In Igor, select the Tools menu, and then Gun Editor. Select the gun you want to mod from the list on the left (or create your own from scratch by clicking "New..."). The scope can be improved by increasing the number found next to "Zoom1" (they MP5 has a zoom of 1.5, which is pretty useless). Note that a gun must be set to be a sniper rifle to get two zoom levels.
I'm not quite sure what you mean about the target "staying in the middle, so it's always accurate". Do you mean the ret pips? In the Accuracy area at the bottom of the gun editor you can alter how much the pips bloom when you're at different stances and moving at different speeds (walk, run etc). The smaller the number here, the tighter the pips and so the more accurate the gun. You can also reduce the recoil (above the Zoom parameter) to improve accuracy when shooting full-auto...
Hope that's what you wanted.
Now a word of warning: don't overwrite any original game files. It's a bad idea. If you want to alter things, create your own versions of things separately so that you can always revert to the original, uncorrupted versions if you need to (eg if you have problems/crashes etc). You also need to consider the effect it will have on the gameplay - you need to keep GR a well-balanced game. Giving yourself a silenced gun with pin-point accuracy, a 20x scope and a 10,000 round clip is easy to do, but it takes the fun out of playing because the balance that the weapons have is lost...
I'm sure you'll see what I mean when you start experimenting with things. Have fun...
