U.S. Sen. Frank Lautenberg has a point, to a point.

The Democratic lawmaker from New Jersey says it “simply defies common sense” to allow gun sales to people suspected of being terrorists. He’s right – until he proposes that the government’s terrorism watch list should be the deciding factor of whether someone can buy a gun.

At Lautenberg’s request, the Government Accountability Office recently studied firearms and explosives purchases among terror watch list members. It found that people on the list tried to buy guns 963 times in the last five years, and nine out of 10 times they were successful because nothing else in their background disqualified them.

That sounds outrageous, but consider this: The FBI refused to divulge details about who was able to buy a gun and what their connection to terrorism might be. That itself makes conclusions rather hard to draw.

Then there’s the watch list itself, which has a number of problems, the first being its size. At last count, the list had more than 1 million names representing 400,000 people, and it’s growing quickly. Just four years ago, the list stood at one-fourth of its current size.

It’s also rife with errors. While much of the criticism can be attributed to cases of mistaken identity – such “false positives” have snared the likes of Sen. Ted Kennedy and members of the Federal Air Marshal Service – the list itself is also suspect.

Just last month, the Justice Department inspector general found that the list included the names of at least 24,000 people who may not have belonged on it. Other names that should have been on the list weren’t.

But the most significant concern is the secrecy surrounding such intelligence tools, which doesn’t necessarily make much room for due process.

A person may never find out that a government agency nominated them for the list, and even if he or she does, getting off can prove impossible.

The “suspect” is told to appeal to the agency that nominated him. The catch: The Department of Homeland Security won’t release that information. Civil libertarians have been demanding a better redress system for years, to no avail.

The incongruity of the government being able to stop someone from getting on a plane but not from buying a gun is not lost on Americans. Before Congress attempts to curtail the exercise of Second Amendment rights, it had better be sure that it’s using a threat assessment that is credible and fair.