It was only the second time builder Aaron Hughes, 26, had used the gift from his father, Brian, who was with him at Gilfach, near Rhayader, along with his girlfriend Charlotte Croudace.
Police cordoned off the area and called in a bomb disposal team from Hereford.
Mr Hughes joked that the bomb was "nice to find," but he would have rather have discovered a pot of gold.
It is understood the device was a mortar bomb, but why it was buried on a nature reserve is a mystery.
The trio were walking along a path on the reserve when they made the discovery.
"It was the first time I'd properly used the metal detector," Mr Hughes said.
"I had been out on New Year's Day and I'd found a few coins near a playground - a few coppers and a couple of pound coins.
"The following day on the Gilfach reserve we'd only been there 10 minutes when the detector started beeping.
"I thought I'd discovered a can or a metal lid. I never imagined I would find a bomb.
"I scratched down into the ground and saw the tip of the what we now know is a bomb.
"I wasn't sure what it was to start with, so I started digging down with a spade and flicked it out of the ground and onto the grass. It was about 2-3in (5-7cm) deep.
"We were shocked because it was pretty obviously a bomb.
"We weren't sure whether or not to phone the police because 999 is only for emergencies - but this was sort of an emergency.
"It was a nice thing to find, but I obviously would have rather have found a big pot of gold."
A man using a metal detector he was given for Christmas has unearthed a World War II bomb on a nature reserve.