They are more vertical than a Magpul MOE or MIAD grips. Not 90 degrees like the grip on the Magpul Chassis for instance. I have just become used to it running it on my 5.56 SPRish build, my 7.62 OBR and all training ARs. To me it's a consistency and familiarity thing. If all my rifles have the same grip, it's less for me to adjust (in myself) if I go from one to the other. The Tikka is my first and only bolt gun. All my shooting "knowledge" has been acquired on ARs. Using the same grip everywhere, makes it easier for me.
The AR15 and AR10 accept the same grips.
This is the one I have on my Tikka:
https://www.bravocompanyusa.com/BCM-GUN-FIGHTER-s-Grip-GFG-Mod-0-Black-p/bcm-gfg-mod-0-blk.htm
I use the Mod-1 on guns that allow for the backstrap.