Back in 2000 BGNW (blow guns north west) commisioned nelson to do a run of .62. (this run was Orange in color) The first few pallets seem to have worked well for most. All later paint was total crap, just too much force on the balls. the balls, if you can get them to leave the barrel intact, blow up in mid air ( I ran them w/o a barrell on an old beater SMG that I torched) Some have had luck with the paint out of AGA .62s but thats it.
It comes down to this, the SMG 60 is nothing more than a wall hanger due to lack of quality paint. There was a company doing .55 conversions, but they landed a big contract and sold all the SMG 55 stuff. Converting a 6o to 68 is possible, but its cheaper to buy a SMG 68, unless you have your own machine shop and want to spend tons of time to get everything right.