High on the chest rig at a comfortable angle to draw from keeps the gun snag free when maneuvering around the field. If you are using a single point sling you need to make sure that the rifle can clear the holster when you let it drop out of the way to draw the pistol.
Mine sits just above my mags and does not restrict my ability to quickly extract my AR mags at all.
No question that drawing while lying prone is an issue but rolling onto your side is not much of an inconvenience given the benefit and the ease of access from a high chest mount while doing anything else beside sucking on the dirt. TBH I'm not sure that I've had to draw my pistol while prone. Have to think about that one.
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