It is the Christmas card that has sent shockwaves across the world – and provided a chilling reminder of the size and type of weapons that are perfectly legal to own and carry in large parts of the US.

An analysis by the Guardian indicates the guns in the photograph published by the Republican congressman Thomas Massie are military grade and – in some cases – similar to those used in recent notorious deadly incidents.

The furore began on Sunday when Massie posted on Twitter a picture of himself and what appear to be members of his family, smiling and posing with an assortment of weapons, just days after four teenagers were killed in a shooting at a high school in Michigan.

He wrote: “Merry Christmas! PS Santa, please bring ammo.”

The tweet provoked an immediate response – with other Republicans criticising him for his insensitivity.

Yet Kentucky law allows people aged over 21 to possess and carry guns – and no permit or licence is required for rifles, shotguns or handguns, according to the National Rifle Association.

The calibre and range of the weapons on display horrified one expert. Philip Ingram, a former British military intelligence officer, told the Guardian: “There is no way in a modern society these weapons should be in hands outside law enforcement or the military … They are designed for one purpose: to kill people.”

The weapon held by Massie, bottom left, has been identified as an M60 machine gun, first developed for the US military in the late 1950s and which, with its belt-fed ammunition, became one the best known weapons of the Vietnam war.

A collector’s item, the M60 is relatively hard to buy on the open market, with sales prices estimated by some auctioneers at $69,000 or more.

Machine guns made since 1986 are banned in the US – but US federal law allows any weapons that were registered before 1986 to be traded on an approved basis.

According to Calibre Obscura, a military blogger and weapons expert, the other guns in the photograph are almost certainly semi-automatics, meaning an owner has to squeeze the trigger each time they want a shot to be fired.

At the front, to the left of Massie, a woman is holding an Uzi, the Israeli submachine-gun, which can fire up to 600 rounds a minute.

Massie’s wife, Rhonda, appears to be holding a Thompson M1SB, most likely a replica of the second world war submachine-gun, sold by its original manufacturer in the US.

In the back row, other family members appear to be holding, from left to right, a Spanish military rifle, a Belgian rifle, and two AR-15s – the AR stands for ArmaLite, the original manufacturer. One of the most popular semi-automatics available in the US, its sales have soared over the last decade.

An AR-15 was taken by Kyle Rittenhouse on his notorious trip to Kenosha, Wisconsin, last year, which saw him kill two people and injure a third as he roamed the streets acting as a self-described militia, following public protests after police shot a black man in the back. Rittenhouse was found not guilty last month after a jury concluded that he had acted in self-defence.

Giffords Law Center, which campaigns against gun violence, said official US figures showed that 726,951 machine guns remain listed as in legal circulation.

Over the weekend, Massie was condemned by political opponents, particularly for releasing the Christmas family picture days after the shooting at Michigan’s Oxford high school by a 15-year-old that left four dead. “I promise not everyone in Kentucky is an insensitive asshole,” said John Yarmuth, a Kentucky Democratic congressional representative.