Shermwell 1 and Shermwell 2 - pictures for a discussion on turret rings

One of the limiting factors on the size and power of the gun you can fit to a tank is the turret ring.

Model manufacturers do not always use the turret ring size of the actual tank when designing their models.

The Sherman could be up gunned with the British 17 pound anti tank gun because its turret ring was larger than the Cromwell.

In model form, the sizes are reversed slightly.

As you can see, the Cromwell turret sits on the hull.

The Sherman turret sits flush in the turret ring.