Yesterday rccalzadilla, Quentin and I were discussing Head Space Gauges and instructions on using them. Which the issue of Throat Erosion and the Throat Erosion Gauge came up. So here is my two cents on that subject. Please view the Throat Erosion Gauge that can be purchased from Brownells in the following pictures. The Throat Erosion Gauge and the Armorer's Magazine are the most important tools in the Armorer's Tool Kit. Both can diagnose a lot of what normally would be frustrating problems. However in this conversation we will address the Throat Erosion Gauge. Please see the pictures below! One way to use the Throat Erosion Gauge is to insert it in the bottom of the receiver as pictured in Pic #2. Or you can insert it through the ejection port also it does not matter! First of all make sure the chamber of the weapon is absolutely clean! If not you will get a false reading. Insert the Throat Erosion Gauge Tip into the chamber until it stops. If it is a good or like new Chamber the Throat Erosion Gauge should read somewhere around the second line from the Tip of the Throat Erosion Gauge. Some manufacturers have tighter throats than others but it will always be around the second line + or - 1/4 of a line if in new condition. In Pic#2 You notice that if you look directly down at the Throat Erosion Gauge from a 90 Degree Angle with the upper. You will be using the Aluminum Ledge that is horizontal to the rear of the Pivot Pin on the upper as shown in the Picture as your reference reading point. The second line from the tip of the gauge should be visible at that point while looking directly down at the 90 Degrees on it in the receiver. The Question that was mentioned yesterday, basically do I need to keep checking head space after checking it when the rifle is new? In my opinion unless a Barrel Nut would get loose you do not. Once you confirmed that the Head Space on the weapon is good, you only need to keep an eye on the Throat Erosion after that. The throat of the weapon will wear away before you have a head space problem. And normally, you will notice the weapon loosing accuracy. Since I do not like catastrophic events, "Blow Ups" And Broken Bolts at the Cam Pin or bulged receivers! I change Barrels anytime the Throat Erosion Gauge when inserted reads on the 7th. Line from the tip. You can literally over time tell by checking with the Throat Erosion who is shooting their rifle more than the other guy! It is a great tool to have! One additional thing I failed to mention regarding Head Space is checking head space preferably with the upper off the lower. I have seen occasion where they checked head space while the upper was assembled on the lower and were dropping the Bolt and Carrier on the gauges using the Bolt Catch. Allowing the Carrier and Bolt to go forward with full force. Totally destroying the Head Space Gauges. Use caution when closing the bolt to check head space. Do Not Use a lot of Force. You can also remove the Extractor which will make it easier when checking. Don't forget to put it back On! As mentioned the Head Space Gauges are about $30.00 a piece !!!!