After leaving Pathway, Jon Hammar and Ian McDonough spent $1,400 on the used Winnebago, splashed out another $3,000 to outfit it and mapped a route to Costa Rica, hitting surf breaks in Cocoa Beach, Fla., and in Louisiana and Texas along the way to Mexico. Inside the rolling white beast were up to nine surfboards.

“We begged him not to go, specifically because we were worried about his safety in Mexico, but they were fearless Marines and were undaunted,” Olivia Hammar said in an email.

McDonough, a U.S. citizen who’s lived off and on for three years in Argentina, said he and his friend were wary of dangers as they approached the Los Indios border crossing, which links Brownsville, Texas, with Matamoros, Mexico.

“We had enough gas in the vehicle that we were going to make it to southern Mexico before nightfall,” McDonough said. “We weren’t going to stop.”

The issue of the shotgun came up near the border.

“I told him that we probably shouldn’t take the shotgun with us,” McDonough said. “And he said, ‘No, I’m going to get it cleared with customs at the gate.’ So I said, ‘That’s fine. As long as it’s legit.’ ”

The Customs and Border Protection agent said it was all right to take the shotgun, McDonough said, adding that the agent told them: “ ‘All you have to do is register it.’ So they gave us a piece of paper and said, ‘This is your registration. You’ve got to pay this much.’ They gave us the piece of paper to give to the Mexican authorities.”

As soon as the Winnebago lumbered over the bridge and they handed over the form to Mexican agents, trouble began. The two spent several days in custody, separated from each other. Mexican authorities eventually freed McDonough, perhaps because of his Argentine residency, and he walked back to Brownsville.

On Aug. 18, Mexican prosecutors leveled serious charges against Hammar. Curiously, it wasn’t the type of shotgun that broke Mexican law. It was the length of the barrel, which the formal citation said was shorter than 25 inches, although a discrepancy has emerged over how the barrel was measured.

“It’s a glorified BB gun,” Olivia Hammar said.

Indeed, Mexico’s criminal groups routinely wield AK-47 and AR-15 assault rifles, high-powered .50-caliber sniper rifles, rocket-propelled grenades and other potent weaponry. If Hammar had any intention of causing mayhem, using his great-grandfather’s proud firearm would have been like Daniel Boone and his muzzle-loading Tick-Licker fighting a modern U.S. Marine.

Back in April, the Dallas truck driver, Jabin Bogan, carrying 25,000 pounds of ammunition in his 18-wheeler, said he got lost in El Paso en route to a delivery in Phoenix. When he lurched to a stop at the Mexican border, asking to turn around, a Customs and Border Protection agent told him it was impossible. He was told to enter Mexico and make a U-turn. He had no passport and couldn’t speak Spanish.

The ammunition was openly displayed on nine pallets in the truck, most of it of a caliber unsuitable for the AK-47 and AR-15 rifles favored by Mexico’s cartels.

Mexican prosecutors charged him with crimes that could have brought more than 25 years in prison.