I put together a video explaining the need for an AGR and also how to use it. Following that I'll post some photos to further illustrate the concept(s).

One bit of perspective first. As far as I know. ALL receivers will cause a gap between the end of the mouthpiece shank and the start of the leadpipe (hidden inside the larger receiver). If the insertion depth of your mouthpiece creates a good gap on the horn you use, then the AGR doesn't matter. But keep in mind that different mouthpiece brands have a different insertion depth (the amount of shank that disappears into the receiver), which changes the gap. If you try a new mouthpiece and don't like it, the new gap may be the problem. Then the AGR does matter.

First is a photo of my calipers showing the distance from the end of the shank to the insertion mark on the mouthpiece. That is what I refer to above as "insertion depth."

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Then I put a thin wooden rod in the receiver until it hit the end of the actual leadpipe, at which point I marked it with tape.

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Here is my calipers showing the distance I just demonstrated on the stick.

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Now to show you how large the gap actually is. I took the original insertion depth reading and overlaid the stick & tape. If you subtract the measurements of the insertion depth from the distance to the leadpipe, you see this setting creates a gap of 3.33mm. (Keep in mind these are a bit "finger in the wind" measurements.)

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I hope this helps give perspective on what the AGR is about. For another look at this, see my previous blog post: