I have often wondered how many different variables may exist in extreme velocity spreads. Let's say for example you are loading a 300win mag (this is just for the sake of discussion) and lets say you are using 180gr pills,..and are getting a 50fps spread. How would the experienced loaders go about determining the cause,..or what steps would you take to eliminate that spread? Different powder (slower of faster) ? adjust neck tension? change primers? Adjust seating depth?

This is definately not a comprehensive answer, but, if all other factors are controlled, powder charge can have a LOT of influence upon extreme spread. When I test my reloads I usually check out several strings of cartridges identical except for the powder charge then select the one that performs the most consistantly for use as a "standard" load. Note that this does NOT necessarily mean that it will have any particular "desired" velocity.

There certanly are many factors that contribute to high spreads, but most could be traced to powder charge and it's volume in the case. Weighing the powder charge carefully is a big step, however, the volume of the case has a big influence too. It is not uncommon to find cases that vary in weight by 1 to 3 grains, maybe more. Since we know the external size/shape will be held fairly close, the difference is the INTERNAL volume/capacity of the case. Weighing and segregating cases is one of the first steps in preparing cases for competition.

Neck tension is critical, but consistant tension is too. I make sure the inside of case necks are clean, and you can feel the difference when seating bullets. Good load density can also help. So many things.....

I usually try to load a progression of increasing powder charges (deltaC). Usually you will see the velocity change (deltaV)go flat just below the maximum charge. At this point the velocity is least sensitive to powder charge and will tend to give you the least ES. Then adjust seating depth for best groups.

If you get a point where deltaV/deltaC is small and the point of impact change is also small then you have found a really good sweet spot.

Does not always work, but it's a plan. I'm not the inventor of this method and don't remember who it is attributed to.

Example:

70 gr/2250 fps/0x,0y
72 gr/2300 fps/1x,1y
74 gr/2350 fps/2x,2y
76 gr/2400 fps/3x,3y
78 gr/2420 fps/3.1x,2.9y ******
80 gr/2470 fps/2x,2.5y

The one with the stars is a sweet spot, and you should try seating depth changes at and near this load. (Most are not this easy.)

I hate to sound stupid, but what are the x and y values and what are they telling you ?

Actually, I know that case weight doesn't mean squat when you are trying to understand the internal volume of a case, and are discriminating between good and bad by a few grains! I can show you cases which vary by as much as 10 grains in weight, but have IDENTICAL internal volumes!

Scott

1. Don't worry too much about extreme spread. Loads with a standard deviation of 35 fps will still nicely do 1/2" 5-shot groups in a good gun. For 5-shot strings, divide MV extreme spread by 2.33 to get standard deviation.

2. If the powder does not fill the case, spread will be greater.

3. All other factors constant, as the barrel heats up, pressure and muzzle velocity will rise. For best consistency, shoot at some selected barrel temperature.

They represent the Point of Impact (POI) on the target as the load changes. When the change in velocity and change in POI both get small for a constant charge step then you have a double "sweet spot".



(The assumption is that the scope settings are not changed during load development.)

Don , what range is the example above for or at what range would you use this method. Does the 3.2x mean that this bullet shot .2" higher than the last one ? Thanks , Bill.

Yes, X and Y are in inches. I usually use X for horizontal position and Y for vertical, but it makes no difference which is which in the example. You are looking for places in the powder progression where incremental charge has the least effect on POI and velocity. The range is usually 100 yards, so that velocity changes have less effect on the X,Y position than barrel "harmonics" or barrel motion in response to ignition. (And it stays on a smaller target.)

Sorry I explain it so poorly, I wish I could find links to the original article, or remember the name of the originator. The method has been used for years.

Don, your explanation was fine. I think I got it now, thanks.

OK,....I have my light gun in 300RUM tubed with a 30" 1:10, and am pushing 200gr SMK's with 88gr of H1000. The gun does .25moa at 100yds,..but shows velocity deviation too great for my liking. I get some highs and lows on the impact at 1000yds,..which I attribute to the spread. My groups are quite often tight,..tight!!! But there is the high or low flyer,..not indicating my wind reading ability to bench technique. Then there are the groups which obviuosly don't experience such a spread,..and they are beautiful. But to be consistent,...I need to pull the spread down,.so I am looking at options.

The necks are turned and loaded with "s" series bushing dies with correct bushings. the runout is .001" or less. The brass is match prepped ans seperated,...but I KNOW this damn spread is exagerating these groups.

I would suggest that you read "Rifle Accuracy Facts". It is the best book ever written on what is really
going on in a rifle. If yoy are getting 0.25 groups now tou will be playing around with the "funny" things.
Good Luck!

