Assuming identical weights of powder, what are the next most likely culprits in shot to shot velocity variation that contributes to higher than acceptable SD/ES?

Does it always follow that super tight groups are a function of consistent shot to shot velocity? What's your goal for SD/ES for maximum accuracy?

Thanks!

Roger

NEck tension & less than 90%+ density loads. I have found some very accurate loads that SD+40-50fps, but usually the tighter the SD, the better the groups. Get it into the teens & single digits is really nice, but not necessarily the most accurate.

just for interests sake a few years ago I took the chrono out and tested loads for uniformity. Using a 243 I tried weighed and measured charges of both ball and stick powders. By far the most consistent loads were measured ball, some of the test runs (5 shots each test) were within 5 FPS of eachother, then came measured stick, then weighed ball and furthest out was weighed stick. Some of the weighed stick loads were 200-250FPS apart. all in all I suppose I must have fired about 200 rds with various powder/bullet types. I didn't test the accuracy at the time, but did it just to see about weigh vs measuring powder charges. Found out that the bench resters are right when they say not to weigh

When I see a wide range of velocities from the same charge, the first thing I investigate is neck tension from the brass case.

Take a once fired case, and try to slip a bullet into the neck. If it won't go easily, you have found the problem.

Garrett

The underlying culprit is ALWAYS inconsistent ignition. The bullet starts moving at different points during the start of combustion, and a varying amount of powder travels down the barrel after the bullet.

Case design and primer characteristics can address this. Ideal is when the flame of the primer hits the base of the bullet (that way, all the powder is burning before it goes down the barrel).

If you can't change the case shape, you need to increase the amount of pressure inside the case before the bullet starts moving. That way, the combustion is sure to be started throughout the charge before the bullet starts moving.

To ensure this, you need a primer that isn't so vigorous that it shatters too many kernels, but vigorous enough to consistently ignite the powder column. You need a powder charge that is uniform within the case (i.e. a full case), and you need a high bullet release tension.

The primer question is, pretty much, a try and see proposition.

Bullet release tension can be manipulated by increasing the neck tension (smaller bushing or expander ball, or softer brass), crimping, or seating into the lands. Seating into the lands, for example, increases the bullet start pressure about three fold (from the 3,500 PSI range to well over 10,000 PSI). HTH, Dutch.

Interesting responses. Ive found the single greatest variant to be the type of powder I use in any given case and the lowest S/D's Ive recorded have been with ball powders of all things.

Of all the variables I must agree with Dutch about inconsistant ignition. But as to SD's which are high but accurate... not so at long range were shot to shot variation can cause big differences in drop. Yes highly variable loads can shoot super short range groups, but it's a ballistic impossibility at longer ranges.

Wstrnhuntr, I believe you should consider the possibility that it wasn't the powder, but the powder/primer combination. Changing the powder improved things -- but changing the primer could have achieved the same thing.

I too, have seen the lowest ES in strings with ball powder running at medium pressures. Why? Beats me..... FWIW< Dutch.

I have found that very slow powdera give larger than average MV SDs. I have found, for example, that H4831 in a RUM is pretty consistent, but certainly not the fastest. I have alao found absolutely max load in magnums often group well, but exhibit high SDs.

quote:

Originally posted by Wstrnhuntr:
Interesting responses. Ive found the single greatest variant to be the type of powder I use in any given case and the lowest S/D's Ive recorded have been with ball powders of all things.

The problem with ball powders is that they are extremely temperature sensitive. So while they might be consistent today, they won't give the same velocity 6 months from now.

Theoretically at least the smaller SDs should yield the best accuracy.....That's never been the case in my experience.....as a matter of fact I'd not draw any correlation between SD and accuracy at all.

quote:

Originally posted by rogerinneb:
Assuming identical weights of powder, what are the next most likely culprits in shot to shot velocity variation that contributes to higher than acceptable SD/ES?

