Vice President Bush sloshed through mud and rubble here today to inspect the ruins from Sunday's guerrilla bombing in which at least 219 U.S. marines were killed and more than 75 wounded.

Earlier, the marines in the peace-keeping mission had come under a two-hour barrage of mortars, rocket-propelled grenades and small-arms fire along the eastern perimeter of Beirut International Airport.

There was no firing as Bush, in a white shirt, gray trousers, flak jacket and helmet--the chin strap hanging loose--glimpsed the remains of the bloodiest bombing in Lebanon's violent history.

For journalists on the scene, the vice president sought to emphasize, as President Reagan had in the immediate aftermath of the disaster, that it would not alter the U.S. commitment to assist Lebanon.

"We're not going to let down friends," Bush said. "We're not going to let a bunch of insidious terrorists shape the foreign policy of the United States." But, viewing the destruction of the building that housed the marines, the charred fatigues, duffel bags, ravaged jeeps and tattered magazines, Bush confessed surprise at the enormity of the ravages.

"I hadn't expected this much destruction. You've heard about it. You've read it but until you feel it, see it, see the sides of those reinforcing steel bars . . ."

The sounds of jackhammers competed with the remarks of the vice president. Workers broke up concrete of the second floor of the former four-story headquarters of the Battalion Landing Team. Later, a Marines spokesman said it might be another two days before the salvage operation reached the basement. There was no hope of finding more marines alive.

As Bush trampled through the mud, made by workers hosing down the site to contain the dust, officers at a still intact headquarters a few yards away continued the feverish effort to account for the living and the dead marines.

Afterwards, Bush traveled by helicopter to the presidential palace, changing into a suit before being received with pomp by honor guards and a band playing "The Star-Spangled Banner" and "Stout-Hearted Men."

Bush was greeted by Amin Gemayel, president of the sharply constricted Lebanese republic that--even within the confines of the Beirut region--is in jeopardy politically by the bombings. The French contingent's compound was bombed almost simultaneously. The French toll given today was 54 dead and four missing and presumed dead.

"I don't look for any radical shifts" in U.S. policy toward Lebanon, he said. That policy "hopefully can be a catalyst for bringing peace to a troubled land," Bush told reporters during his three-hour visit here.

"Under this president, believe me, it is not going to happen and I'll take back this report of heroism and cooperation" of the marines and others in the multinational forces and "I know it will make an impression on him," Bush said.

Few of the marines here saw the vice president. Informed of the trip, troops on the embattled front lines said they were heartened that he was concerned enough to travel here but some worried about his exposure to the dangers they daily face.

"I think it's pretty nice that he came over here but I don't know if it's a good idea to take a risk," said Lance Cpl. Robert Diffenauer, 19, of Pickneyville, Ill., as he peered through the slits of a sandbag bunker out to sniper positions beyond.

For the marines, the lingering effects of Sunday's explosions were a mixture of grief, shock and frustration.

"I think we ought to start shooting first," said a 22-year-old marine in a platoon facing one of the Moslem Shiite slums from which recent attacks have been launched. "We see something that doesn't look right, shoot first, cut it dry instead of keep letting these people get an upper hand on us."

As two fellow marines nodded in agreement, he said he wanted revenge for the death of his friends killed in Sunday's explosion. Asked what that would accomplish, he said, "personal satisfaction."

Other marines were less severe in their comments. "There's probably a more settled determination," said Lt. Arthur W. Harris, 24, of La Grange, Ga., "a grim determination, maybe even a bitter determination to accomplish the mission."

Marine spokesman Maj. Robert Jordan said of morale, "I think many of us came here with very optimistic views, very altruistic motives. I think the altruistic motives are still there but I think some of the optimism and idealism has dissipated and we now are resolved to do the best job we can and complete this mission."

The Marines on the line here listen with a mixture of pride and foreboding to accounts of their comrades in the U.S. invasion of Grenada.

"Most of them here are kind of happy that it's happening," said Cpl. Diffenauer. "They think it's saving our face a little bit." But the foreboding arises from fears that the new commitment of Marines in the Caribbean will mean that those here might have to stay longer.