Scott, Please explain how the internal volume remains he same. The external size and shape will be exactly the same after fire forming. If there is a weight difference between cases, and the outside is the same, where is the difference in weight? It is physically impossible to be the same, internally, given these parameters. By sectioning some cases, it appears the place which differs is in the web area. This is the thickest section of brass.

I think it hard to argue that a consistant powder charge/volume will produce the most consistant velocities. By weighing the powder, then insuring the case consistancy, will give the smallest velocity spread.

Ask a 1000 yard benchrest shooter what he does to reduce velocity spread. When you are persuing the smallest group at long ranges, low velocity spread is paramount, since a 25 fps difference will equate to something like two inches of verticle at 1000 yds.

At 1000 yards, 50 fps can be significant.



Rem RUM brass is notoriously inconsistent. Has Lapua or anybody else started making high quality aftermarket brass?



If the brass has not all been fired the same number of times it will not have been work hardened the same amount, and the bullet grip will not be the same. You can anneal the brass to try to regularize it, but you have to have a consistent annealing process.



Did you deburr the flash holes?



I always got the lowest ES by using a powder that was about 2% compressed when loaded.



My best 1000 yard range loads always had the bullets against (or even into) the lands. This gave the most consistent ignition and made the bullet retention in the case less of a factor. These were not the fastest loads as I had to drop the powder charge a bit to combat the pressure rise. Drop your charge by at least 5% when you go up against the lands.



Good luck,

In my experience, the shape of the cartridge case is one of the most important factors in ES. I have shot strings with an ES below 5 with my 223 WSSM, and strings above 150 ES with my Whelen.

ES, contrary to what was said above, is mostly a factor of consistent ignition. Getting the powder to burn, thoroughly, before it moves into the barrel and the heat/pressure drops is the key.

To do that in a long case, (i.e. a powder column over 1.25" long or so), without much of a neck, you need plenty of neck tension, or a bullet hard into the lands. You also need a primer properly matched to the charge, so it doesn't dislodge the bullet when it detonates, doesn't overignite too much powder, and has a long burn duration. Since you can't see any of these factors, it's still a trial and error process, however. JMO, Dutch.

Before you spend a lot of time and effort, please consider this.

A rifle that averages 1", 5-shot groups, will routinely produce groups as small as 1/2", and as large as 1 1/2", just by normal, random variation--no actual change in the performance of the gun or shooter.

The ratios "scale" in this case, so you can divide the numbers by 2 or 3 or whatever to get numbers that fit your gun. Pick 2 for convenience. If your system averages 1/2" 5-shot groups, you will get groups as small as 1/4" and as large as 3/4", even if you are doing everything perfectly consistently.

Just because it's human nature, if we get a few 1/4" groups, we tend to think that is what the gun is really doing, and the rest is just technique. It's probably not true.

Three shot groups are extremely weak statistically. You will see even less consistency there.

So it may be that you are already at the point of diminishing returns, and any additional accuracy is going to be very difficult to get.

Don,...flash holes were deburred,....all cases are on the same firing, all cases were seperated by weight with ~1% variance, powder charges are all weighed seperatetly as well. My 200grSMK's are AT the lands,..no jump.



I do think RL25 or Retumbo may help in this case,..as they will certainly provide a greater load density.



Also,..the rifle has gone .25moa 5 shot groups (.5moa is terrible for this gun),..and any variation is easily attributed to me as I can always tell when I pull a shot,.even before seeing it on paper. The barrel is just coming into it's sweet spot at 188 rnds down the bore,..little if any fouling after 40rnds at the last match,..and a 3" group at 600yds in a 15mph wind with gusts and lulls. It is the vertical factor that keeps these groups from going into the 1's quite a few times,..so I am quite sure it is ES.



No,...the damn brass guys decided to spend time producing WSM brass but no RUM brass yet. only rem or that contracted fed brass that is a 500rnd minimum order.



I guess some new powders filling more of the case are in order.



Dutch,.....ifin I go to a RL25 load that goes into the 90+gr range,..can a 210M primer be used to control the ignition? I know the 215M is a fierce bunch of flame,..maybe that will help??? or how about CCI magnums??? or WLRM??



thanks for all the input so far.

If you're convinced that it's not just normal random variation, then perhaps the following will be a help--

1. Weigh and sort your bullets. In my much smaller 223, that brings SD in about 5 fps.

2. Make a .3 lb weight for your muzzle. As the bullet accelerates down the barrel, the barrel lug presses backward against the stock, which creates upward force on the barrel. The weight reduces the distance the tip of the barrel moves.