Thats a massive help! Thanks for all the info, now I know what to get for most of the blaster and what to try for the main body ones!I used 4mm grub screws and an M4 (7.5mm head size 4mm thread size) button head hex bolt in the top of the receiver. I couldn't work out whether the big bolt in the circular recess at the bottom front of the magazine was an M4 or an M5 hex bolt (in the end I used the bigger size). For all the other smaller hex bolts that I replaced or added on mine I used M2 (3.8mm head size 2mm thread size) socket cap hex bolts) and recessed them inside a 6mm diameter styrene tube.
That said, I haven't actually replaced any of the moulded in bolts on the main body of the gun, which look to my eye to be slightly bigger than M2 (M2.5? M3 maybe?) and seem to be a mix of socket cap hex bolts and button head hex bolts.
I have no idea those sizes are accurate to the original - as I say I eyeballed it for the most part - but it may help!