Understanding Velocity SD/ES Variation

[DenverDave]

Hello,
I am new to reloading enjoying it and learning a lot. I am hoping someone can help me with a question I have about why velocity SD and ES varies with different Powder charges with all other variables being the same.

For example load A has a 40gr charge and a high SD/ES while load B has a 41gr charge and a low SD/ES. Both load A and B use same bullet, powder, primer, brass, seating depth and neck tension.

Thanks

 

[GhengisAhn175]

It has to do with pressure curves starting from inside the case to the muzzle end.

Explosives always scale geometrically, not linear. (Example you cannot say .1 grain of X powder gives say 10 FPS so that means going from 42.0 to 43.0'will give me Y FPS)

that being said what happens at the muzzle end is important too regarding how much turbulence is created on the exit for the bullet.

Wayyyyy to much for 99% of us to worry about. All we need to know is :
-theres a node my rifle/ammo combo likes. find it
-neat full case fill is desired (decrease surface area for more controlled burn) *exception working with reduced velocity, subsonic or fire forming loads).

explosives are also finicky... the more you go away from molecular integrity (i.e. Liquid explosives vs solids) the more variations there are.

The only other thjng thing I'll mention about the "explosion"
Is temperature scaling with pressure to achieve detonation velocity.

When you increase pressure (i.e. neck tension, seat depth) you increase the rate of burn (det velocity)
When you increase temperature (i.e. Hotter primer, more powder) you increase the rate of burn. (Det velocity)


Thinking about that , the moment the firing pin hits the primer and you're building pressure, you're also at some point expecting relief in pressure (ie bullet exiting case mouth fully, bullet going down the barrel)
likewise you're also relieving temperature (spit of flame dies, bullet being farther away from ignition source which increases volume, etc).
So you have to tweak until you find a good balance.


Again, almost everything I typed above is fruitless and pointless. The only take aways should be find your rifle/ammo node.

 

[DenverDave]

It has to do with pressure curves starting from inside the case to the muzzle end.

Explosives always scale geometrically, not linear. (Example you cannot say .1 grain of X powder gives say 10 FPS so that means going from 42.0 to 43.0'will give me Y FPS)

that being said what happens at the muzzle end is important too regarding how much turbulence is created on the exit for the bullet.

Wayyyyy to much for 99% of us to worry about. All we need to know is :
-theres a node my rifle/ammo combo likes. find it
-neat full case fill is desired (decrease surface area for more controlled burn) *exception working with reduced velocity, subsonic or fire forming loads).

explosives are also finicky... the more you go away from molecular integrity (i.e. Liquid explosives vs solids) the more variations there are.

The only other thjng thing I'll mention about the "explosion"
Is temperature scaling with pressure to achieve detonation velocity.

When you increase pressure (i.e. neck tension, seat depth) you increase the rate of burn (det velocity)
When you increase temperature (i.e. Hotter primer, more powder) you increase the rate of burn. (Det velocity)


Thinking about that , the moment the firing pin hits the primer and you're building pressure, you're also at some point expecting relief in pressure (ie bullet exiting case mouth fully, bullet going down the barrel)
likewise you're also relieving temperature (spit of flame dies, bullet being farther away from ignition source which increases volume, etc).
So you have to tweak until you find a good balance.


Again, almost everything I typed above is fruitless and pointless. The only take aways should be find your rifle/ammo node.

Thanks for explaining that. I am just trying to wrap my mind around cause/effect. This helps. Appreciate the time you took to explain.

 

[supercorndogs]

Thanks for explaining that. I am just trying to wrap my mind around cause/effect. This helps. Appreciate the time you took to explain.

+2

 

[918v]

There is an optimum pressure range for a load where the ES is going to be low. Sometimes the optimum load generates too much pressure in the case you're using. You see tiny groups but the ES is high. So you lower the powder charge but the groups get bigger. What to do? Get a bigger case with more room inside. Or go down in powder charge to the next lower node.

Sonetimes your pressure sure is too low. You see tiny groups but the ES is high. But your velocity is low and the next higher node comes at too high of a cost in pressure. What to do? Get a smaller case with less room inside. The decrease in internal volume will raise the pressure of your load into the optimum range.

If you weigh eight cases made by different manufacturers you'll see they vary in weight by a huge margin. You can't simply interchange them because they have different capacities. The classic approach has been to start low and work up. But that doesn't always yield the optimum load because you run out of case capacity.

The point if if all of this is don't get stuck on using just one brand of case. There are some loads that work best in heavy, low capacity cases. There are some loads that work best in light, high capacity cases. Pick the right case for the load.

 

[CharlieNC]

Get Quickoad and you will be able to see pressure and velocity curves. And you will be able to utilize Optimal Barrel Time optimization techniques, and join those debates.

 

[Deleted member 10043]

You need to understand that two sets of small data is not enough for any sort of answer. Next time it could be the exact opposite. Just way too many variables at play. Needless to say, just go with the best load you loaded.

 

[Themagundaog]

What is considered low sd I'm getting sd's from 8-10.2 is that considered good?

 

[MarkLeupold]

Usually SD's under 10 is what guys are shooting for, yes.