In my experience thus far, you won't be able to go to a varmint contour unless you get a new stock that is either already inlet for a varmint barrel, or has enough meat around the channel (inlet for a standard sporter) to allow you to open the channel further.
You may be able to get a "heavy sporter" or a "lite varmint" contour to fit, but you will be really thin around the barrel channel.
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