Mike,
While I agree with those that say the Tin Cloth Filson stuff is waterproof and tough, HJ is also right with the comment about the stuff being stiff enough to hold you in shooting position. If you just want to walk thru briars while it's pouring rain, Tin Cloth is for you. If you want to quickly mount your gun on a flushing quail while in the pouring rain in the briars, I dunno what you need. Drop the briar part and I'd say get a thin raincoat.