Point of Impact Changes With Different Loads

[TwoEyedJack]

Most of the hunting rifle load development I have done has been with the .270 win and .338 win mag. My .270 is an old Remington Sportsman 78 (very similar to a Model 700ADL) restocked with a walnut Remington 700ADL stock, glass bedded and free floated. My .338 is a Ruger M77 Mark 2 with the newer synthetic stock.

With both of these rifles I was able to sight the rifle in for one load, and similar loads would hit pretty much the same point of impact at 100 yards. In the .270, I have tried 130 gr. bullets from Nosler, Speer, Sierra, and Remington. None had a point of impact more than an inch or two away from where it was sighted. Even a 150 gr. Partition hit in the same spot. The .338 shoots factory 250 gr. Partitions (Federal factory load) to the same poi as my 225 gr. Hornady SST reloads.

I bought my first .308 last summer and have been trying out different recipes. The rifle is a Ruger Hawkeye stainless laminated. It has been skim bedded and free floated. Today I tried out two different loads, both using a 155 gr. Sierra Match King. The first one used 51 gr. of Win760. It went 1.125" for five shots, about 1.5" high and 1" left from point of aim. This is one of the most accurate loads I have tested so far.

The next load was the exact same bullet, casing, casing prep, primer, OAL, etc. but using a stout dose of Reloader15. This load was off the paper. At least 8" lower and 4" left. I noticed this same phenomena a few months ago when the rifle was sighted in for factory 165 gr. CoreLokt and a Herters 180 gr. load printed 6" lower.

Is it normal for a rifle to throw different loads to such wildly different points of impact? Have I just gotten lucky with my .270 and .338?

 

[rcmodel]

You got lucky 2 out of three!!

Barrel harmonics are closely tied to the burn rate of the powder.

You got lucky twice with good barrels with no internal stress.

You struck out the third time.

As most all of do with rifles, most of the time.

rc

 

[jmr40]

I must be luckier than most. Based on my experience I think it is related more to the load than the rifle Over the years I've rarely had a rifle that shot far enough off at 100 yards to matter regardless of bullet weight. I have 2-3 rifles in 308, 30-06 and one in 300 WSM. I zero them all at 100 yards and with 150,165, and 180 gr loads they are all close enough at 100. At 200, 300, or farther the windage is still the same but heavier bullets hit predictably lower.

The one exception is the Barnes bullets. I've experimented with them and have gotten good accuracy, but they tend to hit 2-3" higher, and 2-3" to the right at 100 yards with the same zero. I get the same results in several rifles and in 3 cartridges.

 

[Newtosavage]

I found the same thing from my .308 when going from H4895 and W748 to Reloder. The Reloder powder rounds were WAY high compared to the other two.

At 300 yards, it's the difference in 2.5 MOA (7.5") on my scope.

I also get a change in windage between my H4895 and my W748. About 2 MOA.

So, for me at least, the elevations are the same between two of my three powders and the windage is the same between another two of my three powders. Go figure.

 

[Grump]

Um, don't draw any firm conclusions until you go back to that other load and confirm that it still hits the same POA.

As far off as you describe IME has always been a sign of something really major being suddenly way off. Even really, really reduced loads in the rifle have not gone off the paper at 100 yards compared to other known full power loads.

 

[Bart B.]

rcmodel,

Barrel harmonics, or the resonant and harmonic frequencies they whip and wiggle at, are the same for every load fired. That's because the barrel has the same weight and dimensions and metal properties all the time. Just like a metal tube in a xylophone; makes the same note every time it is hammered both hard and soft.

They don't change with loads; physically impossible.

 

[R.W.Dale]

rcmodel,

Barrel harmonics, or the resonant and harmonic frequencies they whip and wiggle at, are the same for every load fired. That's because the barrel has the same weight and dimensions and metal properties all the time. Just like a metal tube in a xylophone; makes the same note every time it is hammered both hard and soft.

They don't change with loads; physically impossible.

What does change with burn rate is where in this wave form the barrel is pointed when the bullet clears the muzzle which can greatly affect POI

I have some rifles that are fairly resistant to poi change with different loads. Others the difference can range into FEET

In my experience in longer barrels firing slower rounds in barrel dwell time seems to play a factor.

 

[Bart B.]

Yes, what does change with burn rate is where in this wave form the barrel is pointed when the bullet clears the muzzle which can greatly affect POI. Same thing happens when you change the charge weight in a given load.

But adjusting the sight for a different zero for each load fixes that. Unless you shoot different loads with the same sight setting.

What's "barrel dwell time?" When the muzzle axis is at the top or bottom of its vertical whip cycle?