how much effect can you expect by shooting several groups with warm barrel? or maybe a more appropriate question is how long should i let my barrel cool between 3 shot groups?

It depends on the weather. On a hot day it needs more time to cool. If the barrel feels hot to your hand it is time to let it rest a bit. A warm to the touch barrel is not too bad, but slightly warm is better. I believe that they cool best if you rack it out of the sun, bolt open, muzzle up. Rifles like your new magnum can heat up quickly, even in the winter.

Al rifles react to repeated firing and barrel heating differently. And any contact with the barrel and stock will alter accuracy as the barrel heats and expands. Also as previously stated, temperature plays a key role too in the cool down time. I've seen where it is normally suggested to wait at least 1 min between shots and 5 - 10 mins between groups.

It warms, it walks. Like Ridge stated, wait 5 to 10 minutes. My magnums got real warm after 3 shots.

It depends on the outside temp but I wait a min. 10 minutes. I take more then 1 rifle to the range this allows time to spread out groups.

Part of trying to find a load that consistently shoots well is to eliminate variables. One of those variables is barrel temperature. Every gun reacts differently as the barrel heats up. By allowing the barrel to cool to a similar temp after every shot, you are eliminating barrel temp from the equation. As for time to wait between shots? Depends on your gun and outside conditions, is it cool or warm outside, is it windy or calm..........Just try to wait long enough that the barrel has cooled to a point that it is similar to previous shots.

Depends on barrel contour also.