Let's talk about Optimal Barrel Time

K

kilotanker22

New member
So I have been thinking a lot about this lately while trying to squeeze the most out of my rifles and ammunition while also trying to hone my own skills and constantly work on fundamentals.

I have noticed a few things. Now I don't have the tools, or mathematical skills to measure any of this in a scientific way. However I have seen several Instances where increasing velocity or decreasing velocity actually gave me horizontal dispersion instead of vertical. I have also noticed that with any of my 24 inch barrels, even though some are much faster with the same load than others. When I adjust the powder charge, or the seating depth in an attempt to compensate for the pressure difference. I have noticed that about the time that I get back to the nominal velocity for that load, all of the 24" barrel rifles shoot considerably better.

Also I have noted in my Savage Tactical Rifle that with any 140 class bullet with either H-4350 or Reloder 16 At 2800 fps shoots right between .7-.9 moa on average. However when my Magnetospeed Chronograph is hanging off of the barrel two things happen.

1: Group sizes are literally cut in half! With the same loads. I have tested this theory over hundreds of groups in several different rifles. With each rifle and load the results have been very similar.

2: Point of impact shifts 4-5 moa high and horizontal dispersion is virtually non existent. In all of the rifles with those loads, without the Chronograph. Vertical dispersion is very low while the largest dispersions are always horizontal. With the Chronograph, POI shifts high while eliminating the horizontal dispersion.

I have trouble finding any other explanation than this being due to barrel harmonics and the added weight of the chronograph reducing variation in the antinode moments of the barrel at the muzzle. There is less variation at the muzzle at the moment the bullets clears the barrel therefore I see less variation on the target.

To continue testing this theory I ordered a muzzle brake to use on these rifles. I chose a brake that weighs roughly the same as my chronograph. It will be interesting to see if I have the same or similar results. I plan to use the data already collected as a control.

Also worth noting is that the barrel that shows the least amount of POI shift when the chronograph is attached is also the heaviest profile barrel. Also the barrel that shows the largest POI shift is the lightest profile barrel. I have found the same result when measuring the horizontal dispersions of each rifle with the same loads. When the chronograph is not attached.

What is your take? I look forward to hearing from Bart B and Unclenick on this. Is what I am finding a coincidence? Or is there maybe something there?

My Brake will be delivered tomorrow, so I plan to try to test the first rifle on Friday.
 
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I would just put a weight on the barrel behind the muzzle that equals that of the chronograph where it attaches.

Adding a brake puts the weight at a different place. The muzzle won't vibrate the same and accuracy may be worse.

Better yet, get an adjustable tuner.

http://www.ezellcustomrifles.com/home-3/pdt-tuners/

Thinner barrels bend and wiggle more because there's less metal resisting being bent from external forces. This happens when barrels are fluted.
 
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The following two targets were fired today at the range. These were all identical rounds. I have not done a seating depth test with this load yet. A near max charge of H-4350, CCI 200 and Hornady ELDM 140 grain bullet, exiting the tube with an average velocity of 2814.

The first target was shot without the brake. This is a pretty good representative of what I normally see with this load and rifle. Although to be fair when I shot this target it was a little breezy, but not "windy" targets were all at 100 yards.

The second target was shot with the brake. The group high on the target in the center was shot with the brake and the magnetospeed.

All groups were 5 rounds with the exception of one. On the second target in the top right corner is a ten round group. I did not plan to shoot a ten round group, but I had exactly 10 rounds left when I was finished shooting. For this reason the only 10 round group shot today was with the brake attached. The 10 round group in the top right corner of the second target measures .89". I did not bother to measure any of the 5 round groups. I certainly can measure them if someone wants me to.
 

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Sounds like you know what you're doing. (Especially asking for comments from Bart B. and Unclenick.)

Bart's suggestion about the adjustable tuner sure seems like that would be the way to go.

