velocity variance

[Vaughan]

I'm just back from a range session testing new loads for the 308W at 100m. I had loaded 45gr of Varget behind 180gr Norma Oryx. I had tried this load at 0.010" off the lands last sumemr and thought it was a bit wild and went to 180gr NABs instead.

This time I set up my new chrony and found that at 0.040" they were running at about 780m/s (measured 4m from the muzzle - a bit slower than I was expecting but thems were the numbers) and the 5-shot group was about 1.5" not spectacular but certainly "minute of moose" and better than I had consistently managed with this bullet before. At 0.03" average MV was again 780m/s with really low ES/SD and grouped below 1". I can not remember exactly but the ES for that 5-shot group was less than 3m/s. So I was very happy.

The odd thing, to me, was that when I tried 0.020" and 0.010" the ES/SD went WAY up with the ES over 15m/s for the 5-shot group and the groups opened up again. The groups were not crazy bad, and part of the problem is obvisouly me, but still they were noticably worse.

I assume this is caused by pressure variations was I sneak up to the lands but is that right? I did not expect such a dramatic change just from goign from 0.03" to 0.02" off the lands Where these bullets contact the rifling was measured by full length resizing a case fired in this rifle, splitting the neck of, then jamming the bullet in by closing the bolt and then measuring using a Sinclairs bullet seating gauge. I will repeat a few of the loads and see if the result is the same but perhaps some of you wise folk can 'splain it to me..... Basically I'm very happy with the load I have (although I might try squeezing in another 0.5gr of Varget) but I am curious

[Corjack]

There are all sorts of things at play when you start sneaking up on the lands, you increase case capacity, you loose neck tension and you change the pressure spike. Frankly about the time I think I have things figured out with one round, another round gives me fits. I am going to order a set of Redding, bushing type dies and get Quickload so I can study where the optimum pressure spikes are for a certain cartidge with a certain powder. Guessing all the time is burning up a lot of powder and bullets.

[skeetshot]

There are all sorts of things at play when you start sneaking up on the lands, you increase case capacity, you loose neck tension and you change the pressure spike. Frankly about the time I think I have things figured out with one round, another round gives me fits. I am going to order a set of Redding, bushing type dies and get Quickload so I can study where the optimum pressure spikes are for a certain cartidge with a certain powder. Guessing all the time is burning up a lot of powder and bullets.

How can Quickload tell you what the optimum pressure spikes are ? I would think there would be the major factor of the chamber dimensions and leade location of that particular barrel that would influence that.

I guess there is no option but to burn some powder and document

[Vaughan]

By coincidence one of the Blaserbuds here ran my details through Quickload the other day. The program seems to think I'm seriously over-pressure with 45gr of Varget behind 180gr bullets. I have my doubts. I worked up from 43gr and see nothing untoward on the fired cases. Nevertheless, I will back off a little and play around some. I have bullets and powder to spare and I doubt the animlas will notice the difference if I end up using a grain or 2 less powder.....

/Vaughan

[skeetshot]

By coincidence one of the Blaserbuds here ran my details through Quickload the other day. The program seems to think I'm seriously over-pressure with 45gr of Varget behind 180gr bullets. I have my doubts. I worked up from 43gr and see nothing untoward on the fired cases. Nevertheless, I will back off a little and play around some. I have bullets and powder to spare and I doubt the animlas will notice the difference if I end up using a grain or 2 less powder.....

/Vaughan

Vaughan, I find it difficult to believe that without some input of barrel/chamber/leade conditions any program can accurately predict except in most general terms, specifics of any cartridge.

Maybe we are interpreting the results incorrectly or not understanding the software.

No harm in backing off a little, but how would that predict the velocity variation you documented.

[Vaughan]

Skeetshot,
I do not believe for a moment a program can predict or model accurately the outcome of a particular load in a particular chamber. But it may well give good guideance for what is a safe starting point for new combinations of cartridge/powder/bullet, especially if no load data is available, so I think the program has merit. Especially if, as others suggest, it is a good appxoimation of their practical experiences.

I was using Hodgdon's load tables and their suggested max load. Quickload predicts it is certainly max, maybe even into unsave territory. So, both are telling me "be careful" but neither explains why I get the velocity variation when I creep the bullet forward. That data comes from field testing with my chrony and that just says "stay back a bit and you will be fine".

So IMO I have a safe max power load and a good seating depth, yielding consistent accuracy and stable MVs. Now I'm just messing about trying to edumacate myself a little..... If the program suggest I am near the border of what is sensible, it seems reasonable to back off a little and field test again to see what effect this has on MV and accuracy. If I could measure the actual pressure the load was generating, that would be interesting, but I do not know how this is done or what is needed to do it.

[skeetshot]

You're right, Vaughan, you have a safe max load, but the tables and software cannot confirm that.

