Part of a British bombing raid on Saddam Hussein's invading forces inside Kuwait in January 1991, MacCormac and his fellow Jaguar fighter jet pilots were "taken aback" by the extraordinary expanse of dust and damage the weapons created.
Their radios were unusually silent as they returned to their base in Bahrain.
"Even though you know exactly how it's going to happen, seeing it operate for the first time provides an immediacy and intimacy that you don't get by reading about weapons," MacCormac said in an interview.
"That was the reason why we were all fairly thunderstruck, really, by what we'd seen."
MacCormac, 51, retired in 2010 after a 25-year career in the British air force, and now leads a Danish aid agency's efforts clean up the remnants of deadly cluster bombs in nine countries.
He is also an advocate for an outright ban of a weapon critics say is indiscriminate and inaccurate with a long history of maiming innocent civilians, and has no useful place on any modern battlefield.
MacCormac will be among those testifying Wednesday at the Senate foreign affairs committee, urging the Harper government to close what they say is a loophole in a controversial bill to ratify an international treaty outlawing their use.
The hearing will mark another chapter in a grinding legislative battle over the bill, which has exposed the Conservative government to broad international criticism from the likes of the International Committee of the Red Cross.
Canada signed the Convention on Cluster Munitions in 2008, but has yet to ratify the treaty because of a contentious clause in its bill that would still allow Canadian Forces personnel to be indirectly involved in the use of the weapons.
The government argues the bill, in its current form, is needed to preserve the military's ability to participate in joint operations with the United States, which opposes the treaty, and still reserves the right to use the weapons.
Last fall, the Commons foreign affairs committee allowed a one-word amendment that removed the word "using" in a key part of the bill, but retained it in other sections. That meant the bill now clearly prohibits Canadian military personnel from directly using the weapons, but doesn't completely rule out less direct involvement in combined operations.
"The bill was amended very, very slightly, but we think there are some areas where it can still be tidied up because there are still references to 'use' in the bill and we'd like to see those removed," said Paul Hannon, the executive director of Mines Action Canada.
The senators will also hear from Louise Bradach, whose son, 21-year U.S. Marine Cpl. Travis Bradach-Nall, was killed in southern Iraq in July 2003 while helping to clear cluster bombs that had failed to explode — a U.S.-made weapon fired by his own comrades in arms.
"If you really, really look inside your soul, I cannot believe that you could say, 'No, we need to use loopholes,'" she said in an interview.
"I would say, 'No, hard and firm, we're not using it,' I don't want to be part of saying, 'Yes, it's OK to use it.'"
Last month, at the international meeting of states party to the treaty, Canada announced that it had destroyed its stockpile of cluster munitions — a sign that amendments to the bill are still possible, Hannon said.
"It's the first time in years I heard Canada applauded when they made a statement," he said. "And there was a solid round of applause for Canada when they announced they had destroyed the stockpile."
The Canadian Forces have never used the weapon, and a directive from the office of the chief of the defence staff issued several years ago prohibits their use.
Hannon is also buoyed by the fact that the United States recently announced that it would no longer use anti-personnel mines, a cousin to the cluster bomb, except in the Korean Peninsula.
"Specifically, the United States is aligning our APL (antipersonnel landmines) policy outside the Korean Peninsula with the key requirements of the Ottawa Convention," said a statement last month by the U.S. State Department.
Canada spearheaded the international treaty to ban landmines in the 1990s over the objections of its key ally, the U.S., which has ignored that treaty as well. But it has kept a low profile in the ongoing campaign against cluster bombs.
MacCormac said he would like to see Canada take a global leadership role as it did with the Ottawa Convention by passing an airtight cluster-bomb bill.
"That does not for me involve creating situations where it is possible for members of a state party to engage in acts that the convention itself seeks to prohibit," he said.
"From a clearance perspective, there is very little difference frankly between leftover landmines and leftover cluster munitions. They both cause death and injury to parties many years after the event."