For testing main guns place target at 20km so that the gun can be seen to elevate.
If gun elevates correctly and gun flash appears to be in the correct direction then adjust the rotation of startshell in *.msh.
If the gun flash appears correct then roty and rotz are probably correct.
Try rotating rotx 90deg 180deg and 270deg to see the effect.
With target at 20km you are not likely to achieve a hit but the shell splash should be close to the target.
Its easier to see the effect of adjustments with a distant target.
Before starting these tests check the pivot point alignment in Max.
Pivot points must be located on rotation axis.
For turrets z-axis is vertical pointing upwards, x axis points in the direction that the turret faces.
For guns x axis points in the direction the gun faces, z axis is horizontal, y axis is vertical pointing downwards.
These must be correct before you export the mesh.