ES of slower powders

[Toolbreaker]

Typically when developing a load for a given cartridge, I tend to favor the slower burning powders for obvious reasons (case fill ratio, higher velocities, lower pressure per powder volume, ect.). Something I've observed myself and read about, but never really understood was that these slower burning powders can (not always, though) produce somewhat higher velocity spreads over a given number of rounds fired than that of a faster powder. Why is that? What are the mechanics behind it? Thanks,

Edit: Just to clarify, I'm not referring to huge velocity swings. More along the lines of 10-20 fps over another powder.

 

[Jager]

That's an interesting notion, one I've never read about and never observed or sensed.

Perhaps because a given rifle cartridge / bullet weight has a reasonably limited range of powders that are optimum for its use. And most of us tend to stick to powders within that "appropriate" range. I don't know that I have ever developed a load for a given rifle/bullet combination with powders of widely differing burn rates.

There are a million things that affect the consistency of ammunition. My guess is that any ES/SD variance that folks might observe is up to those other things. One thing's for sure... If faster powders were inherently more consistent/accurate than slower powders, we'd all be all over that.

But I'm all for learning something new. Can you point us to any references?

 

[SBS]

If true, perhaps because we would normally use slower powders in larger cases (larger vs. bore diameter). Usually the more powder capacity behind the same bore diameter, the less inherently accurate the cartridge is and the wider the ES. Be interesting to know if it may be case capacity or if slower burning powders actually burn more erratically since they burn over a longer period of time.

 

[ballisticxlr]

A physics based hypothesis: If pressure builds up over time and you add more time then you get more chance for time varying non-linear processes like heat dissipation and the creation of chemical compounds from simpler ingredients to affect things. As an exaggerated example: If you touch the loud lever and the powder instantly and completely combusts inside the case before even the projectile is let slip from the neck tension then there's no variance in how much of that powder heat gets turned into pressure. It's essentially all of it because it was an instantaneous conversion from solid to gas. The pressure will change with the volume as the bullet cruises down the bore and the barrel will eat some heat and some heat will be lost to things like chemical compounding. If you have all of that heat loss going on while the powder is still combusting and the powder is in fact burning in the barrel rather than in the case then the amount of energy you get out of it can be more or less from shot to shot because of those time varying processes that eat energy. It's the same reason a nail head is hot as balls after being sunk with a couple super hard hammer blows but it's cool to the touch if you use a hydraulic press to slowly push it into the board. The time spent slowly pushing it in allowed the heat that would have built up in the nail do dissipate. Change how fast the press pushes the nail in and you'll get different amounts of heat buildup in the nail that directly relate to how long you made the nail sinking process take.

 

[JohnKielly]

All slower powders, or just those with larger kernels? Larger kernels have less surface area, so any variation in the ignition capability of the primer will be exaggerated.

 

[ballisticxlr]

All slower powders, or just those with larger kernels? Larger kernels have less surface area, so any variation in the ignition capability of the primer will be exaggerated.

This is a really interesting part that nobody talks about probably because there's no easy solution in small arms chamberings unlike in artillery and even just stuff like 20mm and 30mm cannon. I love ball powders because they meter so well but they also suffer commonly from generous SD's because they have so much surface area that burning can be a little random in large cases like belted magnums and even .308 derived chamberings. I've long wondered if something like a flash tube might not really help consistency by lighting the powder column along its whole length more or less simultaneously. I suspect, though I have no evidence really, that a flash tube would really help things of this sort. Problem would be, the holes in the flash tube would have to be so small to be smaller than the granule size of something like BL-C(2) that it'd probably get all cloggy on the first firing and then you'd need a special tool to decap the brass and that would probably then have to be done in a separate die from resizing.

 

[Toolbreaker]

Thanks for the replies. I was a little reluctant to post this thread, mainly since I have never done any type of formal testing on this and have made these observations over time across different atmospherics and with different cartridges. I thought what I have seen could have very well just been a culmination of background noise. Possibly even choice in primer, although that's a whole 'nuther can of worms.

@ballistic's post above does hit upon the subject of speed of combustion at a certain volume within the barrel. I had assumed this may have been a factor, but honestly I just didn't know.

