Ok, there were three models fitted to Shermans.
Mk. IA
Mk. IA*
Mk. II
The IA, firing an 18 grain ballistite cartridge had a fixed range of approx 150 yards.
The IA* was developed to give a variable range. This was achieved by fitting the adjustable gas release valve in the chamber and using a 55 grain cartridge. It is a modification of the Mk IA rather than a new design.
Early models had the valve retained by a Jubilee clip and these were restricted to using the 18 grain charge. Later models had the valve retained by a U bolt and could use the full charge.
In Sherman, the vented gasses were released into the turret where ''they have no ill effects on the crew''. Quite what the crews concerned thought about that is not revealed!
As mentioned, in Cromwell, gasses were vented to the outside via a pipe.
The Mk II, also known as the Campbell bomb thrower, was a new design that used an adjustable concentric sleeve (3) to set the range and gases were vented coaxially up the barrel.
There were two types of bomb used, the Mk I which was filled with HCE. This smoked from firing, hence leaving a trail back to the tank. It burned for 1 1/4 minutes. Early bombs had aluminium tails and could only use the 18 grain cartridge, later bombs used Mazak or steel fins and could use the 55 grain cartridge.
The Mk III bomb was filled with white phosphorous and was fitted with a no 151 percussion fuze so only started producing smoke upon impact.