Causes of extream spread

[Link]

If everything being equal, case weight, powder charge, jump, bullet, primers etc. what would cause ES? I know there are mostly 2 nodes in a tune, and you have to find the sweet spot in your rifle. Now how does ES effect this? How can ES be high if everything is equal ?
thanks Link

 

[Alex Wheeler]

Excellent question. How is it that you can load very good ammo and one powder charge produces good es and another doesnt.? Looking forward to this.

 

[dannyjbiggs]

Beats me...I dono. I just load 'em, an pick out the good ones.

Dan

 

[jimbires]

tagging in , I'm keeping an eye on this thread .

 

[savagedasher]

tagging in , I'm keeping an eye on this thread .

The only thing that changes the bullet speed is a difference in pressure . The reason the Es change with powder amounts Is the powder isn't burning at the with the same ignition pressure Many of time when you Change amount Of powder it's a simple as a ignition change or I should say primer
My self I use seating depth to lower Es .
In raceing when you lean the jet size down you back off of timing. Or ignition . Larry

 

[jvw2008]

The only thing that changes the bullet speed is a difference in pressure . The reason the Es change with powder amounts Is the powder isn't burning at the with the same ignition pressure Many of time when you Change amount Of powder it's a simple as a ignition change or I should say primer
My self I use seating depth to lower Es .
In raceing when you lean the jet size down you back off of timing. Or ignition . Larry

 

[jvw2008]

The OP started his question with the qualifier "with ALL things being equal" which elimates changes in charge, case capacity, seating depth, neck grip (tension), primer variance, etc. If all those variables are held constant, then other considerations for ES would include ambient temperature of the charge (how long does the case sit in a hot chamber), changes in barrel friction (also heat dependent), and especially recoil management (a free recoil round will be slower than a hard shouldered round). I believe the fps difference in types of recoil management can be as much as 20 units.

 

[Rsadams]

I get this also its hard to believe sometimes...I have wondered if case volume comes into play but it just doesn't seem like it should be as much as you see sometimes... Makes you wonder if spending the time to match everything is worth it sometimes... I pulled down 2 federal GMM .308 rounds today and there was half a grain difference in the charge... Yet I sit and make sure each charge is perfect..

 

[Bart B.]

The only thing that changes the bullet speed is a difference in pressure .

A popular belief. And true as far as the bullet speed relative to the rifle is concerned. However, rifles are not motionless while the bullet goes through their barrel unless they're hard mounted and don't recoil back while powder gas pushes the bullet out. Newton's laws explain why; the old equal and opposite thing.

A given rifle and ammo lot can produce near 100 fps average velocity spread across several people shooting the rifle hand held against their shoulder. The velocity extreme spread and standard deviation will also be different for each person.

That rifle resting on bags in touched by humans will shoot that same ammo at a lesser average fps but have much lower extreme spreads.

Clamp the barreled action fixed in place and that ammo will have the highest average velocity and lowest ES and SD numbers. ES and SD numbers are often 1/4th that obtained with the rifle held against ones shoulder.

Us humans are not a 100% repeatable platform to hold rifles while bullets go through their barrels. This is why benchrest rifles are virtually untouched by us while bullets go through their barrels. Hold them any way and velocity numbers start to change in bad directions as well as where the muzzle points as the bullets leave.

There are some who believe the shouldered rifle and barrel are motionless until after the bullet leaves, then the jet effect of gas leaving the bore moves the rifle.

 

[Bart B.]

I know there are mostly 2 nodes in a tune, and you have to find the sweet spot in your rifle.

I don't understand this claim, but I see no significant accuracy change in ranges through 300 yards over a 5% spread in charge weights. Yes, zero's will change but group size not much at all. A 308 Win shoots 168's virtually equal in MOA through 300 yards with all rounds with 41, 42 or 43 grains of IMR4064. A slight elevation change in zero is normal. That's the norm for all the 308 Win barrels I've worn out.

Any group's shot count less than 15 is not very meaningful; 20 or 25 is much better. The top 10 shooters in a benchrest match typically have a 4X to 6X size across their several 5-shot groups. Everyone else, more spread.

A 1/10th grain change in powder charge weight causes about 6 to 8 fps change in muzzle velocity for 308 Win size cartridges. If a load has a 40 fps ES, A 2/10ths grain change in weight may not make a significant change if you're shooting a few-shot group.

Having seen the same lot of ammo shoot MOA or better through 1000 yards in dozens of rifles with different barrel shapes, bore and chamber numbers, I think if folks shot more rounds in their test groups, they could dump charges from a measure with a 3/10ths grain spread and make no changes less than 1 grain.

 

[Bart B.]

A weak firing pin spring won't fire primers uniformly. Many weaken with age.

Neither will shallow dented primers, pins need to stick out past the bolt face at least .055" to .060" when full forward in the bolt. Measure yours.

 

[CharlieNC]

First of all I think es as related to a tuned load is perhaps two independent topics. There are some who report that nodes can be identified by measuring velocity vs charge weight to find the plateaus, whereby es is minimal at those regions. I have not observed this, but cannot dismiss the possibility.

