Originally Posted by silvertip-co
As to item 1... I think it may be true if the rifling is RH and spin is CW. If its LH rifled barrel and bullet spins CCW then it don't seem like it's gonna drift right in northern hemi.
Coriolis drift will drift the bullet to the right in the northern hemisphere no matter the direction of twist of the rifling. And Coriolis drift will cause a bullet fired in the southern hemisphere to drift leftward, no matter the direction of rifling twist. Coriolis drift is a separate cause of bullet drift from spin drift. Coriolis drift is a result of the earth's rotation. Spin drift is caused by the rotation of the bullet while traveling through the air.
Spin drift will drift the bullet leftward with left-twist rifling, and rightward with right-twist rifling, independent of which hemisphere the bullets are fired in. Whether or not the bullet will ultimately drift right or left will be based on the summation of these two independent sources of drift - Coriolis drift & spin drift.
Purchasing a barrel with left twist rifling and using it in the northern hemisphere would result in the leftward spin drift counteracting the rightward coriolis drift. The resulting horizontal drift could end up being pretty minimal, as the two sources of bullet drift would be in opposite directions.
Almost all rifle barrels are right-twist. It might be possible to order a cut rifled aftermarket barrel with left-twist rifling without too much additional cost. Dunno. Never looked into it.
Like I mentioned previously, learning about the magnitude and direction of horizontal deflection/drift associated with Coriolis and spin drift is much easier than understanding the theory behind each source of drift.