Gloucestershire 161 for 5 beat Middlesex 157 for 8 by five wickets
Gloucestershire recovered from their narrow home defeat to Netherlands on Saturday to beat Middlesex by five wickets at Lord's in a match reduced to 26 overs by rain.
A target of 158 seemed a tall order for an inexperienced side low on confidence, but they won with 22 balls to spare with Hamish Marshall setting the pace with 33 off 22 balls and Benny Howell seeing them home with an unbeaten 45.
Joe Denly and Dawid Malan gave Middlesex a brisk enough start with 45 off the first eight overs before Graeme McCarter, the 19-year-old Ireland fast-medium bowler making his Gloucestershire debut, made the breakthrough.
McCarter went on to finish with 3 for 41 from his five overs while Ed Young, the 22-year-old left arm spinner, stifled Middlesex's middle-order with 3 for 26 in six overs, his best figures in one-day cricket.
McCarter, who has made one appearance for Ireland in the ICC InterContinental Cup, took his first wicket when Denly, who had made 25 at a run a ball, had his stumps spread-eagled as he tried to drive.
Young then had Malan caught at long on for 22 and, after Chris Rogers had been caught behind off Howell, he picked up two more important wickets when he had Neil Dexter caught at long-off and Paul Stirling caught behind.
Middlesex were struggling at 90 for 5 but they were revived by Gareth Berg and John Simpson, who put on 47 in seven overs for the seventh wicket before McCarter had Simpson caught at long-on.
Gloucestershire made their intention plain from the start with Chris Dent hitting two fours off the first three balls from Tim Murtagh and then falling leg before. Undeterred, Marshall and Howell then took 22 in two overs from Corey Collymore and had added 53 in six overs when Marshall was lbw trying to sweep Ollie Rayner's second ball.
Kane Williamson did not get very far before he was lbw to Steven Crook but Alex Gidman kept up the momentum with 26 off 21 balls and Ian Cockbain weighed in with 18 off 19.
At the other end, Howell kept a cool head, stroking 45 off 48 balls with a six and three fours and settling the match with a boundary through midwicket. It had been a strangely low key performance by Middlesex, who were pipped for the semi-finals on net run rate last season.