WASHINGTON (AP) _ A dissident faction is trying to oust the top official of the National Rifle Association in a feud just about as ferocious as the organization's legendary confrontations with advocates of gun control.

The NRA's shootout at the O.K. Corral will take place April 25 at the gun lobby's convention in Reno, Nev. The incumbent executive vice president, J. Warren Cassidy, will try to retain his seat against a challenge from former NRA board member Neal Knox of Rockville, Md.

The NRA's former chief spokesman, John Aquilino, calls the power struggle ''bitter and destructive.''

Cassidy said in an interview he does not view the Knox challenge as a struggle over philosophy. But his opponents portray him as too willing to compromise on gun control.

''Two disgruntled former employees have decided to take their unhappiness with NRA out in the public and media,'' he said of Aquilino and Knox. ''It isn't any more philosophic than that. Two disgruntled former employees shooting from the hip.''

The fight's outcome will likely determine the future militancy of the 2.7 million member organization, a lobby so powerful that many lawmakers cringe at the prospect of crossing it.

''We're talking about shades of extremism here,'' says Barbara Lautman, chief spokeswoman of Handgun Control Inc.

Besides the Cassidy-Knox fight for the top operating job, NRA members are now voting by mail on four proposed by-laws proposals, including two that would radically change the method of choosing the executive vice president. The proposals would:

-Give the 75-member board of directors the right to elect the executive vice president. It's the only proposed change supported by the board, and approval would mean a return to the procedure used until 1977.

That year, the system was changed to call for election by eligible voters attending the NRA convention the year the vice president's five-year term ended.

-Allow all voting members to select the executive vice president by a mail ballot and reduce the term to two years. This is Knox's main proposal.

-Give the board the right to suspend the executive vice president. Such a move would trigger a recall election, during which members would decide whether the official or the board members trying to remove him should be ousted.

-Require NRA directors to uphold the Second Amendment to the Constitution, which says the right of citizens to ''keep and bear arms'' should not be infringed. This is aimed at preventing gun control advocates from joining the NRA and trying to undermine the organization, Knox said.

Knox, who was fired as an NRA board member in 1984 over policy disputes, has campaigned for more militancy.

In an interview, he said Cassidy and the current board members have ''paid more attention to getting along with the politicians than in getting along with their membership. It's time the politicians started listening to us. I have seen a reluctance on the part of the NRA leadership to exercise the muscle NRA has developed.''

Knox blamed the leadership for not blocking a bill that passed Congress last year banning the manufacture of armor-piercing bullets. He said softness by the NRA created a climate of compromise during negotiations on last year's McClure-Volkmer bill, the first major revision of the 1968 federal gun law.

Many items sought by the NRA were ''given up voluntarily,'' Knox said, and the problem was compounded by a successful last-minute amendment that banned manufacture of machine guns for purchase by individuals.

Cassidy has indeed taken a more compromising approach. He refused a request from machine gun owners to try to kill the entire McClure-Volkmer bill on grounds that other provisions were important to the diverse NRA membership.

Aquilino and his entire 17-member public education section were abruptly fired last May by Cassidy's predecessor, G. Ray Arnett. Two weeks later, Arnett was fired by the NRA board in a dispute over his leadership.

Aquilino, who is not part of Knox's campaign, started his own newsletter in January, The Insider Gun News. The four single-spaced pages called the Knox- Cassidy battle ''bitter and destructive,'' and had nothing but scorn for the current NRA leadership.

Aquilino said NRA membership dropped 334,000 since last June and contended the organization is ''losing money, losing members and is in total chaos. The real disaster is in the management.''

In a clipped, gossip-column style, Aquilino wrote in the newsletter, ''Drop likely to bring new wave of bad 'pr' from media-smart anti-gun politicos and groups. Press will eat it up. NRA protests to the contrary will sound feeble. Bad signal to send new Democratic Senate.''

Cassidy disputes the membership figures, pointing out the total of 2,767,000 at the end of 1986 was only 39,000 lower than the previous December.

He said the June peak figures are always unrealistically high because of winter and spring promotions. He contended a dues increase from $15 to $20 a year, plus a change in the method of counting members, were responsible for the slight decline.

John Snyder, the former associate editor of NRA's magazine, said the internal problems may well stem from the diverse NRA membership.

''Within the NRA, you still have a large part of the organization interested in proficiency of target shotting. They're not that interested in the political aspect of the right to keep and bear arms,'' said Snyder, now head of the Citizens Committee for the Right to Keep and Bear Arms.

Within the politically oriented group, there's a dispute over tactics, said Snyder, a lifetime NRA member.

Some members, he said, prefer getting along with those who hold political power. Others ''attack anyone who disagrees,'' and try to defeat them for re- election.