I took my virginia flintlock rifle out again today and while shooting noticed that it was shooting five inches high at 100y. Two weeks ago it was dead on the bull at 100 and a week before that it was two inches high. My barrel is pinned into the stock and while the stok is very thin and easily flexible in the forend when the barrel is dismounted I'm starting to think that the shift I'm experiencing is due to himidity.

On a given day it will shoot in the same spot for the entire day but when I bring it out on a different day it will be in a different spot.

This time of year humidity fluctuates wildly. I have seen fluctuations between 35 and 85%

I have a hard time accepting that my stock is warping enough to flex the barrel like this but it is the only thing I can think of.

What have you guys noticed with your full stocked rifles?

Is this issue normal?

How can I correct it?

It makes a difference. Try some of the apps like strelok to help with some of that variance.

I've tracked with my rem 700 that it shoots .7" at about 60 degrees @ 100yds. The hotter the barrel gets, the further high and left it flings rounds.

My scoped AR cold hits high .5" cold @12, and walks straight up when it gets hot.

At those distances with both rifles it doesn't make a difference, but when you start to back it off,  3.5~ moa turns into a miss pretty fast.

Try taking notes and looking for patterns.

I believe you are referring to a POI shift based off of the barrel heating up. This is something that I know about and understand. But not the issue that I'm describing with my muzzleloader. On a given day it will group consistently all be it in different places on paper.

From going back and looking at historical weather it seems that the lower the humidity is the higher the gun shoots. I don't know if my observations are correct however.

It may be also that the lower the humidity the more quickly or energetically the black powder I'm using burns.

Is your barrel channel sealed?  If not unpin the barrel on a day it is shooting well and brush some polyurethane down the in letting.  Another thing to try is to get a rattail needle file and extend the pinholes in the loops under the barrel horizontally just a fraction to give the wood some leeway to move