Obviously a barrel obstruction of some sort, be it overlooked bore sighter or plugged muzzle. I don't know how to deal with intentionally introduced obstructions, other than careful inspection, but on rifles with GI style flash hiders, I use shoot-through muzzle caps, occasionally seen at gun shows (check the mil-surp vendors) or from Gun Parts Corp., which has recently only had them available in rather comical looking red color, vs. GI issue black. On hunting rifles with conventional sporting muzzles, I cover the muzzle with electrical tape --- fold a single layer across the muzzle, then wrap around the barrel to secure the ends. Keeps mud, snow, rain, twigs, bugs, etc. out, and air compression in front of the advancing bullet blows the tape out of the way with no affect on accuracy. Well, maybe some positive affect on accuracy, as it helps keep the muzzle from getting beat up, as it might if you're clambering around in rocks with a slung rifle, as I sometimes am...