Variation in each cartridge component, variation in the way the cartridge fits the chamber, variation in all the chamber and bore dimensions and variation in the consistency of the Striker's Impact.

quote:

Does it always follow that super tight groups are a function of consistent shot to shot velocity?

Most people seem to think so, because that is one thing they can measure with their chronograph. Only problem is (as some have pointed out) just because you have low SD/ES does not necessarily mean the groups will also be small.

quote:

What's your goal for SD/ES for maximum accuracy?...

My goal for SD/ES is to never concern myself with them. Shoot the groups with the Test Loads, pick the best grouping Loads and reshoot them to verify the smallest groups. No chronograph is used and thus no concern for low SD/ES.

If your goal is to have Loads with a low SD/ES, then the chronograph is certainly the way to go. But, if you want the best groups, you just need to shoot and see what groups the best.

This just happens to be another of those nice things anyone can verify for themselves if they put in enough Trigger Time with and without a chronograph. Eventually it becomes clear that having a low SD/ES does not ALWAYS mean that particular load provides the best accuracy.

For rational distance hunting, Hot Core is right, SD really is not relevant. For short range Benchrest, SD is usually not even a concern.

That said, once you get to 500 plus yards (which few, if any hunters do), velocity variation DOES have a significant impact. If you are besotten with long range varminting, as I am starting to be, then SD becomes another variable.

Thing is, SD and short range accuracy are not mutually exclusive. By building a robust load, you can easily have both. I once built a 35Whelen load which had nearly 200 fps in ES. That's rediculous, and certainly enough to influence shot placement even at 300 yards. Simply seating the bullet deeper reduced the problem, and did not effect accuracy enough that I could tell the difference. FWIW, Dutch.

NOTHING IS ALWAYS in reloading or the accuracy of a gun...Most of us are slaves to technical bull s---t, and it just ain't so....you have to try it in your gun, then you will have a correct answer..I have had guns that shot under an inch with a lot of variation in MV from shot to shot and I have had the opposite.

Sectional Density IMO has more to do with bullet performance on game than anything else, as it equates to penitration, the most important thing in the killing of game...

It also has to do with stabilization of a bullet at extreme ranges, which I am not much concerned with from a hunters point of view...a high SD bullet seems to hold its velocity better at extreme ranges, but only from a target shooters point of view I think.

Ray, I think you are reading SD as Sectional Density, instead of Standard Deviation. FWIW, Dutch.

Dutch:

I agree with you totally about SD and long range shooting. You can see vertical stringing at long range. I had a .300 RUM load that shot bugholes at 100 yards, but would throw flyers at long range due to velocity variation.

Unfortunately, most "long range" shooters never shoot past 500 yards...until they see a deer.

quote:

Originally posted by Hot Core:
My goal for SD/ES is to never concern myself with them. Shoot the groups with the Test Loads, pick the best grouping Loads and reshoot them to verify the smallest groups. No chronograph is used and thus no concern for low SD/ES.

If your goal is to have Loads with a low SD/ES, then the chronograph is certainly the way to go. But, if you want the best groups, you just need to shoot and see what groups the best.

This just happens to be another of those nice things anyone can verify for themselves if they put in enough Trigger Time with and without a chronograph. Eventually it becomes clear that having a low SD/ES does not ALWAYS mean that particular load provides the best accuracy.

For those of you that are interested, I did not mention anything at all about "Distance". If you are shooting up close or way out yonder, what I posted above is still what I would recommend. Look for the best group, irregardless of the distance.

It seems we often forget what we are actually trying to accomplish and get mired down in lots of secondary trivia that then become concerns. For those of you who want small StdDev/ES, I'm all for you having it. Best of luck to you.

But for those of you that want long range accuracy(or short range accuracy), the only way to know you have it is to shoot the groups and see what they do. That is the only thing that matters.

Interesting responses! Thanks guys.

However, I don't think it could be said that there is no correlation between SD/ES and accuracy--at least statistically speaking. That would mean velocity could vary infinitely with no systematic effect on accuracy. I suppose the relationship could be nonlinear?