This practice was probably more part of the 'Mutiny Legend', fomented by the press in Britain, as was the wholesale 'dishonouring' of European ladies. Doubtless, some rapes did occur (it's in the nature of war when civilians are involved) though the official enquiry never found clear evidence of any, and the same can probably be said of the 'blowing from guns'. I've never come across any account of the latter from the accounts of the 'Mutiny' in, around and during the Relief of Lucknow. A 'good, old British hanging' (which as my generation was taught never did any harm to anyone) seems to have been the favourite execution method in that theatre. It was probably more repulsive to the victims as, in rural areas, the bodies would often be left hanging, to be eaten by wild pigs.
Since this was never an official policy, and certainly went against the Royal and Governor General's proclamations (1858) of the end of hostilities and amnesty for all those not found guilty of murdering British subjects (including loyal Indians), my guess is that this practice was 1) less prevalent than often assumed 2) very much a 'private' undertaking and therefore the mechanics likely to have varied from place to place and to the 'aesthetic preferences' of those carrying out the 'executions'.
I would have thought that roundshot would have been more practical, since the shot could have been recovered for future use. Gunners amongst our members, might be able to enlighten us as to whether powder alone would have produced the desired effect i.e. total dismemberment.
Sorry if I offend anyone by my rather light-heated treatment of this topic, but I always associate it with the wonderful, comic scene in 'Flashman in the Great Game', where 'Flashy' is lashed to a gun and about to be 'blown' only to be saved by the chance presence of his fellow rider in the Charge of the Light Brigade, Clement Heneage V.C., in the firing party. Flashman, of course, does not share the belief in martyrdom which a Moslem would have had, nor of rebirth, which would await a devout Hindu, and I think this is one of the best accounts of what it must have been like to be subjected to such a punishment if you had no consolation of religious or other beliefs, especially if you were innocent as Flashmen, for once, was in this case.