Small powder charge gives 1/2MOA HORIZONTAL movement - wierd

[pat fulghum]

Took my 6x47Lapua out today to get it tuned in, but I ran into something very peculiar I got about a 1.5" HORIZONTAL shift at 300yards ( 1/2 MOA)


The 4 targets are 2" in diameters and were shot at 300 yard.
Each of the first 4 targets have 5 rounds each, they where shot in round-robin....
Shot 1 at Target 1, Shot 1 at Target 2, Shot 1 at Target 3...

Can anybody explain this Left to Right shift between Target 1 and 2?

Brass was all from the same lot and all loaded today so reloading processes where almost identical.
Note: there is a typo Upper Left Target is 39.1 gr (H4350) NOT 31.1
Quickload and OBT theory says I need 39.4gr H4350 (Ba Calibrated)

I'm happy with 1" groups at 300, but the left to right shift scares me.

Thanks in advance

 

[Spike A]

Barrel harmonics? Just a big guess....

 

[savagedasher]

Barrel harmonics? Just a big guess....

Way more then barrel harmonics . It's complete gun and rest harmonics .
You have more then a powder change showing with the targets . Larry

 

[pat fulghum]

Savagashooter....
These are all H4350...
TYPO on first target 39.1 not 31.1

 

[savagedasher]

Savagashooter....
These are all H4350...
TYPO on first target 39.1 not 31.1

That's not a small change I would consider it a huge change . Larry

 

[Patch700]

Shooting over flags or just plinking? For a joke, try typing in all your variables into a program designed to give you a drop chart.. Now see what it says for how much drift you would see with a mere 5mph 90 degree crosswind at 300yds.

Things can make you scratch your head if you get caught in a switch more than once.

 

[pat fulghum]

This was shot round robin on very calm conditions. With sun out and 40 degrees

This shift isn't weather

Shot off solid concrete bench
Front rest was a Seb NEO with rubber feet to eliminate slipping.
Rear bag is a Edgewood gator.
Pretty much free recoil (slightly touching shoulder to meet f-class rules) with 2 oz trigger.

Rest adjustments was never touched except via joystick.

I have good clean fore/aft movement in the bags and I'm able to hold on target about 3/8" (@300 yards)

Never adjusted scope (vortex golden eagle at 60x) after first of 3 sighters before test started.

I tried do to all rounds 50-60 seconds apart capturing velocity via lab radar and writing it down. This gives my eye some rest time and to simulate a twenty round relay.

I didn't post the 6br 2 load test that I did immediately after shooting the 6x47...
the 6br was less than 3/4 at 300yds

So I don't know what else this could be

 

[savagedasher]

Toms target is what you should see with powder change .
I don't know how he test but I would bet all test was done without more then 10 seconds between shots. The case was never let to. Cook the conditions were as close as you can get between shots . The barrel temperature was as near the same as you can get from shooting. Larry
.

 

[LHSmith]

Simply placing flags would have addressed the condition variable? IMHO, without using flags -you are just plinking.

 

[CharlieNC]

Round robin shots should have neutralized weather and shooter influences. Vertical looks fairly stable across the board. Horizontal is stable for the three upper charges, so you may want to push in that direction. Have you optimized seating depth? That may help stability as well. The groups will be tighter once you shoot back to back vs round robin.

 

[Steve Ladino]

Groups seem to consistent to blame on shifting winds but look like groups caused by minute changes occurring at the bench...rifle shifting in the bags or maybe face pressure on the stock changing....trigger finger position inconsistencies, tight front bag.........

 

[Keith Glasscock]

Look at that seating depth. That pattern looks a lot like what you get as you twist on a tuner. Was something touching the barrel? I've had the forend stop just touch, and man does that cause some chaos!

 

[Jay Christopherson]

re-shoot it and you'll have your answer...

 

[Alex Wheeler]

I had a barrel that would go up the target with powder charge increases, then stop and start to move left. There was stress in that barrel. It would also walk on you if you got it hot. If you kept it cool it grouped very well.

 

[SheepDog]

Just a reminder here that barrel harmonics are three dimensional. A barrel can throw rounds up, down left or right and anyplace in between. Find your barrels "happy spot" and it will be fine.

 

[pat fulghum]

I will reshoot in next few days and report back
Thanks all!

 

[Brians356]

What's your bench setup? How do you shift your aim between targets aligned horizontally? My hunch is you could be applying varying side forces on the rifle when shifting between targets. I'm not convinced there's a strong enough correlation between the powder charge changes and POI shifts to account for it.
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[dkhunt14]

I don't like round Robin for two reasons. Over a 20 shot string like you shot is a long time for conditions to change, I prefer to see a small round group with a flier. I also think moving a gun from target to target is not conductive to accuracy or time of string. If I wanted to do it round Robin I would color the bullets and use same aiming point. I also would reduce to 3 shots till I find want I am looking for. To me the idea of shooting different charges is looking for the place they overlap. Then I will test with what looks good. 3 shots is plenty to see that. Matt

 

[jfseaman]

I'm happy with 1" groups at 300, but the left to right shift scares me.

It all looks normal to me. Your 39.3 looks pretty good for 300yards.

POI will shift with different charges. Up, down, left or right. This is normal.

 

[Brians356]

POI will shift with different charges. Up, down, left or right. This is normal.

That makes sense, in retrospect. I concentrate on vertical dispersion early in load development, ignoring any cross breeze, and at the bench concentrating on holding consistent vertical when shooting. I shoot off a Jamison-style heavy tripod portable bench, and it does introduce barely perceptible side-to-side movement (no portable is as solid as a concrete / cinder block bench!) If I see all shots overlapping in a horizontal line, but spread out left to right something less than an inch, that's ok. Once I narrow down the load, I can shoot off the county range's concrete benches to finalize the tuning, but there will still be a little windage.

I've sometimes wondered if any of you hold off for crosswind when doing load development (at 100 yards, anyway)?
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