A BIG white cannon points straight at you as you enter M.L. Van Nice's show at Gallery 10. But it is made of paper. And it's armed with a piano keyboard, not a trigger.

It is peace, not war, that Van Nice is touting. She makes her point by transforming two rooms in the gallery into a fictitious war museum containing, according to the show's title, "Artifacts of the Anglo-Armenian Wars."

No matter that no such wars ever took place, nor that her "artifacts"--strategy maps and charts, medals and trophies and guns--are pure invention, many masterfully carved from thick mat board, gessoed and painted a stark, flat white. The pointlessness of war is made plain through the cumulative experience of this show.

And therein lies the magic. Though most of these objects are clearly bogus (there is always a twist that makes them unreal, such as the finial protruding from the musket) one tends to take them seriously for reasons that have less to do with fool-the-eye than fool-the-mind.

For example, each one is labeled in meticulous calligraphy that appears to say something important, but is, in fact, indecipherable. Sometimes there is wit, as in the Swiss Army knife with the electric plug attached; sometimes horror, as in the slashed canvas of a war map, stitched together like a wound.

Van Nice's strength--apart from her extraordinary craftsmanship--is in her ability to give just enough visual information to conjure a real object, but not settle for easy cliche's as she does in her medals. These and nearly all of the larger objects in the front gallery lack subtlety and add enough clutter to break the spell of what was meant to be a single-theme installation.

What is missing here is the powerful restraint of "Museum Piece," her splendid installation at Washington Women's Arts Center last year. What is present despite the clutter, however, is the artist's extraordinary talent and imagination. The show continues through June 18 at 1519 Connecticut Ave. Hours are Tuesdays through Saturdays, 11 a.m. to 5 p.m. Margaret Dowell Richardson

Also on view at Gallery 10 is a large installation by Margaret Dowell Richardson, made from white Pellon (used by seamstresses for linings) cut into a lacy, open-work pattern and stretched over the wooden frame of a truncated pyramid.

Titled "1983: Fortress Study," the piece was designed, in the artist's words, to "comment on the ineffectiveness of protection." To do that, it would have to evoke the sense of both fortification and fragility. This piece lacks any monumental aspect and comes off as little more than a lacy stage prop. Art Deco at Krainik Gallery

If you've ever seen one of those floor ashtrays from the '20s, held aloft by a sleek bronze female nude, you'll recognize their photographic counterparts in "Art Deco Nudes" now at Krainik Gallery.

Warm and sepia-toned, these photographs and photogravures manage to come up with the same streamlined female form, but here with live models--a neat trick carried out in the service of a distinct esthetic. Posed against drapery as if for the idealizing brush of Titian, these often boyish, always anonymous figures are rendered as classic and eternal--sensuous but never sensual. They are also as flawless as chrome car-hood ornaments (though considerably softer) thanks to the miracle of dramatic lighting and tender retouching.

The purpose of these images was art, not passion--or so it seems, with a few exceptions. For the most part, these often beautiful photographs were conceived as decorative objects, and they were widely published in portfolios during the first decades of this century. Some are German, others from the French publication "Nus." Two of the best are by Roman Vishniac, who continues to make and exhibit photographs. These pictures date from his youth in the '20s and have Cubist-inspired backgrounds.

This intriguing photography gallery is at 1224 31st St. NW and is open from 10:30 a.m. to 6 p.m., Mondays through Saturdays. "Art Deco Nudes" will continue through June 28.