I've built a 100m tunnel range with 2 foot diameter pipe. Works great to catch the noise and no wind so reducing one more variable for load development. But I've noticed my zero point is different on my range compared to my local club range. IF I zero sop on at my range at 100m, my POI is 3/4" low out at the club range. This different is consistent with 3 different rifles (22BR, 6.5 SLR and 284) Initially I thought it might be barometric conditions, but there isnt enough diff (997-953 mHg) to make that amount of different. Then I got to thinking that maybe the enclosed space is affecting the bullets flight path? Could that be a factor?