Anyway, this velocity variance thing is something I struggled with in the 243 Win, and while I did not find relief in backing off on powder charge, I got it by turning the necks to a uniform thickness and using the Lee collet die to provide uniform neck tension. I found after doing that, the seating depth while critical for accuracy was not critical for velocity variance. Cannot explain why, but that is what I found.

I am getting another 243 barrel and it would be interesting to see if the same parameters would hold for that barrel.

However, in the various 30-06 rifles I have had, I found considerable difference in seating depths, powder charges, velocities and groups, hence my comments.

[Vaughan]

You're right, Vaughan, you have a safe max load, but the tables and software cannot confirm that.

Agreed, I guess only not blowing my fool head of confirms that.....

[Corjack]

I think quickload is just a helpfull guide, to help establish boundries for start and max loads. It beats just totally guessing, especially when I have discovered that changing seating depth, also changes case capacity, making the data in reloading books, not very accurate.

[skeetshot]

When I started reloading for my then newly acquired R 93 barrel in 243 Win, I started with a "medium -safe" load of 37 grains of H 414 in Partizan cases. I found considerable difficulty in extracting the fired cases from the rifle. I even had to tap the R 93 bolt with a mallet

Some of you members may recall me writing to you looking for help and wondering what had happened.

Yet when I shifted to RWS cases, I could take the powder charge all the way upto 44 grains without any pressure signs or extraction difficulties.

But the manuals only mentioned to start at the "low end" and "carefully work your way up".

The point I'm trying to make is there are too many variables here to know what is a safe load or, to even fully know what is an unsafe load.

So where does that leave a rookee reloader

The matter of velocity variation is even more confusing. I believe that a combination of consistent neck tension, consistent powder/primer pressure and playing with seating depth are required for reducing this variation. And one of the better techniques is the "ladder testing method" where one optimises each variable one at a time.

The beauty of the Blaser R 93 barrels is that they are so well made that they benefit by these techniques, many of which are used by Bench Rest shooters, and if the shooter can do his part, will give some spectacularly accurate groups. Using 87 grain Sierra heads, 41 grains of Sochem 355 (almost IMR 4350) with properly prepared cases and a lightly fouled barrel I am able to achieve 5 shot groups at 150 meters often less than 1/4 inch with the 243 sometimes. But I weigh my stock with lead shot and use a 14 x scope and had to go through several hundred rounds before I achieved this.

So what. . .? for me the journey was and continues to be as much fun as the destination.

[Vaughan]

I believe that a combination of consistent neck tension

This I need to learn......

[Spartan]

The matter of velocity variation is even more confusing. I believe that a combination of consistent neck tension, consistent powder/primer pressure and playing with seating depth are required for reducing this variation. And one of the better techniques is the "ladder testing method" where one optimises each variable one at a time.

So what. . .? for me the journey was and continues to be as much fun as the destination.

I tried out Dan Newberry's Optimal Charge Weight Load Development (OCW) technique recently to identify an optimal charge for the 7mm-08. The optimal charge of 43.5gr of AR2208 was not pioneering work on my part as it turned out to be the charge my buddy had recommended.

Newerry also speaks of the the Audette Ladder Test and the use of Velocity spread to predict accuracy.
See, http://optimalchargeweight.embarqspace. ... 4529811360

Newberry writes about velocity spread and it's use for prediction of accuracy - "Uniformity of velocity (meaning low extreme spreads of velocity) are definitely not an indicator of the OCW zone. We are actually finding that in many cases the OCW zone does not have the tightest numbers--at least initially. Fine tuning of the recipe with seating depth variations and primer changes will improve the velocity consistency, but simply shooting a succession of graduated charges over the chronograph and looking for a tight velocity spread will not lead you to the OCW."

There is an interesting line of literature with detailed instrucitons - see also, Chris Long's longitudinal shock or acoustic waves theory....not simple harmonic bending vibration modes - that can be followed with Newberry's OCW, if you are so inclined.

I expect the journey will be interesting and I hope will also prove to be enlightening - although, for me the jury is still out on his theory until I do more load development for various powders and calibres .

[skeetshot]

Fun, isn't it

Absorbing,educative yet frustrating sometime, but it will make you a better shooter

And yes, that 'round robin' technique is the way to go

[Hawk]

"Where these bullets contact the rifling was measured by full length resizing a case fired in this rifle, splitting the neck of, then jamming the bullet in by closing the bolt and then measuring using a Sinclairs bullet seating gauge."

I suggest you buy the Sinclair threaded case sold to go with the seating gauge. Without the case screwed onto the tool you easily could be creating unnecessary variation. You should also have the sinclair ogive bushings. Using cartridge overall length is not a good proxy for distance of the bullet ogive from the lands. I suspect the velocity and group variation you are seeing if from the bullet being jammed into the lands when you think it's off the lands. If you want to test this, blacken or color the bullet on a completed round and load, then unload. If it's touching the lands you should see four or five marks on the bullet from the lands. Just one man's opinion.