can anyone explain this POI change?

[lrgoodger]

I shot 15 rounds each of 40g V-MAX and 52g Sierra Match Kings in the photo below with one five shot group of each at distances of 110, 200 and 290 yards (don't ask - the terrain dictated those distances). Groups 2, 4 and 5 are the SMKs. They all three centered vertically on the left side of the second smallest ring, directly under the point of aim (POA) at all three distances, as one would expect.

The 40g V-MAX, however, kept moving to the right as distance increased. It grouped far better than the SMK, but I can't understand why the horizontal walking of the groups with distance. The groups were fired alternately at each distance and nothing was changed between any of the six groups from the bench except the point of aim.

The only thing I can think of is spin on the lighter bullet pulling it right, but it seems way too much for these short distances. Note that I numbered groups 5 & 6 wrong, because the V-Max was shot first and landed clear on the bull to the left. Because of the distance the bullets dropped, I moved the POA for the 52g SMK to the top edge of the paper as marked, and they impacted right in the bull at the bottom directly underneath the POA.

Any explanation for this when no scope settings, rifle rest, or anything else at the bench was changed would be greatly appreciated.

Ron

 

[6BRonwheels]

What kind of wind did you have during your shooting? I would guess that wind is the problem here. That is why I shoot heavy bullets, less wind drift.

 

[lrgoodger]

Oops! I said clear to the left in the original message when I meant clear to the right!

Anyway, there was a light, gusty wind that day, which I watched as I shot. The 1.18" group at 290 yards should tell you something. The wind would have had to be very consistent to do that. I thought about wind, but there was not nearly enough to account for moving the 40g bullets 3 inches while flying over the additional 180 yards from 110 to 290. Also, if wind had done that, it would have moved the 52g bullets noticeably as well.

I know a receiver not threaded straight will cause groups to walk in the direction the barrel is pointed off the receiver center line as distances increase, but the 52g bullets flying straight at all three distances tell me that that is not the problem here. I'm at a loss, unless the vibration pattern of the barrel is enough different between the two loads. If I'm getting some figure 8 type muzzle movement with the 40 grain and the bullet is exiting at the far right swing, maybe that's it. Any other ideas?

 

[Gunsmoke]

What kind of scope,, Rest,, Rifle ???
AS.. starters ??????

 

[lrgoodger]

The rifle is a Rem 700 VLSS .223 heavy barrel stainless. The scope is a brand new Shooter's Edge 8-32x50. It's a Chinese Nightforce knockoff I wanted to test. The 110 yd groups show it is holding it's own. Heck, I'm not complaining about the 1 1/8" group at 290 yds, either! The rest is homemade from oak. It has an 80 lb weight on it and the gun stock is clampled into the rest, so there is no wiggle at all when I touch off the 1 1/2 lb trigger (changed the trigger spring). When I built the rest, I was going for as stable as I could get (machine rest) for testing potential accuracy without shooter error affecting things. I have no trouble shooting 1/4" groups with it at 100 yds with a gun capable of doing it. This is just the first time I've ever run into an unexplainable variation in vertical POI drift with increasing distance between two different loads shot from the same setup.

 

[thane]

Sometimes little position changes between groups can affect bullet impact.
Once while shooting with a buddy using his REM 700 .223 taking turns, we both shot similar groups but not the same impact point.
Repeatability in set-up and approach is vital. "Good in... good out"
I have experienced the same problems as you are now and should have more grey hair than I do. You will sort it out.

Thane

 

[6BRinNZ]

Hi, did you use a scope level when shooting these groups? Could rifle cant have been an issue?

 

[tuck2]

Changing your position while looking thru the scope can cause a group shift . Do you have a scope with AO so that you can set the scope for the distances to the targets ?

 

[lrgoodger]

I understand how critical repeatability is. I try to keep everything exactly the same for each shot. I don't use a scope level, but I use two targets side by side and make sure the horizontal crosshair on the scope bisects the adjacent bull for each shot. That ensures that cant is the same each time. The AO on the scope is changed for each distance, of course. I might buy that that will change the POI at each distance (a bit, but not as much as the target shows), but it would affect both loads and not just the V-max.

I want to thank you all for your responses. I may get it sorted out when the weather improves in March and I can do some more testing. It was zero degrees here in Michigan last night. These groups were the last ones I shot before our winter weather set in late in November.

 

[Bryan Litz Ballistics]

I don' think there's anything mysterious going on here.

Different bullets will group at different POI, in an angular relation. The barrel harmonics of different weight bullets and different barrel times dictate the angular relationship.

For example, it looks like the 40 grain Vmax bullets grouped about 2" to the right of the 52 grain Sierras at 100 yards (2 MOA). So I would expect them to be about 4" to the right at 200 yards (still 2 MOA), and 6" to the right at 300 yards (still 2 MOA). From how I interpreted the targets, this looks to be about what happened.

Cheers,
-Bryan

 

[25AI260]

thinking a day of shooting.. you shot the 110 yrds first... an then the others
starting with a cold/'clean' barrel 2 different powders an bullets
changing position leads to parallax an mother nature an the wind
finally the nut behind the trigger 'shooter fatigue' -just my .02$-

other thoughts...
the one thing i wonder about is you didn't mention tracking.. going to zero at 110yrds,
did the bullets shoot where they started out ?

 

[vrscdx09]

I don' think there's anything mysterious going on here.

Different bullets will group at different POI, in an angular relation. The barrel harmonics of different weight bullets and different barrel times dictate the angular relationship.

For example, it looks like the 40 grain Vmax bullets grouped about 2" to the right of the 52 grain Sierras at 100 yards (2 MOA). So I would expect them to be about 4" to the right at 200 yards (still 2 MOA), and 6" to the right at 300 yards (still 2 MOA). From how I interpreted the targets, this looks to be about what happened.

Cheers,
-Bryan

I agree

 

[lrgoodger]

Thanks, Bryan. You confirmed that my statement below, in my second post on this thread, was correct. Now it makes perfect sense.

"I'm at a loss, unless the vibration pattern of the barrel is enough different between the two loads. If I'm getting some figure 8 type muzzle movement with the 40 grain and the bullet is exiting at the far right swing, maybe that's it."