Someones pointed out to me that if the gun had the ORIGINAL Mk1 type barrel nut, then it wouldn't have happened because that nut had a block to prevent for and aft movement of the piston extension if the nut were raised. But this block was removed during the first simplifications to the gun in 1939/40. On the basis that '.....with the proper and correct training, such an event (leaving the barrel nut undone) or using the gun without the barrel nut fitted never likely to occur'. The wise old trials engineers of 1940 didn't cater for enthusiastic amateurs though.....................