Note: Contested against a published dressing: MISMATCH. Three independent published dressings of John Barr's emerger agree with each other and disagree with our all-blue-dun version. Source: FlyGuysNLies — "Barr's Emerger Fly Pattern - Recipe & Fishing Tips" — https://flyguysnlies.com/flies/barrs-emerger/
Reference Recipe
Hook Standard dry/emerger hook, 16-22
Thread Gray 8/0
Materials
Tail Blue dun material (hackle fibers)
Abdomen Blue dun gray material (fine dubbing)
Thorax Dark dun gray material (fine dubbing)
Legs Blue dun rubber legs (a few hackle fibers)
How to tie it
Mount the hook and start gray thread, running it back to the bend.
Bind a short, splayed tail of blue dun hackle fibers about half a shank long.
Dub a slim, tapered abdomen forward to the 60% mark using half the dubbing you think you need.
Tie a few blue dun fibers along each side of the thorax as sprawling legs, reaching the hook point.
Dub a slightly darker gray thorax over the leg butts up to the eye, keeping it plump.
Whip finish and add a tiny drop of cement; leave the fly untreated so the abdomen hangs in the film.