BBob
WKR
If it's not headed or guided in some way down the center of the hole it's not going to cut everything evenly around the existing hole...it's going to cut everything offset the amount the reamer is off the center of the hole. In other words it cuts offset to one side cutting more off one side than the other leaving lands shorter on one side and longer on the other. It's as if you drilled a new hole slightly offset from the original hole. To add: If the reamer is offset by only a few thousands the throat would be obviously visibly offset with shorter and longer lands. It needs to be guided in a pretty precise manner.I do understand that part of it, thats why its so hard to imagine how the reamer would leave only 2 lands behind...
When a land might only be .003" of an inch tall you can easily see how little the reamer can be off and cause concentric throat issues.
Typical 6mm bbl: .243" groove, .237" bore.
.243-.237=.006, .006/2= .003
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