Puzzled by POI Shift

[WaltNoVA]

I have a 16" Criterion barrel 6.5G purposed as a long-gun that I recently installed an Atibal MRD V2 RDS for CQB utilizing a Magpul offset 1:00 mount.

I zeroed the Atibal at 25 meters with Wolf 100g steel case, bench-rested and canted 45-degrees, resulting in a .75" group. I then switched to Hornady Black and the POI with 3 rounds using the same zero as for the Wolf was 5.5"L and 2.5"H, also while bench-rested. I re-zeroed the Atibal with the remaining Hornady Black (about $6 later) and had a .5" group at 25 meters. I ran out of Wolf steel case to re-check POI with the zero set for the Hornady Black.

Is it possible for POI to shift 5.5"L and 2.5"H between the 2 different rounds, only at 25 meters?! My concern is if the Atibal is losing zero .

 

[Coffee_and_Pipes]

This is based off a small sample size of rounds down range, but if you were able to shoot smalls groups with each type of ammo, it sounds like the mount is doing its job.

I imagine there’s a big difference in quality between Wolf Steel Case and Hornady Black (I haven’t tried either). Add in a 23 grain difference in bullet weights - Hornady Black in 6.5 Grendel is listed at 123 - and that kind of POI shift seems possible. It might be surprising at such a short distance, but I wouldn’t jump to an issue with the mount as the first possible cause.

 

[Saclee Rhino]

Try testing again with a few more quality grade rounds of similar weight to the hornady black, rather than testing cheap vs costly and see if there is a just as big POI shift. Could just be the difference on light weight cheap ammo vs heavier match ammo, 100 grain pill vs a 123 grain pill, there will be a POI shift, but I must say the amount your getting at 25 meters is odd. Different Velocities will also be in effect. Just a thought.

EDIT:
Ok so

Wolf
Bullet Weight: 100
Bullet Style: FMJ (Ball Round)
BC: 0.390
Muzzle Velocity: 2,620 FPS
Muzzle Energy: 1,688 ft-lbs.

Hornady
Bullet Weight: 123
Bullet Style: ELD-M (Tipped Jacketed Round)
BC: 0.505
Muzzle Velocity: 2,580 FPS
Muzzle Energy: 1,818 ft-lbs.

23 grains in weight difference.
0.115 BC in difference. (Should not make any noticeable difference at 25 meters)
140 fps in velocity difference.

The Hornadies higher BC means it will have less drag through the air, so even though it has a lower velocity it is probably shooting a flatter trajectory than the wolf even though it is heavier which would explain the higher POI, also the ammo is manufactured to way higher tolerances and standards than Wolf. Not sure about how the shift can be 5.5" Left though.

Wolf goes from 2620 to 2563 FPS at 25 meters, where as the Hornady is going from 2580 to 2536 FPS. 57 FPS Drop vs 44 FPS Drop. (Still should not make much of a noticeable difference I would not think.

 

[schlaiek]

Wolf steel cased ammo probably isn’t the best benchmark for anything accuracy/consistent related. Other than function testing why waste a criterion barrel on very iffy ammo?

 

[Precision Underground]

By offset mount do you mean turn the gun 45 deg to see the rds? If so make sure the sight is directly over the barrel each time or you will get crazy poi shift. 5.5" left and 2" high sounds like the gun was under rotated.

 

[Scotts556]

By offset mount do you mean turn the gun 45 deg to see the rds? If so make sure the sight is directly over the barrel each time or you will get crazy poi shift. 5.5" left and 2" high sounds like the gun was under rotated.

After a quick google foo. If you are in fact using a offset RDS, there will be a POI if you dont hold the rifle at the same time every time

 

[WaltNoVA]

Yes, the offset means turning 45 deg to properly view through the RDS. Based on your much appreciated inputs, the point is made that the POI will indeed shift if under/over rotated. I'll have to come up with a consistent method for shooting offset with the RDS. I like the location of the RDS mount; manipulating the on/off/brightness intensity single button by reaching underneath the scope is super easy with a gloved support hand.

Come to think of it, a 11.5" SBR 6.5G with a Vortex Razor-E 1-6x has very little POI shift, if at all, at short distances of 20-50 yds using the same two types of ammo, zeroed with the Hornady Black. Nice tight center mass hits. The difference being both types of ammo are shot at 12:00 (no cant) with the SBR as opposed to a 45 deg cant with the big brother's offset RDS.

Mark at PF bored and nitrided both Criterion barrels, the nitride making for more robust barrels. Plinking with the SBR is just plain fun with the low-cost Wolf steel case. The folding stock adaptor makes the SBR a great little duffel bag gun still supersonic out to 800 yds.