waterman,
Good questions all, but I'm afraid I have no definitive answers to many of them. The wall in question was about 5 ft. from the bench I was shooting off and is about 10yds. long, and 7 ft. high. It's used to separate the 25 & 50 yd. shooting line from the 100 yd. range, so that both ranges don't have to be shut down when the pistol shooters want to look at their targets.
The first time this happened I just put it up to a bad day at the range. Two days later it happened again. I don't know why I thought of it, but the idea that Franklin Mann had shown in his book that placing a plank under the muzzle and extending it outward caused a deflection of the bullet. His conclusion was that you had to have the muzzle of your rifle extend beyond the end of the bench or bad things were gonna happen.
Normally I shoot from the 2nd bench out from the wall, but on this occassion GWArden was using that so I moved my stuff over to the 3rd bench and the groups returned to normal. When shooting off the 2nd bench groups are all normal. Now we're not talking about a 1/2" or so difference, we're talking groups getting blown out to 3 & 4" from a normal under an inch.
Was this anomaly caused by the wall, or something else. I can't say for sure, but that's how I look at it. It might not be the wall itself causing the problem but the air moving around the outer end of it and coming back in, in a swirling motion. Prevailing winds are usually from the 9 o'clock side and the rifle benches are on the 3 o'clock side. As we know the first 25 yds. or so from the muzzle are the most critical in getting the bullet stabilized as it's motion moves from a center of form to the center of gravity. The inference could be then that this swirling action could be causing an unusual irregular amount of disturbance to the bullets flight.
Of course the next question would be how come the other benches aren't affected. Another answer might be that the wind coming around the wall causes a slight vacuum to occur which is not particularly strong but enuf to affect a bullets filight near the wall but would disipate rather quickly as it moves out away from it.
There's a coupla answers for you which might be the cause, or none of them are. Until some actual testing is done to try and figure it out that's the best I can give you. In the meantime I don't shoot on the bench next to the wall!
If either of the above is true then a .22 bullet would be affected more than a heavier one.
As to any effect indoors..... I've never noticed any, but then most indoor ranges have fairly enclosed firing points so one point would give the same effect as any other. Also you don't have any cross air current of any consequence since most indoor ranges pull the air down range from the firing point.
PETE