The 400m zero for an AK74 /5.45x39 is very simple and is designed to give the rifleman the ability to hit out to 400m without knowing any hold overs; additionally the rifleman can take the same point of aim at most any distance and have a reasonable chance to score a hit without having to know the actual distance to the target. The intent of the 400m zero is remove the guesswork from range estimation and to eliminate the need to adjust or change anything on the rifle or scope during the heat of combat. When used correctly the procedure enables a very simple and quick ability to shoot accurately at most ranges.

While SVD marksman have the ability to adjust the elevation and windage via the turrets on the PSO, in practice the philosophy is instead to shoot quickly using hold overs and not adjust the turrets unless the distance to the target is very long range or enhanced accuracy is needed. Speed and relative accuracy are what's important...the ability to quickly hit a man size target at practical engagement ranges is most important and helps maximize the value of the SVD during engagements.

Modern Russian optics for the AK74M and AK100 series follow this trend and simply eliminate the traditional PSO style turrets from the optic completely. Instead they are replaced by screw driver adjustable elevation and windage controls that are zeroed once and then left alone in the field. They are also typically covered by a screw-on cap and often recessed to prevent them from being accidentally disturbed.

How to use the 400m Zero:
Unlike the common 25m or 50m zero and learning hold overs for 5.56x45, the AK74 is instead zeroed at 100m distance with a preset difference in point of aim vs point of impact. This difference in POI vs POA allows the shooter to take the same point of aim on the target out to 400m... the shots will land higher or lower depending on distance but should still be hitting a classic 1.7 or 1.5m tall target somewhere from the upper chest to the waist without the rifleman having do to anything other than aim center of mass (The belt buckle, according to Russian and former Soviet doctrine)

The procedure is to shoot from 100m and have the rounds hit about 22cm higher than point of aim. Typically a 44cm circle is drawn and the shooter aims at the bottom of the circle. If the rounds land in the middle the point of impact is considered 22cm higher and the optic has the proper offset for a 400m zero. Note that certain data cards show 22cm while others show 24cm, both for AK74. Additionally you could zero for 300m as well (see below)

An alternative procedure for the AK74 is to use the already zeroed iron sights. Again starting at 100m the shooter can fire a 5 shot group with the rear sight leaf set on "4" or " п". This should put the the rounds about 22cm higher than they would have landed if the rear sight was set on 1. If this is the case the optic reticule is simply dialed onto the 5 shot group. A second 5 shot group is fired using the optic to confirm zero.

PO 3.5x21P showing 400m data for AK74 and RPK74, and PK01-VS data card showing 300 and 400m data

 

As you can see from the data cards there are actually two distances to zero the AK74....300m zero by using a 13cm higher point of impact at 100m or 400m zero by having point of impact about 22cm higher at 100m. The difference at 100m determines at what distance the 5.45 round drops to match the point of aim, and this convergence is considered to be where the rifle is zeroed at. 

The battle sight setting on the Russian AK74 "п" corresponds to 400m (actually about 440m)

PO 3.5x21P Datacard info:

AK74 400m zero (POI is 22cm higher than POA at 100m)
50m = +9cm
100m = +22cm
150m = +32cm
200m = +37cm
250m = +36cm
300m = +32cm
350m = +20cm
400m = 0cm
500m = -64cm

RPK74 400m zero (POI is 18cm higher than POA at 100m)
50m = +9cm
100m = +18cm
150m = +27cm
200m = +31cm
250m = +31cm
300m = +27cm
350m = +16cm
400m = 0cm
500m = -52cm

PK01-VS Datacard info:
AK74 300m zero (POI is 13cm higher than POA at 100m)
50m = +6cm
100m = +13cm
150m = +17cm
200m = +16cm
250m = +11cm
300m = 0cm
350m = -17cm
400m = -43cm
500m = no data

AK74 400m zero (POI is 24cm higher than POA at 100m)
50m = +11cm
100m = +24cm
150m = +33cm
200m = +38cm
250m = +37cm
300m = +32cm
350m = +20cm
400m = 0cm
500m = -65cm