Well, maybe. I thougt it was because with nothing to hold it in place, COW starts spreading, like shot from a shotgun. So it hits that front plate if you have an expansion chamber. There is a lot of energy in COW at really close range -- I blew a hole through a Ford Crown Vic hubcap when I was fireforming in the garage at the old house. But maybe its just the gas velocity/pressure.
IIRC, I've also fireformed with a cotton ball, but always took the brake off. I'm not quite interested enough in the notion to maybe ruin another brake testing your theory...But if you are, I'd be interested in the results... ;-)
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I looked awhile for the link where Lynn showed a picture of the tuner he made, and the results of testing, in responding to Vibe's test. After about five pages, I'd had enough. As Wilbur said,
Is either of you under the opinion that the other is suddenly going to agree?
Just go to Lynn's profile & spin though the 20 pages or so of "view recent posts." As you know, Lynn & I don't care for each other, and I have my limits.
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Edit:
I do remember Dave Toloey's post on this, but wasn't sure what he meant. As Vaughn pointed out (chapter on "Muzzle Blast"), the gasses always overtake & pass the bullet. Nothing good happens then, either, but IIRC, Vaughn didn't have any solution -- or even any way to predict whether or not there would be a problem.