Personally OBT and Quickload have reliably identified poi target nodes in my limited experience. Tuners work. Both meaning harnessing barrel harmonics is what provides the poi nodes, not minimization of es.

Even though you strive to make the factors like volume, charge wt, etc perfect there are always some differences. Operating at a node minimizes the effect of the resulting as on down range poi via the harmonic tuning. But in addition the condition of the barrel changes shot to shot, the exact shape and surface condition of bullets vary, etc,etc. Many variables beyond our control. The identification and use of a node to desensitize the output (poi) to the inputs (charge wt, etc) is interestingly not only appropriate for shooting, but also applies for many industrial processes as well.

 

[mgunderson]

Absence of ES makes nodes irrelevant, but doesn't answer the question.

 

[billlarson]

If everything being equal, case weight, powder charge, jump, bullet, primers etc. what would cause ES? I know there are mostly 2 nodes in a tune, and you have to find the sweet spot in your rifle. Now how does ES effect this? How can ES be high if everything is equal ?
thanks Link

1) case weight no good... volume better
2) powder charge must be exact... +/- .02gr. That`s hundredths...not tenth`s... esp. true in long range....
3) to jump bullets you must have straight cases
4) primers.. uncontrollable.... get a good lot........
to get good data... you gotta have a large test lot..... at least 20 shots....
in my opinion neck tension is of paramount importance.....
different guns will give different speeds and sd`s and es`s..... same ammo....
OMHO... let the flames begin.....
bill

 

[mikecr]

I don't understand this claim, but I see no significant accuracy change in ranges through 300 yards over a 5% spread in charge weights. Yes, zero's will change but group size not much at all.

What you don't describe well is is ACCURACY -vs- PRECISION..
A zero change is directly affecting accuracy.
And increasing shots in grouping is doing nothing to ES. That's SD.

I'll throw in neck sealing. Faster the sealing, lower the ES. Also, striking pin protrusion was mentioned, there is no credible rule of thumb there. Best is what testing demonstrates as best, and I suspect it changes with factors like primer brand, seated crush, striker spring, and trigger sear location. The trigger itself can affect ES, and so can the bolt in different ways.
Neck tension, loaded case capacities and chambered load densities, flash hole variances, pocket depth variances, charge variances. Many recoil factors, and temperatures(barrel and ammo).

Also, there are different kinds of 'nodes' here. Powder burn nodes, seating nodes, primer/striking nodes, barrel timing nodes, barrel vibration nodes, recoil nodes, and an extreme pressure node come to mind. All summing to pure abstract, with no bit of it predictable on paper.
Some having no connection with ES.

 

[Bart B.]

What you don't describe well is is ACCURACY -vs- PRECISION.

nor does 94.45% of all posters on shooting forums. I'm not part of the 15.55% of people on this forum who are perfect. Besides, I thought this was the accurateshooter.com forum. Not the accurateandpreciseshooter.com forum.

A zero change is directly affecting accuracy.
And increasing shots in grouping is doing nothing to ES. That's SD.

I must not have made clear my charge weight spreads. Fixed that post to clear things up.

 

[Bart B.]

2) powder charge must be exact... +/- .02gr. That`s hundredths...not tenth`s... esp. true in long range....

How many long range competitive shooters cut that last tiny particle of powder to a fraction of its size to get charge weights uniform to 1/50th grain? Wouldn't that be hard to prove with a cartridge which typically has a 6 to 7 fps change in muzzle velocity for 1/10th grain change in charge weight?

Never heard of such meticulous detail in ammo loading. Especially when I and others have shot 308 Win ammo with .003" bullet runout in new, unprepped cases with a 4 grain spread in weight, 3/10ths grain charge weight spread that shot about half MOA at 600 yards. In a few dozen barrels with different shapes plus different bore, groove and chamber dimensions.

 

[dmoran]

@Bart B.
Not doubting that ammo is capable of 1/2-MOA, but how consistently?
Are you saying it will "agg" 1/2-MOA or better?

My own "match ammo" goals for LR-BR are: charges to with in 0.04-hundredths, case volumes qualified within 0.4-tenths, and 0.001-thousandths max to both seating depth and runout (on the bullet). Also will note: that I thrive for 1/4-MOA vertical dispersion or better at 1k from 5-shots.
Donovan

 

[Down South]

Beats me...I dono. I just load 'em, an pick out the good ones.

Dan

Same here!.......

 

[Bart B.]

Not doubting that ammo is capable of 1/2-MOA, but how consistently?
Are you saying it will "agg" 1/2-MOA or better?

Half MOA is what they feel it shot inside of with 15 to 20 shots in a string based on how far shot holes missed their calls. They were all top ranked Palma match folks from different countries. 20 rounds were grabbed at random as the fell out of the second of two Dillon 1050 progressives loading them then tested in a Win 70 based rifle clamped in a machine rest. They printed 2.7" at 600 yards.

I thrive for 1/4-MOA vertical dispersion or better at 1k

How often does your 1K yard groups do that?