Where do people hunt here that you can go prone to take a shot? I have drawn at least 10 moose tags in my short life and they have all been filled with me kneeling while shooting.
I figured every hunter on here could shoot a 6 inch, 3 shot group with irons, at 100 yards while standing. If I could not do that I wouldn't go hunting. Am I being unreasonable or arrogant in thinking this?
There are a few things to consider here.
Its sometimes difficult to imagine how well you might do if hunting in country you're unfamiliar with. The western prairies and deserts, the mountains, the northern tundra and sea ice all present possibilities for an individual to make a longish shot on game if thats what he's interested in doing. Whether the game is shot at 100 feet or at a half mile makes little difference to the animal, it would rather live.
I tend to abide by the rule that if you can get closer, then get closer, and if you can get lower, then get lower. A few years back I belly crawled up to a caribou and could have killed it with a knife had I been so inclined. But while it was fun I wouldn't say that was a skill necessary in order to make caribou hunting fair chase, although around here caribou are more about groceries than about sport and for some fair chase is the limitation of their snowmobiles over any particular piece of ground.
There are days here where even with a proper kneeling position; that is with my butt resting on my heel, with my support arm placed directly under the forend, with the pad of my arm behind the elbow resting on my knee, and with the sling tight around the back of my support arm, the wind batters me so much that I can't hold to make a 100 yard shot with a 2X scope or irons. Sitting is only slightly better, but from prone I could shoot without the wind interfering, provided I have a clear view of my target.
IMHO, shooting groups is a poor measure of field marksmanship, but we continue to do it. Lets examine your example of a 6" 3 shot off hand group at 100 yards with irons. A 6" group might be sufficient to kill a moose size game animal, but in order for it to be sufficient, the rifle must be sighted so the group is centered on the point of aim, in other words, no bullet ever strikes the target beyond 3" from your point of aim. When we shoot groups, regardless of whether those groups are made up of 2 shots or 20, they never form a concentric donut a fixed distance from the bull. Some hit the bull, some hit the 7 ring, and given a competent marksman, the majority of shots strike closer to the center, yet the group still measures 6". That is why rifle competitions often employ scoring rings, although measuring the distance from the center of the target to each bullet hole and dividing the total by the number of shots fired would work as well.
But in hunting, its the first cold bore shot that matters. If you want to shoot a group to test your prowess as a field marksman, put up a target at whatever range you feel is appropriate, and each morning go out and fire one shot at that target from your preferred position, with your hunting rifle, loaded with full powered hunting ammo. Don't even look at the target for a week, because that will interfere with your concentration. Then after 7 days retrieve it, and measure your group size; that group might mean something because it shows what you can do on demand under the conditions of which you conducted your test. Even better is to use a target with no aiming mark, as game doesn't come with a bullseye painted on it, and would force you to bisect the target with your scope, or use the width of your front sight to find center if you're shooting with irons. Each shot fired should be broken as quickly as possible, as is the case for a shot on game, which as a rule won't wait around all day for you to shoot it, the primary difference between shots on paper and shots on game is the open ended nature of the time you have to make the shot on game; because you never know how much time you'll have, you must shoot quickly.
But the best course for training or testing is to put up targets in your actual hunting area, where you could engage the targets while walking along a trail. Scoring is +1 for each hit and -2 for each miss, and -5 for shooting a don't shoot target, with no score added or subtracted for a target that is passed up. Once you fire on a target, you must continue to shoot until its hit or until you're out of ammo. There is a reduction to the score if you take too long to complete the course, and the course should include barriers that prevent just one position from being used, that include obscured targets, and include don't shoot targets. You should never be able to shoot the course clean, or it was not demanding enough. This was the premise behind the Kenyathelon, which unfortunately failed to catch on, but which was perhaps the best tool a hunter could use to test and improve his field marksmanship.