For those wanting the 500 kills w. mounted weapon ach. in Red Dead. There is a save spot at a house in El Matadero with a Gatling on the roof. I did get the ach. in SP. Took me 2 hours as having a high honour stat to start means shorter attacks. You may need to raid Casa Madrugada and slaughter some innocents Don't get caught on the road, use the mini-map to watch the ladder at your back and save downstairs when you can. In SP getting killed means a reload to last save, in MP you respawn with stats intact which makes the EL Presidio method in MP possible. I'm quite sure that is correct. Unsuspecting passers-by do count towards the ach. They are technically 'enemies' when they die.? Check progress in stats and add cannon and Browning MG for your total.
I had earlier disproved net rumours that you could get this in SP mission mode. That is absolutely incorrect info. The El Presidio method in Public Free Roam that asskickers like Namrah and JPL use means starting the count from scratch and you face all active enemies so the SP is a bit easier. More rumours report you may need to kill at least one person at El Precidia to get this to pop but I did not have that glitch in SP. Now for 10 mins. as Public Enemy No. 1.
Remember kids this is 'just' a video game and you should never, ever shoot 462 people with a Gatling Gun in the real world!