Velocity and harmonics

[kiwi3006]

I went out on the weekend to check my long range dope as I had dropped my powder charge by a grain.

Using 7mm rem mag, 72gr H1000, federal case, federal 215 primer, 168 berger

Over the chrono the load had two velocity areas one from 3051-3058 and the other 3031 - 3037.

At 300 yds the 3031 load shot consistently 2" lower than the 3058 load.

Checking with my ballistic program the 20 fps difference should only have made 0.4" difference.

The same pattern showed at 600 yds.

I think the impact difference is more a barrel harmonics effect rather than just a velocity difference.

When selecting this load 72 gr gave a vertical 0.8" group at 100 yds, 72.5gr gave a horizontal 1.5" group.

I think I might try 72.25gr and see what happens.

What do you think?

Stu.

 

[BobbyL]

Things like seating depth, neck tension, can also affect you POI. Everything you change will affect this due to the barrel harmonics and velocity as well.

Other thing is those programs are not 100%....

 

[kiwi3006]

I agree with your comments Bobby, however all the loads were made identically, full length sized case, powder weighed to the individual grain on a beam scale, the seating depth was identical on all loads, seated with a Forster match/benchrest seater.

I think though that the loads at 3030 fps were leaving the barrel when it was going down and the 3050 fps loads were leaving the barrel when it was going up.

I'll have to keep looking for the node when the barrel is "still".

Stu.

 

[justgoto]

What do you think?

If matching velocities correlated to the whipping barrel that consistently, there would be no need to shoot and examine groups during ladder testing. You'd only need to shoot your rifle into a backstop then read the chronograph.

 

[kiwi3006]

Justgoto, Yes you are right, but what is the velocity? Have you read any of the threads on OCW? Basic theory is that vibrations travel up and down the barrel. The trick is finding the velocity that has the projectile exiting the barrel when the vibration is back down at the breech end.
This is exactly what the ladder test is trying to find, the velocity range that has a minimal effect on POI.

Stu.

 

[justgoto]

but what is the velocity?

If your theory holds true, the actual velocity is immaterial. You are talking about each individual vibration, not the dynamic of the node.

"Your theory" being:

I think though that the loads at 3030 fps were leaving the barrel when it was going down and the 3050 fps loads were leaving the barrel when it was going up.

Which denotes the bullet exiting consistently in mid vibration... An exact point of that specific vibration consistently.

 

[Oliveralan]

Barrel harmonics are really hard to predict. A slight difference in muzzle velocity means the bullet is leaving the barrel during a different position during the vibrations. If your previous load was too hot, you may want to just work up a new load a little bit, say from 70gr up.

 

[kiwi3006]

Oliverlan read this link on OCW Optimal Barrel Time Paper

He shows that barrel vibrations are really easy to predict.

Stu.

 

[groper]

a short answer to your original question would be simple. There is no way 20fps ALONE would cause impact 2inches lower, it would go against the laws of physics otherwise. Something OTHER THAN velocity HAS TO cause this POI shift.

In my experience, i have reproduced similar phenonmenons myself, the reasons are often mysterious BUT, in the end there IS always a reason.

Did you shoot the groups with BOTH loads on the same day, at the same time, same barrel temperature?

Tell us more about this rifle, what type of barrel is on it length, profile etc is it bedded and free floated or uses foreend pressure etc?

 

[kiwi3006]

Groper the loads were all shot on the same day same conditions, 905 mb 10 degrees C.

Rifle has a 26" No5 contour barrel, free floated, harris bipod, rear supported by rolled up jacket.

The load was 7mm rem mag 168 berger, 72gr H1000, federal 215 primer, federal case. This load as stated produced two velocity groups one from 3031 - 3037 and the other from 3051 - 3058. This was over 12 shots. There was no pattern as to which velcoity appeared ie they did not get faster or slower as the shots were fired.

The 3030 velocities grouped together and the 3058 velocities grouped together about 2" higher at 300 yds.
The same pattern showed at 600 yds except the separation was 5".

Stu.

 

[Mikecr]

This is a really interesting investigation.
If I understand correctly, your 72.0gr load is producing a velocity spread of 3031 to 3058(ES-27).
With this you're getting double grouping miles apart.

Well, nobody can predict internal ballistics with a slide rule.
And I don't know if your out to understand something here -more for the learning -than result.
But I think most experienced in load development would suggest moving on with this load.

Even if it's the best you've reached so far with this barrel, and fantastic, how would you ever rely on it for distance shooting? I mean which group will it be hitting today, or tomorrow, or with the very next shot? You need a combination which is more tolerant to inevitable velocity trends.

I'd try to eliminate one of them groups with seating adjustments, firing pin striking, primer change, or start over with another powder if necessary..

 

[groper]

Groper the loads were all shot on the same day same conditions, 905 mb 10 degrees C.

Rifle has a 26" No5 contour barrel, free floated, harris bipod, rear supported by rolled up jacket.

The load was 7mm rem mag 168 berger, 72gr H1000, federal 215 primer, federal case. This load as stated produced two velocity groups one from 3031 - 3037 and the other from 3051 - 3058. This was over 12 shots. There was no pattern as to which velcoity appeared ie they did not get faster or slower as the shots were fired.

The 3030 velocities grouped together and the 3058 velocities grouped together about 2" higher at 300 yds.
The same pattern showed at 600 yds except the separation was 5".

Stu.

Well if you are 100% sure about the reloading process, brass prep, component uniformity and loads being 100% identical in every way except the powder weight, then i guess id be putting it down to barrel harmonics also.

In one of my rifles, i varied the seating depth by only .015 and it changed the POI by about 3inches at 200yds just as an example... at the end of the day, it dont matter where one load groups compared to the next, so long as you find an accurate, low ES load is all that counts ay

 

[kiwi3006]

Mikecr, yes it is an interesting phenomenon. The ES spread was 27 fps, but there were no shots between 3038 and 3049. I got a bimodal velocity spread with two groupings with a 7fps spread.
When I looked at the chrongraph i could predict where on the target the round hit. I have never noticed this before, usually the velocity did not determine impact point, ie two impacts next to each other with a 20 fps velocity difference while the third hitting away, but having a near identical velocity to the first shot.

Yes I will be looking at a different load. The reason why I had to change loads was my original load of 73gr increased velocity from 3087 to 3120 fps (same bottle of powder, primers etc, the barrel is only 200 rounds old) When developing this load 72.5gr gave a horizontal 1.5MOA spread (3100 fps) while 72 gr gave a 0.7MOA with more vertical spread. I think I will look at 72.2 gr and hope it produces a rounder group at aroung 3080 fps.

Groper the powder weight was as near to identical as I could get. There would only have been one or two individual powder kernals difference in loads.


Once I have got the powder weight fixed I will then work on seating depth. I might also try WLRM and CCI 250 primers.

Oh well, more shooting.

Stu.

 

[groper]

ah i see what you mean now, i thought you were shooting 2 different loads!

Do you weigh and sort your brass or have you checked base to shoulder measurement etc? Are you sure your brass is 100% uniform?
Do you sort your projectiles by bearing surface length? This could easily change the velocity by 20fps...
and how are you measuring your powder? I gave up using an electronic scale as i discovered it could throw anything upto .4grains varience...