**I think I read somewhere that its Minute Of Angle?**

that is correct

**And that it means 1" at 100 yds?**

for all intents and purposes yes. to be exact it equals 1.047" at 100yds. So you can see why everyone jsut rounds down that 47 thousands of an inch to make it 1" for easy calculations.

**How do you measure groups? Center of bullet hole to center of bullet hole or outer edge - outer edge?**

measure the 2 furthest bullets holes from outside edge to outside then subtract the bullet diameter to give you the c-to-c measurement which is the standard for measuring groups.

Keep in mind that a hole in target paper isn't exactly the true bullet diameter either. It actually measures smaller. So depending on how detailed you what your group measurement keep this in mind. If your curious of how small, just measure the diameter of a know caliber size bullet hole in your target paper under a magnifying glass. But honestly I jsut subtract bullet diameter for my own reference to keep it simple.

**would 1"@ 100 yds correspond to 5"@ 500yds? **

you haven't forgotten everything obviosuly. your right on the money. Or to be exact as mentioned above the true MOA at 500yd would be 1.047x5 or 5.235". So how important is that .235" to you at 500yds?

Hope this helps,

Steve