The Reagan administration, still counting on an economic rebound sharper than that expected by most private economists, yesterday lowered its targets for growth and interest rates this year, and forecast higher unemployment.

The administration said that it expected real gross national product to increase at a 5 percent rate in the second half of the year, while many private economists are projecting growth of no more than 4 percent. Combined with the 1 percent growth in the first half, economic activity would increase at a 3 percent rate for the year.

The administration, in an update of its economic assumptions in April, had said it expected an economic growth rate of 3.9 percent in 1985.

Council of Economic Advisers Chairman Beryl Sprinkel said that the administration based its forecast on faster money growth supplied by the Federal Reserve Board earlier this year and the expectation that businesses will accelerate spending to rebuild their inventories.

Sprinkel would not say what effect the change in economic assumptions would have on the federal budget deficit. However, administration officials have said that slower economic growth so far will increase the deficit by $15 billion in fiscal 1986.

Because spending for some programs is running below earlier projections, the current estimate for the 1985 deficit is about the same as the $213.3 billion shown in the April update, officials have said.

Sprinkel said he did not have deficit projections because they depend on what progress is made on the deficit by Congress. "We're not hiding anything, we just don't have them," Sprinkel said.

Private economists said the administration's forecast was unrealistically optimistic because businesses won't increase inventories at that fast a rate. Moreover, they said, the trade deficit will continue to divert consumers' and businesses' buying power from domestic goods to foreign products.

The latest revision also showed the unemployment rate, including members of the armed forces, would average 7.1 percent this year, up from the 7 percent predicted in April. The total unemployment rate was 7.3 percent in January, and has been 7.2 percent for the last five months.

The administration said the unemployment rate will average 6.8 percent in 1986, compared with the April forecast of 6.9 percent.

The forecast for inflation was reduced slightly, from 4 percent to 3.8 percent as measured by the gross national product implicit price deflator, which measures changes in the prices and types of goods. For 1986, the inflation rate also was projected downward, from 4.2 to 4.1 percent. Projections through 1990 remained the same as in April.

Interest rates, measured by the three-month Treasury bill rate, were forecast at 7.6 percent, compared with 8.1 percent forecast in April. The administration said it also expects interest rates in the following two years to be lower than expected, dropping from an estimated 7.9 percent to 7.5 percent in 1986 and from 7.2 percent to 7 percent in 1987.

The three-month Treasury bill rate is now at 7.23 percent.

The administration did not change its projections of economic growth of about 4 percent through 1990.

Many private economists have said that they doubt the economy can rebound strongly from a 1 percent rate of growth so far this year.

"I think that's extremely optimistic," David Jones, chief economist for Aubrey G. Lanston securities analysts, said about the administration's growth forecast. "There is considerable debate about any bounce back in economic growth at all. I have no reason to believe there will be a roaring second half."

Jones said he expected growth to increase at a 3.5 percent rate in the third quarter and at about half that rate in the fourth quarter.

Allen Sinai, chief economist for Shearson Lehman Bros., also said that "the White House remains overly optimistic" in its growth forecast. "It's very hard for an administration to be candidly realistic about its expectations. An administration is almost duty-bound to present an upbeat view, although it is far from what is happening."

Jerry Jasinowski, chief economist for the National Association of Manufacturers, said he expected economic growth to increase at a 3.5 percent rate for the second half of the year "because of the drag of the trade deficit and business caution on rebuilding inventories. Business got burned in 1984 by carrying inventories larger than the consumer was prepared to buy."

"Well, I wish everybody agreed with me, but I'm accustomed to the fact that not everyone does," Sprinkel said. "We're quite confident that the general order of magnitude that we're predicting is close to what will happen.

"Final demands have held quite firmly," Sprinkel said. "This suggests to us the probability of a rebound of inventory accumulation. It was the decline of inventory accumulation that sapped the second quarter."

However, many economists said that although final demand has been strong, a large part of it has been diverted to the purchase of imports rather than domestic goods.

Sprinkel also said faster money growth earlier this year should lead to a rebound.