What's it trying to tell me?

[DDelle338]

  While working up some loads for my new Encore barrel, I've seen something I'd like to be able to explain. The barrel is an EBCO, 26" heavy, .223. As the powder charge is increased, the groups move to the right and down. While shooting some Noz. BT's, the groups moved the farthest of any bullet I've tried so far. The first group was about 1 1/2" high and left of the bull. The 4th group (of 5 groups) was about 1" right and 2" below the bull. Now in this series, the last group moved to about 1/2" left of the pea sized point of aim. A nice tight group! I have seen this each time I shoot, with several different bullets, powered by H335 and BLC-2. I thought it might be from heat, but have proven that not to be the cause. It has to be the load. But why? What is it? Harmonics?

[303Guy]

DDelle338    Just as an experiment, you might try attaching a 'weight' to the muzzle end and repeating your groupings to see what happens.  It can only be harmonics.

Regards
303Guy

[iiranger]

As you increase the load, the bullet is leaving the barrel earlier before the barrel rises in recoil... yes, in the area of "harmonics." I think it was a military test of .303... anyway the much heavier, slower bullet was shooting way above the lighter, faster bullet. "Time in barrel" had the heavier bullet leaving with alot more recoil "up" and striking higher. Of course, this is subject to being disproved.  Educated guess.  You might have a loose screw in something. luck

[trotterlg]

Faster bullet will hit lower on the target.  Also remember it takes energy to spin the bullet, so the barrel is twisting against something, so it will shoot left or right depending on the twist and how it is held.  Faster bullets spend less time in the barrel and twist and recoil on it harder.  Larry

[303Guy]

Faster bullet will hit lower on the target.  Also remember it takes energy to spin the bullet, so the barrel is twisting against something, so it will shoot left or right depending on the twist and how it is held.  Faster bullets spend less time in the barrel and twist and recoil on it harder.  Larry

I have found two otherwise identical hornet loads (new versus once fired cases) having a horizontal POI shift.  I wondered if the type of action mounting might be the cause but as Larry suggests, twist might be responsible since recoil is so low.  (Aside from the obvious, the once fired load had less neck tension and was loaded with a lubed wad and paper patch).

I would think that the Lee Enfield would print higher or lower due to the way the action flexes under recoil, which is in a vertical plane.

There is a website that shows a movie or simulation of barrel harmonics.  I found it once but was unable to run it.  Apparently, harmonic barrel vibration can be quite complex.

303Guy

[DDelle338]

  My assumption was harmonics, it’s just that most of my other rifles will print higher on the target as velocity goes up. But this is the only heavy barreled rifle I own.  It’s something else to figure out. One of the reasons I like hand loading is there is always something to learn.