I asked you before ST, because I didn't hear that part of the testimony and you seem to have knowledge on guns, if OP didn't hear Darren say the gun was "one up" why did he empty the chamber ? At least that is what the lawyer interviewed by Sky explained, i.e. OP heard Darren say it was one up but didn't check to see if it also had a magazine loaded, so when he received it under the table he emptied the chamber for the one up so as to be able to test the sensitivity of the trigger to see if he liked it, but he didn't realise, because he didn't check, that a magazine was loaded so that on emptying the chamber and closing it again another round was chambered so when OP pressed the trigger it fired.
In any case as a responsible gun owner, which I am not, not a gun owner that is, shouldn't you check everything, chamber, magazine and if there is one a safety catch, before pulling a trigger. It reminds me of when I learnt to drive in respect of going by other drivers waving me on i.e. to always check myself if it is safe to go rather than by someone else's indications. I was also taught the same as a kid, i.e. never to cross a road just because a driver hand signed me to go but to ALWAYS check myself with my own eyes.