I remember reading about the old Browning BOSS system (it's an adjustable tuner) when it came out and was suitably impressed with the results I read about people getting with it. Disclaimer: I've never had a Browning rifle but it sure tweaked my interest.
 
A barrel is a round piece of tubing.
As such it doesn't just vibrate vertically.
Most of the slow motion videos are shot from the side, showing the vertical motion of the barrel. I have yet to see one shot from above showing the horizontal movement.

Think of it as the striker in your hand, ringing the ole dinner triangle.
 
Most of the barrel's vibration axis is vertical because the recoil axis is above the rifle's center of mass as well as above the buttplate center on your shoulder. This is what tuners control. Muzzle axis swings down first then up and finally back down over the first 1.5 or so milliseconds after the round fires. Most rifle bullets exit the muzzle somewhere in the upper third of the first half cycle. Best accuracy happens when all bullets leave on the muzzle axis upswing so slower ones with longer barrel time have higher angle above the LOS than faster ones leaving at lower angles.

http://www.geoffrey-kolbe.com/articles/rimfire_accuracy/barrel_vibrations.htm

It's a well known fact among competitors that shooting prone slung up will have horizontal shot stringing when the front elbow is moved right and left under the stock fore end.

Same in sitting. I had a 1/2 to 1 MOA right windage zero shift from standing and prone to sitting

At longer ranges, canting the rifle will cause easily seen horizontal shot stringing. How much can be calculated by multiplying the cant angle's sine by the bullet drop at target range.

And several people shooting the same rifle and ammo will have their own windage and elevation zeros.

I've always had windage zeros 1 to 2 MOA left when seated at a bench with the shouldered rifle on bags compared to standing or prone zeros. There's more of my body mass to the left of the barrel/recoil axis making it shoot further to the right.

Browning recently changed their explanation of how the BOSS works. Both original and current versions are physically and factually wrong. Very ironic because they do work very well.
 
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Let's not confuse this thread with the OBT theory about the shock wave from firing going back and forth through the barrel changing the bore diameter at the muzzle upon bullet exit.
 
Cool stuff Bart--thanks for that. Is it correct to assume all of that can shift simply by shifting the different support points (i.e. front and rear bags where they contact the rifle)?
 
stagpanther said:
Cool stuff Bart--thanks for that. Is it correct to assume all of that can shift simply by shifting the different support points (i.e. front and rear bags where they contact the rifle)?
Click to expand...
Yes. That's why benchrest competitors push their front and rear bag rested rifles forward to a stop after each shot.
 
Bart B. said:
Let's not confuse this thread with the OBT theory about the shock wave from firing going back and forth through the barrel changing the bore diameter at the muzzle upon bullet exit.
Click to expand...
The point of this particular test was to see if I could replicate the effect I get when hanging the chronograph off the end of the barrel. My theory was with the same load this gun shoots much better with weight than without the weight, because the weight at the muzzle changes the way that the pressure wave travels down the barrel.

The rifle is fired from a permanently affixed bipod from the same leg setting. I shoot from the same concrete bench, in the same chair and same rear bag.
 
Also the top left group and the group on top in the center with both the chronograph and the brake were after I adjusted the zero. My first three groups were a touch low and left. As you can see them low and left of my point of aim.
 
Shot this again today. Same results, but temperature was 20 degrees higher today.

200 yard targets were around .5" for 5 round groups and a ten round group of .8"and my 300 yard groups were pretty good too one was just under an inch one was 1.4" and my ten round was 2".
 
This season , when ever it opens up I'm staying with the same load and measurements and get back to benchrest shooting for enjoyment. I have shooting friends that are constantly using the chrono , changing loads , seating from jumps to jams . My rifle has almost 4000 through the barrel and still shoots tight groups at 200 yards , when they open up I'll move it to 100 yards before Oi switch it out again . I did alot of reading on reloading but I thing for me I'll keep my sanity and stay with the basics.

Chris
 
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