Ignoring a request from President Reagan, House-Senate conferees on the fiscal 1984 defense authorization bill voted Thursday night to terminate the Army's program to build a new, 155 mm neutron shell.

Sen. Sam Nunn (D-Ga.), sponsor of the amendment to cancel the multibillion-dollar program, said the conferees' decision marked "a decided shift in emphasis . . . from short-range nuclear artillery to improved conventional systems."

In a letter to Sen. John G. Tower (R-Tex.), chairman of both the Armed Services Committee and the House-Senate conference on the defense bill, Reagan said he was "gravely concerned" about earlier Senate action to eliminate funding, and urged that it be restored.

The United States is producing two other neutron weapons--an artillery shell for the 8-inch howitzer and a warhead for the short-range Lance missile. Both weapons are being stockpiled in this country rather than being shipped to western Europe where they are politically controversial.

Earlier this year, Congress barred production of the 155 mm neutron projectile until a NATO country agreed to its deployment. Thursday's action by the conferees, if approved by Congress, would supersede that action.

Congressional sources said yesterday that Reagan's letter was the only White House written communication to the conferees and was used repeatedly by supporters of the neutron shell.

In another action, the conferees approved an amendment under which the Pentagon would be prevented from ordering full production of a major new weapons system until it had been tested under simulated battlefield conditions and the results reported to the secretary of defense and Congress.

The provision, versions of which were attached to the fiscal 1984 defense authorization bill by both the House and the Senate, would establish an independent director of operational testing with authority to establish criteria for such tests.

Defense Secretary Caspar W. Weinberger wrote Tower earlier this month that he opposed this provision because "it would further lengthen and complicate the weapons acquisition process."

Congress originally passed the language as a reaction to recent Pentagon purchases of weapons that either were not tested in simulated battlefield conditions or appeared to fail such tests. Among them were the M1 tank, the Copperhead laser-guided artillery shell and the Maverick infrared-guided bomb.

Richard D. DeLauer, undersecretary of defense for research and engineering, recently said the proposal "will add nothing to what we are already doing" because "operational testing comes well after you make an initial production decision."

DeLauer acknowledged there have been problems with new weapons in production not working as advertised, but he blamed this on contractors unable to make their initial production models measure up to those made for development tests.

He said Congress does not appear to understand the testing process, which begins with development tests during the design of a new system, follows with quality tests involving tough laboratory-created standards and finishes with operational tests by troops who will use the weapon.

In another action, the conferees approved a 4 percent pay raise for all military personnel except beginning enlistees, grade E1, with less than three months in service. The raise would take effect April 1 unless Congress approved raises for Civil Service employes earlier.