Sinclair International sells a good one. It is a pull button, very easy to use. If you want to go CHEAP, you can go to Lowe's and get the appropriate bolt (longer of course) and just take out one of the two side screws on the left side of the receiver (front or rear) based on your chosen cartridge length. While you are at it, buy a second one wht the normal length. Those allens look MUCH BETTER than those little flats!!!
My XP-100 in 7 BR is one of the last firearms that I would ever sell. I WOULD like to turn mine into a 6.5 WSM, but I am looking at marriage and a home within the next three weeks and need to see how my finances add up.