To find the twist rate of your barrel is easy. Place a tight fitting patch on a loop attached to your cleaning rod. Remove the bolt and insert the patch on the rod down the bore a couple feet or so. At the point where the very back of your action is mark a dot on your cleaning rod with a Sharpie type marking pin. Now pull the cleaning road back our of the bore slowly watch for that dot you made on the rod to rotate back around to the same position it was at when you marked it. Measure between the dot and the point on the back of the action where you originally made the dot. This is your twist rate. One turn in what ever the distance between the dot and action. It takes at least an 8 twist to use the 140gr and longer bullets.
On the use of the Mauser 98 rifle for long range. Don't worry about it. Find an accurate load and go have fun. I have shot better groups with a stock M96 Swede military rifle with open sights at 600 yards than some can with there scope sighted target rifles. As another poster found out when it comes to long range what separates the "men from the boys" is doping the wind. This only comes from lots of experience. Go have fun.
On the use of the Mauser 98 rifle for long range. Don't worry about it. Find an accurate load and go have fun. I have shot better groups with a stock M96 Swede military rifle with open sights at 600 yards than some can with there scope sighted target rifles. As another poster found out when it comes to long range what separates the "men from the boys" is doping the wind. This only comes from lots of experience. Go have fun.