You could probably safely make a chambered howitzer with one by shrink fitting an appropriately chambered breech plug and welding it in place. The breechplug would resist the highest part of the pressure curve.
Although the pressure ratings of hydraulic cylinders sound high, they are usually at least a factor of ten less than the pressures seen in black powder artillery barrels. Unless they meet the wall thickness = bore diameter rule, you need to do all the engineering calculations before you can be sure of a safe cannon when using one.