I filed that event away for future reference, and now with coyote season essentially over and the weather turning nice, I figured exploring that observation further was a good excuse to go shooting.
So, I grabbed an accurate rifle that I had some extra ammo loaded for, scrounged up four different rest systems and headed for the range. The rests included a conventional tripod pedestal rest with sandbag, Caldwell's famous Lead Sled, a Harris bipod, and my good old hunting pack. I stapled up four targets at 200 yards, and after a single fouling shot, fired three rounds at each target using a different rest every time. Starting with the sandbags, I worked my way to shooting off my pack, allowing the barrel to cool down completely between each shot.
For the gun guys out there, the firearm details are as follows: Thompson/Center ICON Weather Shield in .243 Win. and a Sightron 2.5-10X42mm, both of which are no longer being produced by their respective companies.
Note that I was only looking to check for point-of-impact shifts, for which three shots are enough. If I was looking for changes in accuracy, three-shot groups would be largely meaningless as it takes much more shooting to evaluate accuracy than it does point-of-impact. And I do think I'll have to repeat this test one day, looking for changes in accuracy. This time, however, I didn't have enough ammo.
After the shooting was done, I collected the targets and superimposed them in turn on top of an unfired target, marking all the bullet holes with a felt marker. When I was done, the unfired target had 12 black dots on it, representing a group that measured 1.63 inches vertical and 2.45 inches horizontal. This rifle will usually shoot under 2-inch groups at 200 yards, so this was close to its potential. The extra horizontal dispersion is likely the result of a gusty 9 o'clock breeze that I was trying inadequately to compensate for.
Here's a photo of the target that represents all 12 shots.
My conclusion? This rifle's point-of-impact is largely insensitive to the support system used. But why would my rifle show no point-of-impact shift at 200 yards, when my friend's rifle does? Beats me. They are both accurate rifles with fully floated barrels, shooting the same bullet in similar handloads. But, like people, rifles are individuals. What one will tolerate drives another nuts.
I'm going to check some of my other rifles as well, especially my "serious" rifles to ensure they shoot to the same point-of-impact off a field rest as they do from the sandbag rest I normally use for sighting in. It's something else to keep a guy shooting during the off season.