BLR 7 emailed me these photos of some of his groups which he fired with his Browning A-bolt 7mm Mag.

The load was worked up using the OCW load development method, and consists of a 160 grain bullet and 70.0 grains of H1000 powder. Primer was the CCI 250.

My preliminary analysis of this is that you're seeing two principles clearly demonstrated here. First of all, the relation to barrel time of the seating depth of the bullet. As you can see, very slight changes in seating depth altered the POI considerably.

I believe that BLR 7 intends to join this thread and answer any questions and field comments regarding these targets, so I won't go into much detail beyond the obvious.

Here are the targets:

The high shot on target #1 was a sighter, and the POI was dialed down after that. The squares on the targets are in 1/2" increments.



On target #2, you'll see a shot on the 10 o'clock position of the large diamond, which shows the difference in POI with a mere .002" of seating depth difference. Seating depth variations which occur commensurate with a fast "straightaway" of the harmonic whip will move the shots well out of the group. Seating depth variations occurring during the stable end of the harmonic node will not show near as much deviation.



Note that the group to the right on target #2 forms an arc, but is still relatively tight (the bullet ripped the target and makes the group appear a little larger than it actually is, group size is slightly under 1/2").

The precision of the OCW load would seem to be demonstrated here in spades. This recipe is igniting and burning extremely well. So well, in fact, that no matter where the OAL is set at, sub MOA groups are forming.

The group that arcs is likely on a sweeping curve of the harmonic whip, and the smaller groups are likely nearing the better end of the whip.

BLR 7, I would continue with seating depth adjustments until the rifle "stacks" the shots one on another, for the first three shots. I just noted that the maximum length for the cartridge you're using will allow you to go longer, and so long as the cartridges will still fit the magazine you may want to try that. There will be a small node on each end of the harmonic whip.

However, if you simply want to quit right where you are, I think you've got an incredible load and a helluva rifle to shoot it from.

Lastly, congratulations on the trigger prowess. Many folks won't be able to hold hard enough to wring such cogent information from the load and rifle.

In other words, Good Shootin'!

Edit: I forgot to mention that for each seating depth variation, the harmonic whip will play out slightly differently. The relationship of the recoil arc and harmonic whip will change a bit depending on how long it takes the bullet to hit the lands...

Dan Newberry

[ 02-09-2003, 21:29: Message edited by: green 788 ]

Thanks Dan. I would like to add that I set this whole experiment up to prove to myself that the
harmonic whip can be demonstrated by keeping all
other variables the same and only changing the
bullet seating depths. I loaded in groups of 3
rounds each at (top target) 3.225", 3.228" and
(bottom target) 3.233" and 3.236" . There is one
round loaded at 3.231" found by itself on the
bottom target in the outer diamond that I threw in to see if it would seperate from the others.
If you have a box of factory loads, try measuring
the OAL on 20 rounds some time and you will find
greater differences in length than this. The optimal charge weight in this case shows group
seperation at 70.5gr so I settled back to 70.0gr
for this test. Now if I could just get my pics
to download! BLR7

Ok I will try again to attach a pic!

That's a better photo than the ones I posted... Can you get the other target posted in that size?

How long will the magazine let you seat the bullets?

Some of us that are plagued with flyers in our groups might want to take a close look at the seating depth consistency of our loads. These targets should make that case clearly.

Normally the seating depths of the bullets should be pretty consistent if the seater die settings have not been moved. Clean bullets, and a clean seater die would of course be very important for consistency.
And the issue of excessive runout ought to be considered here again--at least from the perspective of how it would affect OAL/seating depth. It would probably take a fair amount of runout to alter OAL's significantly, but it is worth mulling over...

BLR 7, I've gotten three emails regarding these target photos so far, so there is interest out there...
One thing I've been surprised about here lately is the number of non-forum members who come here to read. And some folks who are members prefer to pose their questions via email, rather than "out in the open," and I respect that. Issues such as the one we're discussing here do give many folks pause before posting--totally understandable.
I should mention that my own observations and conclusions about this and any other issue we discuss here are simply that; my own observations and conclusions. If others have contradicting points of view, it would be nice to hear them.

That's how we help each other learn...

