41 Mag Load question

[Todd NE WY]

I have a good one for you guys to ponder. 2 lots of 41 mag ammo, loaded with same brand case, same 210 gr Hornady XTP, same powder type and amount. Velocity Average within 10fps. Here is where is gets interesting, there is a 4-5" vertical difference in impact at 100 yards. Anyone ponder a guess for the difference. Not that I am a great shooter but I pretty much ruled shooter error out. I have a theory but I would like to hear other opinions.
Todd

 

[RL338]

Is the brass new , or do they have the number of firings on them? Trim length the same?

 

[Bruce Mc]

Same lot of powder?

 

[JD338]

Same primers?

JD338

 

[Todd NE WY]

To answer the questions so far, I would have to verify but I think it was the same lot of powder, firings total may not be exact same number but within 1 or 2, these have never been trimmed so length varies somewhat, primer is the same, I would have to check to see if it is the same lot. What mystifies me is the fact there is only a 10 fps difference in velocity but the very large vertical offset.
Todd

 

[KinleyWater]

Wish I could help on this, since it's a really intriguing problem, but the only thing I can think of is a difference in case volume causing a difference in pressure. But, if these are all once fired, that shouldn't be happening with such predictability between lots.

 

[Todd NE WY]

Last night I verified same bottle of powder and I weighed the rounds to see if by chance I had received a lot of underweight bullets, no difference there, in fact bullets are also from the same lot, primers are same brand and size not same lot. Thank you for all the suggestions. This one is weird.
Todd

 

[SJB358]

How do they shoot at 25 yards? Shoot into the same group?