I’ve been doing most of my shooting near Reno, NV at an elevation of 4500 ft. and recently went to CA State Championships basically at sea level. At the California State, I was remarking to a friend of mine from NV that I thought that my breaks were a bit weak in the preliminary event and that I was going to go up a step (0.10) over what I normally used. He said that it could be due to the “Altitude Effect” and that I was right to increase choke at sea level.
It seems to make sense.
1. Pattern spread is due in a large part to the air resistance encountered as the pellets fly downrange.
2. As air density increases, there is more resistance felt. Particularly at higher speeds. (This is one reason that Scott Bredlove et. al. try to break the land speed record and go Mach I in the Black Rock Desert north of Reno).
3. More air resistance will increase pattern spread. i.e. More resistance generates more turbulence which in turn deflects more pellets.
4. Air density decreases at higher elevation.
If the above statements are true then there will be less pattern spread at higher elevation and, consequently, you need less choke to achieve the same effective pattern.
I know that the best thing to do would be to collect good pattern data from the same barrel and choke tubes at various elevations and do some statistical analysis. But I have little hard patterning data. Just anecdotal evidence to support the theory.
So, the big question is, how much does altitude affect pattern density? Should I really tighten up chokes at sea level?
I’ve never done comparison patterning at high and low elevations, but not having done the work won’t stop me from yakking and pretending as though I have some answers.
Yes, I think that you are quite right in saying that patterns tighten as elevation increases due to a lessening of air resistance. The trap shooters who shot in the 1968 Olympics in Mexico City were known to have commented on how much tighter their patterns were at high altitude. As to the actual degree of tightening (or loosening if your are coming down from the mountains), I really don’t know. It will obviously vary with the altitude change and perhaps also with changes in humidity, as that effects air density.
One other thing that I constantly hear from Western desert shooters is how much faster the birds seem on the coasts at sea level. This may be because in the desert’s low humidity and high altitude air resistance is less on the target. This means that it can be thrown a regulation distance with a slower initial launch speed. The thinner air won’t scrub speed off as quickly.
Bottom line is that high elevation targets may be slower and require less choke, but you can’t go surfing when you are done shooting.
The Technoid writing for Shotgun Report, LLC
(Often in error, never in doubt.)