Dan

For this particular bullet (Speer 160gr spbt) in
this rifle the ogive is touching the lands at
3.260" using a black marker coated dummy round.
I could have seated them out longer but I decided
to drop back at least 0.020" or more because earlier loads with this same bullet were giving
some flyers. I noticed that in chambering the
rounds from the magazine some of the bullets were
getting a little pressure mark in the longer
seatings which could cause the bullets to move
from center causing possible run-out errors. I could continue this experiment with longer seating
and not running through the magazine at a later
date. My magazine will allow a cartridge up to
approx 3.300" but that is too long if you want
to unload an unfired round for this action. 3.290"
is maximum for bullets with a narrower ogive.
These are just my findings and observations but
I hope they will help! I have not had a chance
to try this load at 200-300yds. but that will be
next. I would like to try this experiment again
with some of the new Speer Deep Shok bullets
which have a flat maplat and hopefully less
deformed tips as this influences OAL measurements.
I think for every bullet and individual chamber
there is a best seating depth that will tune the
barrel time to the load. I also think that the
optimal load must be found first by whatever
method is best for the individual shooter. Sorry
this got so long. BLR7

Greets fm CO
Pardon me, but I am asking these questions out of ignorance. I do have an electronics/semi-engineering background, so feel free to be technical in your answers.
1. This whip, these harmonics are from what basic frequency and what harmonics??
2. Are these frequencies sinusoidial?? If not what waveshape??
3. At what point in the barrel/chamber/barrel-chamber do these frequencies originate??
4. I am curious if proper barrel length could be computed if these harmonic frequencies are noted?? I totally agree that seating depth variance would be a major factor.
Even OFs are curious!
It appears your experience with Browning is completely different fm mine. Two shots with my Stainless Stalker .338 and the gp starts opening up terribly, the pencil thin barrel, maybe?? I wound up having a custom barrel made and installed.
I would have to make a career out of sighting in and even then I didn't know with certainty that
when I fired the gun the next session or in the field where it would shoot. Thanks Ray

.... is there a possibility that the impact changes are caused by a difference in pressure generated, because of the difference in seating depth? I guess that could be a chicken/egg type of question.. as the change in pressure causes the bullet to have a longer or shorter barrel time, which changes where in the harmonic wave the bullet exits...

Hornet Guy... Varying the seating depth does affect pressure. Pressure will change the barrel time, and the harmonic whip cycle will release the bullets at various points along its path--depending on when the bullets get there...

Newtoot, the barrel--according to all credible accounts I should say--vibrates more or less like a tuning fork would. Since it is being subjected to the projectile passing through it, it likely isn't sinusodial, but I'm not the one to answer that definitively.

I have a sketch of what a typical harmonic whip looks like from the muzzle on page 2 of my website, here: http://home.earthlink.net/~dannewberry/_wsn/page2.html

It is presumed that barrels of different construction and material will vibrate uniquely, and therefore will have their own individual whip cycles.

Most accuracy buffs make the claim that it is best for the bullets to exit the muzzle at a slow point on this harmonic whip, just as the muzzle is changing directions. This gives the largest window of opportunity for bullets with inexact velocities to groups with each other. If such bullets exit in the "straightaway," or fast portion of the haromonic whip, they will "string" or "arc" on the target.

If there is a downside to having the bullets exit on the "end" of this harmonic whip, it would be that they are exiting a somewhat "warped" barrel, slightly only, but certainly not straight. Considering that most rifle bores aren't drilled perfectly straight in the first place, this is probably of little or no consequence.

One curious practice is to shorten a rifle's barrel in 1/8 to 1/4 inch increments (recrowning each time) until it begins vibrating in a "friendly" pattern, with at least one good node. I don't know how much sense this really makes, but I must admit that I've seen some good shooters which had reportedly gone through this process.

I can't really claim to know the frequency of these vibrations, but the origin would be from chamber to muzzle--or so it would seem. Longer, thinner barrels would of course strike larger patterns with their harmonic whip traces.

Heavy barreled rifles will whip less, and would have less of a tendency to throw shots well away from the group when a poorly timed bullet moves down the barrel--for whatever reason.

That said, I've had excellent results coaxing 1/2 MOA accuracy from many sporter barreled rifles. And I prefer sporters for practical uses...

Developing and then fine tuning the load in the proper manner will get you far more accuracy from the average rifle than most custom rifle afficianados believe possible.
BLR 7's results here are encouraging in that regard...

Dan

BECoole,

You're right, of course. Before I got the Stoney Point "nut," I just averaged the OAL, and noted flattened points when I saw shorter OALs. Usually the Nosler polymer tips are pretty consistent, but I too have noted that the Matchkings can vary by .005" or so.

The POI changes initiated by the seating depth adjustments shown on the targets above are eye-opening indeed. Many of us have probably not considered the effect of such seemingly small changes.

One wonders how many flyers are actually caused by slight variations in seating depth...

Dan