The gap that he's talking about is something that I've pointed out for years.
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The drawing above was based on my SOCOM and the gap it had. The gap will vary to some degree but every SOCOM I've ever dealt with had it. I used to do some machining work and of course I applied that to my gun hobby. I picked up a book many years ago about how to build brakes ("The Accurate Muzzle Brake" by Troy Newlon). In that book the author discusses the fact that too large of an inside diameter, as compared to the bullet's diameter, can cause hot gasses to swirl around the bullet and push it around reducing the precision of the rifle. I assumed that the same problem would occur with that gap and installed a Delta P adapter on my rifle in order to eliminate that gap. The end result was a very noticeable improvement in group sizes.
This was a three round test using 168gr Federal Gold Medal Match
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My average groups with all kinds of commercial ammo and handloads are around 1 MOA plus or minus a little but with a couple of specific loads, using a good scope, I can hold under 1 MOA pretty consistently.