More than anything, I was just wondering if slight deviations in es such as that while still maintaining acceptable to good velocity and pressure may be the first signs that the powder choice in on the edge of being too slow for the given caliber and components.

 

[INTJ]

It have noticed that slower powders often produce higher ES when the case isn't full of powder. I have sometimes fixed that by adding more powder. If that doesn't work then I change to the next faster powder.

I am not sure why yet. I find that with a lot of things, that it's easier to know what works than why.

 

[Jager]

It have noticed that slower powders often produce higher ES when the case isn't full of powder. I have sometimes fixed that by adding more powder. If that doesn't work then I change to the next faster powder.

I am not sure why yet. I find that with a lot of things, that it's easier to know what works than why.

The orientation of the powder column in a loaded round can vary if the charge weight percentage is less than 100%. And that will result in varying burn rates, pressure profiles, and velocity.

You know that satisfying feeling when you pick up a loaded rifle round and shake it up next to your ear, hearing and feeling the sloshing of the powder inside the case? Well, that air space between the top of the powder column and the base of the bullet... isn't really a good thing.

Which is not to say that compressed loads are always and everywhere a good thing - they're certainly not. But the static powder charge orientation that they bring is nearly always a benefit.

SAAMI test procedures explicitly specify handling rounds in a particular way, because of that very phenomenon.

 

[INTJ]

The orientation of the powder column in a loaded round can vary if the charge weight percentage is less than 100%. And that will result in varying burn rates, pressure profiles, and velocity.

You know that satisfying feeling when you pick up a loaded rifle round and shake it up next to your ear, hearing and feeling the sloshing of the powder inside the case? Well, that air space between the top of the powder column and the base of the bullet... isn't really a good thing.

Which is not to say that compressed loads are always and everywhere a good thing - they're certainly not. But the static powder charge orientation that they bring is nearly always a benefit.

SAAMI test procedures explicitly specify handling rounds in a particular way, because of that very phenomenon.

I agree, but why does it seem more of an issue with slower powders for a given cartridge than faster? If I had a nickel for every time I have moved to the next step faster powder--with more airspace--and watched groups tighten and ES fall......

Perhaps it has to do with heat and pressure at a specific time in the ignition process?

 

[bozo699]

It have noticed that slower powders often produce higher ES when the case isn't full of powder. I have sometimes fixed that by adding more powder. If that doesn't work then I change to the next faster powder.

I am not sure why yet. I find that with a lot of things, that it's easier to know what works than why.

Spot on!
Wayne

 

[bozo699]

I agree, but why does it seem more of an issue with slower powders for a given cartridge than faster? If I had a nickel for every time I have moved to the next step faster powder--with more airspace--and watched groups tighten and ES fall......

Perhaps it has to do with heat and pressure at a specific time in the ignition process?

Once again Spot on!
Wayne

 

[PigButtons]

I agree, but why does it seem more of an issue with slower powders for a given cartridge than faster? If I had a nickel for every time I have moved to the next step faster powder--with more airspace--and watched groups tighten and ES fall......

Perhaps it has to do with heat and pressure at a specific time in the ignition process?

Not only this post but many in this thread are very interesting, thanks to all who have taken time to think deeply on this and contribute to the discussion.

I can give an example of a powder that addresses the question that INTJ poses. Hogdon LilGun is a fast powder that is very sensitive to the pressure during combustion. If the case is not full enough to get the pressure up to spec, then the ES/SD is all over the place. Get it just right and it is a wonder drug for making bug holes. Just my attempt at furthering the debate.

 

[ebb]

I don't know that I buy into this, I load for 7mmRUM and have a load that has a ES of 6 with Retumbo. Big case, long barrel, heavy bullet. I think that the application has to be right to take advantage of the slower powders.

 

[INTJ]

I don't know that I buy into this, I load for 7mmRUM and have a load that has a ES of 6 with Retumbo. Big case, long barrel, heavy bullet. I think that the application has to be right to take advantage of the slower powders.

I promise you the slower powder ES issue is real, but like the OP said, it doesn't happen all the time.

