Anybody care to explain this charge weight/POI ridiculousness?

ChrisBCS

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Feb 8, 2014
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I started by wanting to do a small OCW 3% below my last result at 57.8 gr. I ended up very lost and shooting my charges at the same diamond to see if I could figure out what was happening... because I am not that terrible a shot.

Low charges are higher than the high charges by 4 inches.

175 gr SMK
.30-06 Sprg
H4350
Win brass

14968967666_7c78041e06_z.jpg
 
I would re do the test and verify your results. I suspect that its a mirage issue and that something changed from first two to second two groups. We don't know how far you are from the target, I suspect 100 yds. Provided you are shooting a light rifle, changing how you position the rifle can effect things, or the barrel was resting on the sandbags and not the stock.
 
they were shot round robin increasing charge weight, not as groups. Despite the round robin, high charges shot low, low charges shot high, that's how they distributed. No mirage. 100 yards. No barrel contact. FWIW, this is a light rifle that I shoot sub-MOA from routinely, including down to 1/2 MOA.
 
Barrel harmonics. Plain and simple, for the 4 in POI shift. Same thing happens to me. Really "squirreled me", at first. Might have some trigger pull or bag issues in there, as well, with the horizontal moves. Could have some neck tension issues, too. Good luck.shoot some more, and see. He'll, I'm still trying to figure this out, and chasing my tail. Where's Yoda?
 
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Yeah the horizontal is all me. Neck tension is consistent though. I was getting really furious. If I hadn't started round robin shooting at the same diamond I would have left completely stumped instead of only marginally stumped.
 
Common to see a "trough" at a node due to harmonics. This would be more clear if you kept increasing powder, as poi would rise again. But 4" is a major change for that small powder increment. What distance? Also all hell can break loose near max pressure.
 
100 yards.

And these are close to beginning loads (55 gr).

More details:

.30-06 Sprg
175 gr Sierra Match King
22" barrel
Brass length: 2.484"
Powder: H4350
COAL: 3.423"
Case weight: 185.7 gr
H2O capacity: 70.7 gr

Previous OCW with higher charge weights:

14674958248_3dc16387e0_c.jpg
 
The scope is straight (or pointed down) and the barrel is pointed up. Almost imperceptibly. The bullet follows an arc which crosses the scope/sight line twice (up then down). I like jelrods post, I've wondered about this causing counterintuitive results with higher velocities but it makes my head hurt.

On long shots max ord can be pretty high up
 
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Well, bullets DO rise relative to the line of sight - at first. Otherwise, with a sight/scope above the bore you'd never be able to hit what you aim at. A drawing of bullet path and line of sight should show the bullet path going "up" from the muzzle, crossing the line of sight somewhere before the bullet reaches its maximum "height" and then coming back down, crossing the line of sight again before it hits the ground at some point way-out-there. I'm pretty sure most if not all centerfire rifle bullets are still going "up" when the gun is zeroed at 100 yards.
 
I was talking about line of sight and trajectory. The more I think about it though I like olfatguy's answer. Sorry for the confusion.

A thought experiment:

For a rifle zeroed at 100 yards with a load of a given speed, if all things are equal (harmonics, etc.) except bullet speed, will a faster bullet hit the paper at 100 yards higher or lower than a slower bullet?
 
Of course. Think about it. As soon as the bullet leaves the barrel, it starts to fall. Imagine that the barrel is pointing directly at the bull and there is a line of sight running through the barrel center to the target. When fired the bullet will drop below that line and your shot will impact low or off the target. In order to cure that little problem, you point the barrel up a little. That imaginary line down the center of the barrel is pointed higher on the target or even above the backer. When fired, the bullet starts to drop below that center line. But since the line was pointing above the target, the bullet rises and then "falls" into the bullseye. There is another line running through the center of your scope to the bullseye, call it an eye-line. Depending on range and geometry, the bullet will probably rise through that eye-line then drop right into the spot where the eye-line intersects the bullseye. The point where the bullet rises through the eye-line is the crossover.

Take your weapon to a place where you can shoot beyond 300 yards. Get a 300 or 400 yard zero. Prop the gun on sandbags remove the bolt, put the scope crosshairs exactly on the bullseye. Without disturbing the gun, look through the barrel. The barrel centerline will be a couple feet above the bull. My 300WM has a 200 yard zero. My 500 yard zero is about 8 MOA above the 200 yard zero -- 8 MOA at 500 yards is a little over 40 inches higher than 200. My 1,000 yard zero is up 28 1/4 MOA, about 280 inches, or about 23 feet above the target.
 
