The only facts in this story are that the prisoner called authorities to his cell and showed them the revolver in question which he claims he found under the mattress. The rest is speculation.
When you stop to think about it, does anyone really believe that the most likely way that it got into his cell is that he had it when he was brought in, and that he carried it in his rectum?
It seems to me that another inmate hiding the gun in an empty cell under the mattress, and it not being found until he was moved into that cell is overwhelmingly more likely.