I took my older Mini to the range yesterday for some accuracy (hahahahaha, what a contradiction in terms) testing.  Mine is in 223.  The first group, from a warm barrel, was a 3" string, then 50 fast rounds (fast enough to get the barrel hot) that pretty much clustered in a 4-5" group, then a 3 round group that measured about 2" (haven't mic-ed it out yet).
Problem was the Mini wouldn't work semi-auto.  It suffered the 'early Vietnam M-16' snydrome and I had to manually operate the rifle.  I suppose that had some effect on my groups - ya think???
The gas piston kept falling out of the gas block.  I simply cannot figure this one out.  I dis-assemble and re-assemble the rifle properly but that darn gas-piston simply will not stay put.
It almost appears as though I am missing something, somewhere.  Some part that would hold the piston in place and guide the operating rod properly seems to be missing but not according to my take-down manual.  The manual shows that all parts are there.
Here's a thought: the gas piston looks like the piston from a auto engine, complete with ring grooves but there aren't any rings.  Do the gas pistons on the Mini's take rubber 'O' rings or something to hold them in place???  
I can't figure this one out.  I like the way the rifle handles, as it is a fun little gun to shoot, but I would like it to shoot.  Thanks for any help.  Mikey.