 

[Alex Wheeler]

I dont think anyone understands ES totally. You can take a very good ES load and add .3 of powder and make the ES higher. You can also effect it with small seating depth changes. I believe its to do with exit timing and muzzle expansion, however have no proof other than I cant explain it any other way. The good news is, ES and group size do not correlate. Shoot groups at the distance you compete or hunt and forget about ES.

 

[INTJ]

I see this slightly different than my friends @Alex Wheeler and @Northridge.

We 1000yd BR shooters have a habit of saying ES doesn't matter, and many of us don't even use a chronograph when tuning at 1000 yds. I know I typically don't. It's just one more thing to drag up to the line and I don't need it.

However, whenever I do check my good loads with a chronograph, they are almost always have ES in the single digits or low teens.

So we have to be careful when we say ES doesn't matter. The stuff we do to load for LR BR, the case sorting and prep, primer sorting, bullets sorting, etc is going to minimize ES. I don't know if any of my LR BR loads, even my initial loads at short range, have ever had an ES of much more than 20.

A hunting rifle is a different matter, especially when we aren't able to shoot it at the distances we will hunt. I have seen sub 1/2" 100 yd groups with ES of 50 or more. I can promise that load won't shoot well at long range.

I and many others have had a lot of success finding a load at 100 yds, then making hits past 1000 yds. To do that, we have to find a load that shoots small AND has a decent ES. The priority is groups size, but the ES ought to be around 20 or less.

Now if we are doing load development at 1000 yds for a hunting rifle, then ES isn't a focus--because we won't have good groups or tuning ladders at 1000 yds if we have a bad ES.

 

[Alex Wheeler]

I still hold to ES doesnt matter even for hunting, and I tune a lot more hunting rifles than BR. If you dont test the rifle at the distance you plan to take game, you really shouldn't be shooting game that far then. But I get your point and I agree with you as well. My point is the load should be tested at the distance and as Leo said, "the paper doesn't lie"

 

[dellet]

If this were other types of propellants, a term would get thrown around that for some reason isn’t associated with loading cartridges, “volumetric efficiency”. If we are talking about an engine, it was determined that 14.7/1 is the magic air fuel ratio for best performance. The computer and sensors have the job of maintaining that ratio in all conditions. A very basic look.

This is no different than loading at the bench for conditions, but without the computer and information inputs, it’s more art than science.

What I think I have found over the years, is that with many powders you can find a fill ratio that is most efficient. Meaning that if 98% produces the smallest ES/SD, you can move the bullet almost anywhere you want and if you keep that fill ratio, the spreads stay consistent. That ratio, determines initial burn rate and start pressures. If start pressures are consistent, they should be consistent all the way down the barrel, leading to consistent exit times and velocity.

The trick is getting that exit time where you want it. A “0” SD at the wrong exit time could be a lot of vertical.

What you are doing with seating depth or small powder adjustments is fine tuning that volumetric efficiency, manipulating the burn rate to its most efficient or consistent point in that particular chamber and day.

In an effort to prove a point that burn rates can be manipulated to extremes with case capacity and bullet weight, I used sub sonic 300 Blackout. 10.5 grains of 4227 under a 208 AMax that netted 1050 fps. In the same rifle, 11.5 grains under a 125 SST nets 1075 fps. and still has enough pressure to cycle an AR.

Being able to tune gas pressure in a load enough to cycle a gas gun, or not, simply by changing seating depth, changed my thinking about manipulating pressures and therefore burn rates.

So in my mind(as twisted as it can be), a high density load of a slow powder, may just actually have a faster burn rate, than a low density load of a faster powder. Where this will show on the target is different exit times at the muzzle. It’s possible to have the exact same velocity, with completely different travel time from chamber to muzzle. It’s all based on acceleration curves.

Just some different observations

 

[JohnKielly]

All slower powders, or just those with larger kernels? Larger kernels have less surface area, so any variation in the ignition capability of the primer will be exaggerated.

What I should have said was larger kernels likely have less surface area per unit volume of the case so any variation in the ignition capability of the primer will be exaggerated.