I was waiting for a batch of barrels to cool that just came out of the Norrell's oven and thought I would do a little math on receiver squaring.
Shooters often talk and read about "MOA" and I'm not sure how many actually know what it is, so I'm going to start there. Simply put, a minute of angle is 1/60 of one degree. A circle divided up into degrees has 360 equal parts, and each degree has 60 minutes in it, so a circle has 21,600 minutes of angle. If you have a circle that has a radius of 300 feet or 100 yards, one minute of angle is going to be just over 1" or 1.0472". Visualize a triangle that is 1.0472" at it's base, and has two equal sides that are 300 feet long. This is the spread of a 1 MOA group at 100 yards. As the range gets longer, the "1 MOA" group gets bigger. A 1 MOA group at 200 yards is 2.0944, or double that of a 100 yard 1 MOA group.
Where the two 300 foot legs of this triangle come together is the head of your case, or basically your primer-pocket. This means that with a 24" barrel, the muzzle only has a + or - of a little over .003" of an inch to stay within a 1 MOA group.
As you get closer to where those two 300 foot sides come together, the amount of error gets less and less to move the point of impact at 100 yards. On an AR-15, if the front of the receiver is out of square by only .001", it will move the POI about 3.5" @ 100 yards. If it is un-square by the thickness of an average human hair, it will move the POI over a foot!
The average receiver that I square is probably about .002", but they run all the way from perfect to .008" or more. Very few are "perfect", perhaps 1 out of 50.
POI is really irrelevant to accuracy. The part that kills accuracy is the fact that a barrel mounted into an un-square receiver will not allow all of the bolt-lugs to touch. The barrel will be tilted with respect to the travel of the bolt, and it's lugs.
Where I talked about "POI shift" in an earlier post, I was referring to a shift in bullet impact when switching from one bullet-weight to another. In other words, if you have your scope dialed in so that 50 grain bullets are hitting dead on at 100 yards, and the POI shifts 3" to the right when you switch to 60 grain bullets, it may be due to an un-square receiver. I have had many customers notice that when they put a new scope on one of my uppers that they normally don't have to move the adjustments much at all to have it dialed in. That is simply because they are square!
Sorry for the long post, and I hope it helps explain things a little...
