaching 100% charge density.

Faster powders hold more latent energy per volume of powder than do slower powders. So the selection of "best" powders are going to be those with a burn rate that pretty much fill the cartridge case without generating an amount of pressure that crosses that max pressure threshold.

The accuracy goal is significantly affected by the unique harmonics of the rifle the cartridge is fired in. Much of handloading involves identifying the nuanced changes to apply to a load, once you're in that high-charge-density-at-near-SAAMI-max, such that the load "goes quiet," resulting in consistently good accuracy.

There are exceptions to this rule-of-thumb, of course. But it's a good starting point in understanding what powders might work versus those that won't.

 

[Emmett Dibble]

Thank you, One Eye Pete. That is what I suspected but then I start down rabbit holes till I get annoyed and leave it be till the next time I am changing powders or loading something new. I was hoping (which is probably why I can't find it) there was some sort of rule of thumb. I have found powders in some guns work with great accuracy with low pressures, and some with high pressures. When you pull up published data with a lot of loads with powders from many manufactures it's shocking to look at the pressure variances from powder to powder. I do understand that some older loads get changed or deleted from newer load data because the old CUP method isn't as accurate as the strain gauges in use now and ugly pressure peaks were found using the newer methods.

I appreciate your reply and will leave this thread up for posterity sake.

 

[Metal God]

When you pull up published data with a lot of loads with powders from many manufactures it's shocking to look at the pressure variances from powder to powder

OK I'm going to butcher this but I'll give it a try . The powders that burn the most consistent are the powders with the higher pressures in the data . Keeping in mind these tests where done in perfect/consistent conditions . Your results will almost certainly not mirror there's do you your tests varying from there's .

Example : lets say for a given cartridge the maximum allowable pressure ( MAP ) is 62k psi . The powder that shows the closest pressure to that MAP in the data is the most consistent as it relates to pressure from shot to shot . So if you have a powder that shows 45gr gets 60k psi and that's the max load . They are saying at 45gr there test showed the pressures averaged 60k psi and never or rarely ever got over the MAP of 62k psi . This means if you see a powder in that same chart with there max load 41gr only hitting 57k psi . That means there 41gr charge although may have averaged 57k psi they saw spikes of it hitting the MAP of 62k psi in some tests . So even though they could get a higher average pressure closer to the MAP they were seeing pressure spikes above that MAP at those higher averages .

So now you have two powders with extreme spreads quite different from one another . Presumably that difference goes both ways . Meaning the powder that allows a charge to hit 60k psi of a 62k psi cartridge has an ES of 4k psi which intern means the powder that only lets you get to 57k psi or 5k psi of MAP has a ES of 10k psi . This is where you get the relationship of a more consistent powder based on how close the data lets you get to the MAP of a given cartridge .

I'll add this is just one tool to use for picking powders and is not universal or intended to be a blanket statement , only to help aid in powder choice if you are really digging deep . The case fill rate matters , burn rate matters , bullet weight matters , constancy of ignition matters etc etc it all matters .

 

[One Eye Pete]

Keep in mind the bullet, the brass case and the rifle barrel itself all have a lot to do with pressure also.
Heavier bullets offer more resistance and for a given powder charge will generate more pressure. That's why for a given powder charge weight goes down as bullet weight goes up.
Bullet bearing surface is also a factor. the more bearing surface the harder to move the bullet down the barrel. (more pressure)
Case capacity also comes into the equation. In general if you have two cases say both 308 win, the one with the smaller capacity (unfortunately not all cases are created equal) will generate a higher pressure for a given powder charge.
Barrels can have slightly different bore sizes. 6mm (243) barrels come in two different bore sizes there are .243/.236 bore barrels (such as Brux) qnd there are .243/.237 bore barrels. Obviously the .236 bore will squeeze the bullet a little more and thus generate more pressure for a given powder charge. Even a new barrel will also have an effect until it breaks in after about a hundred rounds.
Primers can effect pressure to but I ain't even going to go there.
The data in reloading manuals is a very general guide (it is generated with a specific powder, primer case, bullet and barrel) and normally errs on the safe side. This is why the golden rule of reloading is ALWAYS START LOW AND WORK UP for your particular combination.
This is what makes precision reloading and rifle tuning so much fun, trying to coordinate all this crap into an accurate load!!!
This may be very controversial but the best method i have found to tune a rifle is the Eric Cortina method. You can look it up on this forum or on YouTube but basically get your powder charge first then work on your bullet seating depth. The other thing I have found the hard way is that accuracy trumps velocity. The faster moving bullet may be less affected by the wind and have a flatter trajectory but the slower bullet with the load that is in tune with the barrel will hit in the same place far more consistantly.

 

[CharlieNC]

In addition to max pressure, Quickload provides velocity and pressure vs distance in the barrel, among many other useful results. Also the ability to automatically search and compare all powders which give a desired velocity, pressure, barrel time, etc. It is well worth the cost as a screening and learning tool.

 

[243winxb]

Slow & fast powder can have the same peak, of maximum pressure. PSI

Fast consummed quickly.

Slow may well burn to the muzzle. To slow will blow unburnt powder out tne muzzle.

 

[FeMan]

Peak pressure, in-and-of itself doesn't mean much other than the stress it puts on the rifle action and barrel. Its the duration of the pressure that translates to velocity. If you look at a pressure-vs bullet travel chart in Quickload, its the volume under the pressure curve that matters. The greater the volume the greater the velocity you will get.

 

[Emmett Dibble]

Peak pressure, in-and-of itself doesn't mean much other than the stress it puts on the rifle action and barrel. Its the duration of the pressure that translates to velocity. If you look at a pressure-vs bullet travel chart in Quickload, its the volume under the pressure curve that matters. The greater the volume the greater the velocity you will get.

which is one of the reasons I ponder it… I load for several “not so sturdy” actions and would like to beat them up as little as possible.

Not my first or 20th rodeo reloading, and COMPLETELY understand all the variables mentioned in replys above.

 

[243winxb]

Why do some powders make more pressure and less velocity then other powders in the same application, same cartridge/bullet, apples to apples.

Hodgdon data for 6.5 Creedmoor, when comparing IMR 4350 & IMR 4451, is interesting to me. On the burn rate chart, they are next to each other.

The main difference may be 4350 is single based & 4451 is double based, containing nitroglycerin. More energy? The 4451 has more additives, that may slow burn rate, the nitroglycerin is needed to bring it back up to speed.

Comparing maximum loads for both, seems to show 4350 producing more velocity, for about the same amount of powder in grs. The 4451 seems to produce more pressure.

Having reloaded both, i will stay with IMR4350.