Testifying before the Senate Judiciary Committee just after Giffords called on lawmakers to "be courageous" and do more to reduce gun-related violence, Kelly gave a moment-by-moment account of the mass shooting in a supermarket parking lot near Tucson, Arizona, on 8 January 2011, that left six people dead and 13 wounded.
During a discussion of whether limiting gun buyers to ammunition clips with 10 rounds or less would violate their right to bear arms, Kelly told lawmakers why he believed limiting ammunition clips would save lives.
"On January 8th of 2011, a young man walked up to Gabby at her constituent event in Tucson, levelled his gun and shot her through the head," Kelly said, referring to gunman Jared Loughner.
"He then turned down the line and continued firing. In 15 seconds, he emptied his magazine. It contained 33 bullets and there were 33 wounds," Kelly said.
Kelly said that Christina Taylor Green, aged 9, was killed with the 13th shot.
A good guy with a gun
"Others followed," Kelly said. "As the shooter attempted to reload, he fumbled" a second magazine. "A woman grabbed the next magazine, and others tackled and restrained him."
Kelly said he believed that if the magazine had no more than 10 rounds - a limit proposed by Democratic President Barack Obama after the killing of 20 children and six adults at a Connecticut school last month - Christina Green likely would not have been killed.
Kelly also countered National Rifle Association leader Wayne LaPierre's claim that the best defence to gun violence in schools and other public places is "a good guy with a gun" - more armed security.
Kelly recalled how "a good guy with a gun" came charging out of a store that day in Arizona and nearly shot the wrong guy - a good Samaritan who was tackling the killer.
"He admits that he came within about a half a second of shooting the man ... and nearly killing him," Kelly said.
Kelly gave lawmakers much to think about as they consider ways to curb gun violence, including a ban on semi-automatic weapons and the proposed limit on ammunition clips.
Call for action
Gun-rights groups oppose both measures, and have indicated there is little or no room for compromise.
LaPierre, who also appeared at the hearing, voiced support for better mental health treatment and more enforcement of measures designed to prevent the mentally ill from obtaining guns. Kelly and Giffords, who continues to recover from her wounds, support those ideas.
Kelly made a point of describing himself and his wife as gun owners who support gun rights. He said they are political moderates and that Giffords was a Republican before becoming a Democrat.
"We are simply two reasonable Americans who realise we have a problem with gun violence," Kelly said, "and we need Congress to act".