Here ya go, a good read :
ZEROING PROBLEMS WITH THE M1 CARBINE ADJUSTABLE REAR
SIGHT
Casual carbine shooters are frequently perplexed by the fact that the yardage graduations on
the adjustable rear sight usually don’t work properly. With the aperture set at a known
distance, the carbine will shoot either high or low. This is a frustrating characteristic of
most M1 Carbines.
With the exception of late production Inland and Winchester carbines, the vast majority of
M1 Carbines were initially assembled with the two-position “leaf” (or “flip”) rear sight.
The front sight blade was intentionally made too tall, and was then filed down to the proper
height during targeting at the factory. (The Irwin-Pedersen Company was the exception,
using an assortment of front sights with blades of different height; hence the numbers
found stamped on the side.)
The Ordnance Department’s Modification Work Order for installation of the adjustable rear
sight on already-issued carbines, dated 11 September 1944, states: “Carbines should be
targeted if facilities exist and time permits. If facilities do not exist for targeting, the
organization should be instructed to zero the carbine and target it on the range. The weapon
will generally shoot low after installation of the new rear sight, in which case the front sight
should be filed for zero at 100 yards with the rear sight set at the 100 yards setting. The top
of the front sight may then be blackened by painting with lusterless black synthetic enamel.
If the weapon shoots high with rear sight at 100 yard setting, weapon should be sent to
base shop for replacement of front sight.”
Overseas “field replacement” of the “flip” rear sight with the adjustable rear sight began
during the winter of 1944-1945, but the majority of carbines did not have this modification
done until post-war rebuilding. In addition to the adjustable rear sight being added, the
original front sight was pulled off to allow the bayonet lug barrel band to be installed. A
random new or used front sight was reinstalled, but unfortunately for today’s shooters,
there was no requirement for the carbine to then be re-targeted.
Thus, the vast majority of carbines available have a front sight that is either too tall (shoots
low) or too short (shoots high), and the necessary adjustments must be made by the shooter
to find the proper rear sight setting. Finding another front sight, or filing or adding metal –
as necessary - to the front sight blade can alleviate this problem. Changing the front sight
height by .006” (six thousandths of an inch) will change the bullet impact by ½” at 50 yards
or 1” at 100 yards. Be careful! - A little goes a long way
