Hopefully, your range has a cover to keep the sun off the barrel. If not, make some means of shading it, especially if your barrels are black. The exposed portions of your barrel, if in the shade, will heat more slowly and cool more quickly than those in the barrel channel. Unless in the sun, they will also warm more slowly. Heating may cause POI to disperse vertically, especially if shooting a centerfire...and full-charge hunting loads.
I'm also in the group saying to rest your rifle vertically in the shade to help cool the barrel more quickly. Some people have created a forced-air system to cool the bore more quickly, but that may also create uneven barrel temperatures.
Unless there's a good reason to speed things up, take your time and enjoy the "ride". If you're sighting-in a hunting rifle, get it on paper quickly by bore sighting, if necessary, then shoot three shots to confirm, then let it cool and shoot a rifle that doesn't heat up as quickly, like a .223 Rem or rimfire. Feel the temperature of your rifle often, for a centerfire, after a no more than 3 shots and for each shot with a warm barrel. If you can't keep your hand on the barrel, it's too hot!!
If possible, let hunting rifles cool to the touch before testing the POI after the barrel cools.