The giant Raytheon Co. and Beech Aircraft Corp. announced yesterday that they have agreed in principle to merge under a tax-free basis by trading shares.

And in another merger announcement, the directors of Remington Arms Inc. announced yesterday their acceptance of revised merger terms proposed by E. I. Du Pont De Nemours & Co.

Under the Raytheon-Beech agreement, which still must be approved by both companies; boards of directors and stockholders, Raytheon will offer Beech shareholders 0.775 share of Ratheon stock for every share of Beech stock. The Beech stock will the be retired.

With Beech having some 13 million shares of common stock, the total merger will cost Raytheon about $600 million, according to Newell Garden of Raytheon. However, by trading stock rather than buying and selling, the merger will not be taxed.

At the close of trading Monday, Raytheon stock closed at %59.50 per share, while Beech stock closed at $39.60. Arithmetically, Beech shareholders will receive the equivalent of about $46 per share of Beech, a gain of $7.60 a share.

Raytheon, an international technology-based company, employs about 65,000 persons with worldwide annual sales of about $3.5 billion.

Beech Aircraft manufactures single- and twin-engine aircraft, including turboprops, for commercial and military use around the world. It employs about 10,000 persons and has annual sales of about $600 million.

In a separate merger announcement, directors of Remington Arms Inc., a majority-owned subsidary of E. I. Du Pont De Nemours & Co. said yesterday that as a result of negotiations between Remington and Du Pont representatives, Du Pont has offered more liberal merger terms than previously proposed on July 16.

The Remington board, acting through its non-Du Pont members, has considered and unanimously agreed to accept the revised merger terms.

Under revised terms, each share of Remington common will be exchanged for 0.55 share of Du Pont common -- provided that during the 10 trading days preceding the day before the Remington shareholders meeting to vote on the merger proposal, the average closing price of Du Pont common stock on the composite tape is at least $42.25 a share and no more than $47 a share.

If the average price is lower than $42.25, the exchange ratio would be adjusted so that each Remington share would be exchanged for Du Pont common stock having a value with reference to the average price of $23.24 -- provided, however, that if the average price is less than $40, the exchange ratio would be fixed at 0.581 per share.