A thought experiment:

For a rifle zeroed at 100 yards with a load of a given speed, if all things are equal (harmonics, etc.) except bullet speed, will a faster bullet hit the paper at 100 yards higher or lower than a slower bullet?
This is what makes my head hurt...Or maybe if you frame it as definitely pre-crossover (still "coming up" toward the sight line
 
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In a perfect world where the barrel cannot move the faster velocity bullets would hit higher at any distance. Because more variables are involved it becomes more complicated than that. There are some posts on Accurate Shooter explaining it better than I ever could. I'll try to find a link and post it
 
Of course.

Ah, mea culpa, the classic semantic argument of "rise".

A clarified version of my question: are you saying bullets rise without being arched?

An arched bullet trajectory has a negative vertical acceleration the instant it leaves the barrel. So it is fired at an upward angle, so it rises relative to the ground, but it is being accelerated downward by gravity the instant it leaves the barrel.
 
Here is the link. Lrgoodger is the poster with the best answers in it. Beyond that there is a lot of confusion.

Why are my ladder tests backwards (hotter loads hitting lower)?

LRGoodger's answer is the same as oldfatguy, which makes complete physical sense and has zip to do with sight heights or crossovers (LRGoodger says as much in his post). Recoil is not perfect, makes a muzzle rise regardless of zero. The more the muzzle gets to rise before the bullet leaves, the more steeply arched the trajectory is, hence higher POI. Thanks for finding that.
 
LRGoodger's answer is the same as oldfatguy, which makes complete physical sense and has zip to do with sight heights or crossovers (LRGoodger says as much in his post). Recoil is not perfect, makes a muzzle rise regardless of zero. The more the muzzle gets to rise before the bullet leaves, the more steeply arched the trajectory is, hence higher POI. Thanks for finding that.
Not discounting ofg's point. But I'm interested in your thought experiment. If the rifle is zeroed at 200, then shooter does ocw/ladder at 100, would a faster bullet impact higher or lower than a slower one? See reply #50 in the referenced thread.
 

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Chris,

Out of curiosity, why do you start new threads with new information on the same load development for 175 SMK in your 30-06? You should keep updating in your original thread.

Just a thought.
 
Barrel starts to rise with recoil. More powder = less time in barrel = less rise of POI.

OFG


This is why I never shoot at 100 yards for load development. I shoot seating depths at 200, all my other tests are done at 300 (or 600)

I've seen this in 45-70s shooting black powder too. It's most likely due to recoil management, barrel harmonics make bullet diameter variations in group location at 100 yards, nothing as big as this. The only explanation that's ever made sense to me is ...More powder = less time in the barrel = less effect due to recoil.
 
Read the Chris Long Optimum Barrel Time article. I ran three different bullet weights (155-175) and two powders across a wide range in a 308, and found that the nodes occurred very close to the calculated OBT charges. In the low charge range, increasing powder decreased the POI up to the node, and then the POI went up drastically until the second node (which I could only reach for the lighter bullet). Since then I have seen the same thing for several other calibers. To be clear this does not deal with barrel harmonics in terms of the muzzle whipping up and down, but more the shape of the muzzle when the bullet exits. Since the nodes have always occurred near to the calculated OBT, this has been sufficient proof to me to explain what is going on to a great extent. Another interesting aspect which I have not seen addressed so far is that the sensitivity of the POI is significantly different depending on the powder type; so while you can find a node with a given powder, a different one may offer a wider/flatter node. I will post these 308 ladders sometime soon.
 
Ah, mea culpa, the classic semantic argument of "rise".

A clarified version of my question: are you saying bullets rise without being arched?

An arched bullet trajectory has a negative vertical acceleration the instant it leaves the barrel. So it is fired at an upward angle, so it rises relative to the ground, but it is being accelerated downward by gravity the instant it leaves the barrel.

I do not know. I think that the answer is "no" in a practical sense. There may be some deviation at the theoretical level but I think, in most cases, other physical effects like the barrel resonance, wind, and the nut behind the wheel have a lot more actual effect. I suspect that the naval artillery people have a good handle on this. I observe that their accuracy requirements are different from ours for a couple reasons.