I have the same problem as the opening post and I too would like to see an answer. I have the adjustible rear sight and the peep loop must be two full graduation marks to the left to be on the bullseye of the 100 yard standard target.
I have the original M2 style stock, an Italian repro of the M1A1 paratrooper stock, and a modern synthetic sporting rifle stock, and two barrel bands, one with bayonet lug and one without. My problem is not related to barrel bedding: same zero problem no matter what stock or barrel band is used. I have been told my best bet is to retire my 1943 IBM M1 Carbine to the gun collector case and buy a new Auto Ordnance clone.
LAGS: Would it not be easier just to remove the front sight pin and index the front sight, then refit the pin and locating hole ?
Nope, not that simple. Front sight of the M1 carbine is pinned to an indexing key that is a tight fit in a slot in the top of the barrel. Remove the pin, you can drive the front sight forward off the muzzle, then lift out the key which has the hole for the pin. Even if the front sight is canted, relocating the front sight is not easy.
The military tech manual on basic armorer checks advises centering the rear sight with the adjustment knobs, re-zeroing the gun by drifting the rear sight in its dovetail. You must use a special adjustment tool, because the original sight is staked in place by using a centerpunch from the top to displace metal in both the sight and receiver. Just driving the sight left or right with a brass hammer runs the risk of damaging the sight housing, especially the stamped sight housing. Once zeroed, you must repunch the sight into the dovetail to prevent movement.
The usual reason found on the internet for M1 Carbines requiring rear sight extreme windage adjustment to the left was that the 1980s re-importer Blue Sky of Arlington VA stamped their importer mark on the barrels without proper support and bent their barrels. Google M1 carbine Blue Sky bent barrel. I suspect
there.
Other reasons may be the barrel was bent in action (use with a bayonet, used as carry support, used as a pry bar) or the barrel was screwed in tighter when re-headspaced in an arsenal rebuild.
Some folks have reported going to the extreme of supporting the barrel on lead blocks and tapping the barrel with a lead hammer and bending the barrel itself to zero the front sight windage-wise. Lead supports and hammer to avoid damaging the steel barrel. So many ways to go wrong bending a barrel, I shudder at the thought of trying it myself.
A few have found relief by having a gunsmith recrown the barrel. That might cure the problem if the muzzle is off-square, but I can imagine it could compound it in some cases.