45-120 Primer testing results - sort of

[mnzrxer]

I finally had a chance to shoot some loads with the only difference being the primer. All of the load data I had come across for the big BP cartridges listed magnum primers. Since I haven't been able to find any more magnum primers I decided to try standard LR primers. After priming the cases I marked the primers with different color markers so I could tell them apart. Unfortunately, I got interrupted before writing down which color was which and can't say for certain that I remembered correctly once I got back to finish loading. Just another lesson about staying focused on the task at hand and being thorough about recording EVERYTHING.

The base load is Norma 45 Basic brass filled with 120.0 gr (by wt.) GOEX Cartridge topped with a 0.060" veggie fiber wad and topped with a SAECO #745 525 gr semi point boolit sized to 0.459" and lubed with SPG. I used Winchester LR magnum primers in 5 cases and Winchester LR in the other 5.

To help eliminate errors due to barrel fouling, temperature, etc. I alternated back and forth between the two different primers while shooting. First shot was a fouler then started shooting over the chrony for data.

Primer: WLRM WLR
Shot 1 1419.3 1399.6
Shot 2 1387.5 1416.0
Shot 3 1447.2 1461.0
Shot 4 1414.0 1418.0
Shot 5 1442.3 1443.9

AVE 1417.0 1423.6
ES 59.7 61.4
SD 24.5 26.2

Basically, no significant performance difference between the two primers. Only 6.6 FPS difference on average. However, there was a point of impact change of about 5" at 200 yards with the shots that averaged the LOWER velocity impacting higher. I also noticed that the 5 cases I thought had the standard LR primers had a lot of black crusty residue in them that I have not seen with the LRM primers.

Since I can not say for certain that I know which primer was which, I will have to try this test again (just an excuse for more shooting :D) and make better records. The part that confused me the most was when I got back to the computer to enter the velocity data for each primer and found the higher velocity rounds hit the target lower.

[mnzrxer]

Formatting on the data didn't work too well. Here is a screen shot from my log.

[RMulhern]

"The part that confused me the most was when I got back to the computer to enter the velocity data for each primer and found the higher velocity rounds hit the target lower."

Not surprising at all! It's called BARREL HARMONICS! Your higher velocity loads are exiting the muzzle at a different vibrational node!!

[Grapeshot]

"The part that confused me the most was when I got back to the computer to enter the velocity data for each primer and found the higher velocity rounds hit the target lower."

Not surprising at all! It's called BARREL HARMONICS! Your higher velocity loads are exiting the muzzle at a different vibrational node!!

Higher velocity rounds exit the barrel before recoil changes barrel position. This is why slower loads strike high and faster loads stike lower than point of aim.

[mnzrxer]

Man, I wish I had a high speed camera and a pressure barrel for this one! I certainly understand barrel harmonics. And I agree that those harmonics can affect different loads and their point of impact. But, what I find hard to believe, is that two loads so CLOSE in velocity could be that different in POI. I would LOVE to look at the pressure curve for these two loads so I could see the difference in burn characteristics between these two primers (yeah, I am an engineer and find these things fascinating).

Here are some quick numbers that assume a constant rate of acceleration for the bullet (not the case in reality) and a perfectly stiff barrel. For a muzzle velocity of 1417 FPS and a barrel length of 30" (34" barrel, but the bullet only moves 30") it takes 0.0017643 seconds to leave the barrel. For a muzzle velocity of 1423.6 FPS it takes 0.0017561 seconds to leave the barrel. That is a difference of 0.0000082 seconds. If the rifle rotates about my shoulder during recoil a 5" change in POI at 200 yards is a difference of about 0.036" at the muzzle (rifle is about 50" long). It would be nice to know how long it actually takes the bullet to leave the barrel, but to get a barrel position difference of 0.036" in 0.0000082 seconds would be a ridiculous velocity for the barrel to be moving during recoil.

Anyone know what the amplitude of barrel whip might be? Could barrel whip due to harmonics plus movement due to recoil add up to 0.036"? Maybe.

Those of you who have tapped on your barrel to find the nodes, how many did you find? I am going to have to try this myself. When shooting, my cross sticks have been right at the front of the forearm. Maybe this is exactly the wrong spot to be (for accuracy purposes).

These are the things that make shooting interesting for me.

[mnzrxer]

Okay, so I got to thinking a bit more about this resonance thing. I can't imagine the displacement of the muzzle being enough to significantly affect POI, but I can see the vertical velocity of the bullet as it exits the muzzle being affected. The bullet is always leaving the muzzle with some upward velocity to hit a distant target that is at the same height as the gun.

