I haven't met a Rifle yet that I couldn't get something to shoot tight groups out of it.
Oh boy would I love to give you a shot at my commercial Mannlicher Shonauer 30-06 rifle (not carbine). Impulse buy and in pristine condition with factory swingoff mounts (dovetail rail type) and 6x scope. The thing shoots about 5" group @ 100. Tried iron sights with about the same results. The irons were about a foot high which I cannot imagine passed final inspection from factory, being folding hand filed leaves. The gun was probably bought new and sighted in, maybe used a season then put away to be eventually sold with estate. Pulling a tight patch through barrel is smooth and even (no tight or loose spots) with sharp rifling. Fired cases measure as one would expect from a good chamber. The barrel has a band which is secured by transverse screw through forearm (at sling swivel). I've tried a spread of loads and bullet weights but no joy. I have never had a rifle this bad including surplus 303 jungle carbines of my youth ($18 !!). I can see no possible reason for such poor performance. I'm thinking about full bed of entire forearm as a desperation move. I had hoped not to destroy original appearance and function since it is a beautiful machine. Probably better to sell to a mannlicher collector since it really is a clean beauty-100% blueing, stock 98% (some very very minor indentations, not scratches or damaged finish). My hope had been a 2moa rifle to take deer hunting and generate admiration and comments from my buddies!