Slower bullets shooting low and left

[JrodGJ]

I have an odd one that I have not seen. I have a new 300 Blackout AR with free float 18" barrel. I've added a scope and went to sight in. Have zero'd at 100yrd using Hornady 110gr vmax as well as 110gr vmax handload and have a nice tight group all 1 moa'ish. I then grabbed some other factory loads just to see where on paper they hit so that I could compensate. That's where the oddity came in. I expected the shots to ladder down which they did but they also went left. The slower the round the lower and farther left they went.

I took 2 shots with each factory load and still have a nice grouping with each so I can't see how I'm pulling it. All bench shooting - no wind

110gr hrdy vmax - 2476fps - zero'd
120 gr UMC hp - 2175fps - 1.5" low 1" left
144gr ADI FMJ - 2008 fps - 3" low, 1.5" left
150gr Am eag fmj - 1946 fps - 3.5" low, 2" left

Any thoughts on why - I assume barrel resonance but don't know how to correct.

 

[Milo 2.0]

Google arc of a rifle zero, help you understand it, someone here can put it in perspective for you also. Your zero is for the faster rd, the slower ones have to impact low. Now if you were to zero with the slowest load, again all the hotter loads will now be low too, tough for me to explain even though I get it.

Now going off my last sentence, if zeroed at 100 with the slow load, at 300 yards, the hotter loads will impact high, but at 100 will be low.

 

[JrodGJ]

Google arc of a rifle zero, help you understand it, someone here can put it in perspective for you also. Your zero is for the faster rd, the slower ones have to impact low. Now if you were to zero with the slowest load, again all the hotter loads will now be low too, tough for me to explain even though I get it.

It's not the impacting lower part that bugs me - It's the pulling to the left. I'd assume if sighted for the slowest round my fastest would be reversed - High and to the right.

 

[Milo 2.0]

It's not the impacting lower part that bugs me - It's the pulling to the left. I'd assume if sighted for the slowest round my fastest would be reversed - High and to the right.

Ok, sorry, diff load, poi most likely will always change, think about it, nothing is the same.

 

[Milo 2.0]

It's not the impacting lower part that bugs me - It's the pulling to the left. I'd assume if sighted for the slowest round my fastest would be reversed - High and to the right.

One more, just say you started with your 110 zeroed at 100 yards, then decide to find a faster load, shooting at a 1" dot at 100 yards, you could theoretically walk around the dot, but the sequence may be opposite of what you think should happen.

 

[T-shooter]

Could be a combination of harmonics and spin drift. That is quite a bit to the left at that distance. Probably the only thing you can do is to compensate your aim for each different load. You could just use one load, whichever is the best, and zero for it. A muzzle brake may help if you have one to screw on and try. I use one on an AR15, not for recoil reduction, but it helps keep the barrel on target with quick follow up shots.

 

[savagedasher]

So would it be that faster bullet shoot higher right It sound like you’r barrel is timed left Scope correct will fix the problem . This is very common .
Larry

 

[JrodGJ]

So would it be that faster bullet shoot higher right It sound like you’r barrel is timed left Scope correct will fix the problem . This is very common .
Larry

Interesting read on barrel time ... By scope correct you are referring to adjusting windage/elevation for each round?

 

[CharlieNC]

If you were to run a ladder test, varying the charge weight and velocity for a given bullet, you will observe changes in horizontal movement as well as vertical. Part of the harmonic behavior.

 

[savagedasher]

Interesting read on barrel time ... By scope correct you are referring to adjusting windage/elevation for each round?

That why scopes
Is adjusted looks like you have a good shooting barrel by the group size .
Larry

 

[Intheshop]

Probably shouldn't respond....oh well.

There's,what to "do" about it....and then there's,what causes it.Two separate things in my pea brain?

 

[savagedasher]

Probably shouldn't respond....oh well.

There's,what to "do" about it....and then there's,what causes it.Two separate things in my pea brain?

I suggest both or I did my best . After 70 years of shooting it hasn’t changed
Slower is lower . The right left I have found to barrel timing . lol Larry

 

[Intheshop]

OK Larry,do you pick a dbl clicker or a triple clicker.....shock?Yes,they effect a change but it doesn't explain how we got there now does it?Trying to describe a bullets path is a hot topic....which,in the big picture is a good thing.I think I "got it"...barely.So yes barrel timing is at issue,but semantics starts to rear it's ugly head.

 

[50bmg1979]

Could it be because its a right twist barrel and the slower rounds are spinning slower so they dont have the spin drift that the faster rounds have? Just a guess. Im by no means a ballistic expert.

 

[joshb]

I had the same results with my last load test with a new Palma barrel. I tested a six grain spread. As I moved up in charge weight (.5gr) my shot impacts moved up and right. Every third group, I had to adjust my scope to keep shots on paper. I chalked it off to barrel harmonics and the stiff breeze blowing across the range.

 

[cheek9075]

Ok, what is barrel timing???

Fast speed = high right, slower speed = low left, to me that is. I can understand that, barrel twist.

Not understanding barrel timing????

 

[joshb]

Could be two answers, depending on who you talk to. These are my "not so expert " answers.
Most barrels have a slight curve to them. You want that curve in the vertical plane. 12:00-6:00 o'clock. The smith cuts the shank so that the barrel locks into the action with that orientation.

Or:
When firing a bullet, when the bullet exits the barrel. The barrel has a "harmonic frequency" movement in reaction to the "explosion" of the firing. The end of the barrel moves up, down, left or right, etc. Charge weight, powder burn rate, neck tension, bullet seating depth, etc. affect when the bullet exits.

Hopefully, an expert will jump in.
I'm a grasshopper.

 

[rogn]

I read somewhere that a fellow observed the button rifling of a 44 cal barrel, and he reported that it looked like an anaconda swallowing a bowling ball. A lot of gas pressure chasing a bullet down a barrel will most likely cause some barrel gymnastics also. Ive also read the works of some learned people that a bullets progress down a barrel most likely is not a smooth continuous acceleration like falling off a table re: gravity. Its thought to be a spastic collection of discrete accelerations, but somehow repeatable. So it seems downright remarkable that we can get the muzzle pointing in the same general direction shot after shot. Ive had at one time or other a barrel that really disliked a bullet or powder or the combination that would walk the POI around in horizontal directions that made me fear for the livestock or populace of the neighborhood. These were not target quality barrels or rifles, but Ive seen a 6" dispersion horizontally @ 100y with one of the mentioned combinations. A 7X57 with H414 was the worst I d ever seen. I still dont love that powder, but it was likely the rifle and or barrel causing harmonics that were the root of the problem.

 

[T-shooter]

Several years ago I was breaking in a new .308 Savage rifle off a bipod and had several boxes of assorted factory ammo. The light weight (147 / 149 grain) and faster rounds shot wild. This picture was patched together from my targets, all shot the same day. I was breaking in the barrel cleaning in between shots and adjusting the scope as I went on the 1st 3" target. The rest were unadjusted. The Hornady Z-Max and PPU Match would group about 3/4" at 100 yards. The MagTech and Fiocchi patterned like a shotgun.