Quote directly from the notes of 941.29:
In this section, to possess means that the defendant knowingly has control of a firearm.
There is no minimum length of time the firearm must be possessed for a violation to occur. Intention in handling a firearm is irrelevant unless the handling is privileged under s. 939.45. State v. Black, 2001 WI 31, 242 Wis. 2d 126, 624 N.W.2d 363, 99−0230.
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Now my editorial (non-lawyer) comments. To me, having control of something means being able to readily make the object do something that it otherwise wouldn't do on its own. Do I have control of my TV if I have the remote in my hand? Yes, because I can turn it on/off and change the channel. Do I have control of a gun sitting on a desk next to me? No. Because there's no way I can make it do something that it otherwise wouldn't do without taking some action that gives me actual control of it (such as picking it up). However, can control be indirect? I think so. If a gun is in a backpack, and I have control of the backpack, and I can move the gun around by moving the backpack around, then I would say I have control of the gun via the backpack. So just because you're not touching the gun doesn't mean you don't have control of it. The statute doesn't say touch. It says control.
You may wish to read the notes in the statute yourself. Just google 941.29 Possession of a Firearm and look in the official PDF.