[QUOTE] What would be the advantage of making the Knight version with a non-removable vent liner? Doesn't make sense to me. /QUOTE]

Quake, I suspect it's a liability issue. It may be that Knight is afraid that some uninformed person will get ahold of one of their guns (perhaps as a used gun) that has the vent liner removed, and shoot it without a liner. That "just might" cause a little excessive blow back.