While the .22 LR is legal for Field Pistol, I doubt you'll find many shooters who would call that round "reliable" for rams. Probably the most popular chambering for FP is the 22 Hornet. Of course, any centerfire, straight walled pistol cartridge with a case having an overall length of 1.29" or less works well and is legal also.
If you are new to FP, I'd suggest the "shoot what you brung" theory even if that means using a 22 LR & ringing a few rams. That, and talking to the shooters at the match should set you in good stead.
Hope this helps!
Steve Ware