If the barrel whip happens to be moving upward at the time of bullet exit the vertical velocity will be higher and the bullet will hit the target higher and if the muzzle is moving downward at the time the bullet gets there the POI will be low.

Time to build another spreadsheet.

[montana_charlie]

Those of you who have tapped on your barrel to find the nodes, how many did you find? I am going to have to try this myself.
I've been doing it since I learned to spell 'sweet spot'.

Theoretically speaking...
If the barrel has no taper, the primary node should be at the midpoint (between receiver and muzzle). The secondary node would be halfway between the primary and the muzzle.

According to one well-respected gun crank, it is the secondary node that we should be using.

But, when the barrel is tapered, the primary 'sweet spot' falls some distance forward of the center of the tube. How far depends on the amount of taper.
The location could probably be guesstimated mathematically, but that's beyond me.

Similarly, the secondary node is further forward than halfway between the primary and the muzzle. But, I have never been able to 'hear' that one...so I just use the primary.

CM

[mnzrxer]

Well, I played with some numbers last night and still can't convince myself barrel resonance has a significant effect on accuracy. I assumed the muzzle moves up and down a total of 0.005" at 500 Hz frequency. That would only result in a change in initial vertical velocity of less than 1% when I am shooting at 200 yards. The effect would be smaller a longer distance (because the initial vertical velocity has to be higher to hit the target) and more pronounced at shorter distances (where there is very little arc to the path of the bullet).

So, my simple calculations don't explain the difference in POI that I saw. I know people have seen and experienced similar results, so I am obviously missing something in my simple assumptions. Maybe the amplitude of barrel movement is much greater than I thought (although with a heavy barrel it can't be too much). Have any of you seen a technical article on the subject that you could refer me to?

[Lead pot]

:smile:Try this some time.
Have an identical powder charge and shoot 5 rounds, one with a 545 grain bullet and one with a 405 grain and see which one impacts lower.
100 yards is far enough

[Gunlaker]

It's difficult to say what the barrel amplitude is. Without measurement, it's just a guess.

I don't suspect that resting on the sweet spot will mean much at 100 yards. Certainly it'd be dwarfed by the ES on those loads. But at 1000 yards, 0.005" of muzzle elevation equates to about 6" difference in POA. A person with my skills would never notice, 'cause they'd still just miss the target completely :-P, but I'm sure that matters to some of the pros.

With respect to where the sweet spot is, on a non tapered barrel that is not attached to an action or a stock of any sort, it'd be exactly in the middle. Generally you need an action attached to your barrel and probably some wood too. That'll move the sweet spot away from center. It'd be difficult to model, but measuring will once again be the best way to find it:-P

Chris.

[mnzrxer]

:smile:Try this some time.
Have an identical powder charge and shoot 5 rounds, one with a 545 grain bullet and one with a 405 grain and see which one impacts lower.
100 yards is far enough

I don't have a light bullet for the 45-120, but I have done a lot of this type of testing with handguns that have fixed sights. In the case of a 405 gr bullet vs 545 gr the velocity change with the same powder charge weight will be MUCH greater than 6 FPS

[mnzrxer]

Did a little searching and found these two articles.

http://www.varmintal.com/aeste.htm
http://www.varmintal.com/amode.htm

Somebody went through the trouble of setting up a computer model of a bench rest rifle to understand what the barrel resonance and mode shapes did for accuracy. Based on the barrel position at the time the bullet exited the muzzle he projected a POI at 100 yards. Here is a quote of the results:

"From these FEA dynamic pressure calculations, it appears that the recoil and forced deformations are much more important than the natural vibration modes in determining where a barrel is pointing when the bullet exits the muzzle."

His model came up with a larger POI change (up to about 0.75") than my simple calculation based on frequency and amplitude alone, but the result is still far from the 5" change I saw. That model also is a shorter barrel and much higher velocity bullet, so there is much less time for the barrel to move before the bullet exits the muzzle than there is with a typical BP rifle so these effects may be more noticeable with a longer barrel and slower bullet.

[mnzrxer]

By the way, I tapped on my barrel last night and did find two nodes. One is centered about 3" in front of the forearm and the other is about 3" behind the front sight. The "dead" zone on the node in front of the forearm is about 2" long and the one near the front sight is only about 1" long. This leads me to believe that the node near the forearm is for a lower frequency mode shape than the one near the front sight. These nodes were found with only the action and buttstock supported and nothing touching the barrel or forearm.

Next, I supported the barrel where I had while shooting. I could not find a node with the barrel supported this way, so the support definitely changes the barrel harmonics.

[montana_charlie]

I don't suspect that resting on the sweet spot will mean much at 100 yards. Certainly it'd be dwarfed by the ES on those loads.
You picked that up, too...huh?

CM