POI Problem with Faster Load

[C_Ackley]

I have a 45-70 plinking load I fire out of my Uberti 1886 reproduction(1:20 twist, 26in Barrel). I get good groups from them, for my ability, out to 100 yards.
The load is a 405gr MBC cast (.459 dia) over 12gr of Unique, brass is trimmed to 2.100" and the overall length 2.540". Primer is a Federal 210M. According to the manual, velocity should be in the neighborhood of 1050-1100fps.

My problem arose when I bumped the charge up to 14.5gr of Unique in the same configuration, should yield somewhere around 1200fps. The bullets struck around 10in lower than the slower bullets and weren't even hitting the target.
What is happening here?
How does a slightly faster bullet have a lower point of impact?
Any help from the more experienced is greatly appreciated.

 

[SSL]

Timing of when the bullet leaves the barrel would be my guess. Slower or heavier bullets tend to impact higher than lighter or faster bullets because of when they leave the recoil arc. Really noticeable in handguns with heavy bullets also, to the extent that taller front sights may have to be installed to zero the weapon. Only 100 or so fps does seem odd though.

 

[CharlieNC]

I developed a load for my 45-70 doing a standard ladder test and observed the normal response as you described. The vertical point of impact exhibits a sinusoidal response, with a "flat" node across a wide range of powder charge. It sounds like your higher charge is close to a node.

 

[C_Ackley]

I developed a load for my 45-70 doing a standard ladder test and observed the normal response as you described. The vertical point of impact exhibits a sinusoidal response, with a "flat" node across a wide range of powder charge. It sounds like your higher charge is close to a node.

I did notice this behavior across a range of powders when the velocity is within that 1200fps mark. What creates a node and is it different for different firearms?
Should I just avoid that velocity range and stick with the lower charge for Unique and use some other powder to find the firearm's acceptable higher velocity?
I first thought this had something to do with the bullets being transonic, but I really have no clue.
Appreciate the responses.

 

[RegionRat]

Try doing your tests in smaller steps and watching the POI.

You are trying to avoid the charge/speeds where that POI is changing fast.

Ideally, what you are looking for is a few charge steps where that POI shift slows down or even stops. You would then select the load in the middle of that zone for group testing.

When you are very lucky, that speed/charge will give you a tight group, but not always.

Many sporting guns have multiple "nodes" within their useful speed/charge ranges. Sometimes your faster one is the best, but sometimes the next lower one is better.

Testing in finer steps insures you don't blow right past those nodes.

What creates a node and is it different for different firearms?

And yes they can be very different... if we call these nodes structural harmonics and wave forms for a moment, they are different based on the design of the gun.

Some guns are made with very stiff sections and those yield frequencies that are very high with smaller amplitudes. When designed well, they give better performance in terms of accuracy and precision. This is why you see really stiff bbl sections on guns dedicated to state of the art performance like in rail guns.

That doesn't mean you don't find light sporting guns with notable accuracy, but realistically they are rare.

I'll bet that Uberti 1886 is a blast to plink with and probably a good looking example too.

 

[CharlieNC]

Like RR said. Interestingly my 45-70 exhibited a pronounced wide node when shot at 50 yd. Slow and heavy.....

 

[mgunderson]

What sights are you using? Big heavy bullets at slower speeds...... doesn't seem like barrel harmonics to me. Yet!

 

[C_Ackley]

What sights are you using? Big heavy bullets at slower speeds...... doesn't seem like barrel harmonics to me. Yet!

Williams FP71.
Groups with the lower charge load at 100 yards look like this. The higher charge hits so low it doesn't hit the target.
There's always shooter error, I do that a bunch.
*I had to flip the target halfway through shooting cause I had hit the target stand.

 

[LCazador]

I have a 45-70 plinking load I fire out of my Uberti 1886 reproduction(1:20 twist, 26in Barrel). I get good groups from them, for my ability, out to 100 yards.
The load is a 405gr MBC cast (.459 dia) over 12gr of Unique, brass is trimmed to 2.100" and the overall length 2.540". Primer is a Federal 210M. According to the manual, velocity should be in the neighborhood of 1050-1100fps.

My problem arose when I bumped the charge up to 14.5gr of Unique in the same configuration, should yield somewhere around 1200fps. The bullets struck around 10in lower than the slower bullets and weren't even hitting the target.
What is happening here?
How does a slightly faster bullet have a lower point of impact?
Any help from the more experienced is greatly appreciated.

I found the same POI change in my rolling block but ten inches is a lot. I used puff-lon filler with Unique and got much more uniform POI's. I would go with the higher end load if you intend to hunt with it. Otherwise for plinking I'd just go with what shoots best.

 

[C_Ackley]

Recieved my rifle yesterday.
Had the forend bedded to see if it alleviated any of the POI change, no avail.
Top is my plinker at 1100fps~ and bottom is where the IMR4895 load at near 1600fps struck relatively at just 50 yards.

Oh well, I had a ton of fun shooting today in 26F.