If you have an old musket, sword, rifle or other weapon in the attic or basement, you may want to dust if off and check out its value. Some owners are flabbergasted to discover how much their old weapon is worth.

Arnold Marcus Chernoff, firearms consultant, dealer and world-recognized expert, says, "surprisingly enough, with all the books, magazines, pamphlets and articles on today's market about antique arms, many fine guns still gather dust in homes of owners who don't realize their worth.

"Bringing those arms to light would benefit both a collector, who would prize it highly, and the seller, who suddenly finds that what he regards as 'an old piece of iron' is worth several hundred or even several thousand dollars," he adds.

Chernoff appraises old guns and weapons, and if the owner wants to sell, will buy them for 70 percent of the appraised value, refunding the appraisal fee. Chernoff (P.O. Box 344, Deerfield, Illinois 60015) charges $10 for a written appraisal.

Don't assume that an old weapon is a copy; it's a good idea to check such pieces out, especially if you're not familiar with such things. But not every old gun is worth a small fortune. The value of a antique firearm or weapon depends on its condition, rarity, historical importance, type and age. Antique arms are bringing higher prices now than at any other time in history, Chernoff says, because of the number of new collectors joining an already huge fraternity. In 1940 there were about 4,000 active gun collectors in the United States; now there are more than 300,000 he says.

Keeping records of firearm purchases is important, so family members will know what was paid for a piece when the time comes to dispose of it.

Although guns are looked upon today with much dismay, firearms have played an important role in history. Once every man, woman and child knew how to use a gun and depended on firearms for food and protection. Now firearms are collected for their historical value and craftsmanship, rather than for the capacity to destroy.

"In recent years, many leading museums have included firearms in exhibits to depict their vital role in securing our nation's liberty," Chernoff says. "The heritage of firearms, viewed from all its facets, is a magnificent thing.

"There are many reputable dealers throughout the United States," he says. "Upon request, I can suggest those in other areas. Anyone with an old gun should find out what it's worth, to have the satisfaction of knowing that it will find its way into a collection in which it will be prized and appreciated." Q. I am interested in collecting white milk glass and understand that many pieces have been reproduced from old molds. How can I tell the difference between the old pieces and the new? A. White mild glass, popular in the late 19th century, when it was called white opal glass, has been reproduced from old molds since the 1930s. Some companies producing the glass today are considerate enough to mark the copies they make from old molds with company initials that distinguish them as reproductions. But many reproductions are unmarked, and these make collectors cry over spilled milk glass, as it were, if they later discover they paid antique prices for new pieces.

Anyone contemplating collecting milk glass should study examples to learn what to look for. Old milk glass has a softer, less glaring white color somewhat resembling that of white china. New pieces have a brighter, whiter color and a somewhat oily or greasy look. Many milk-glass reproductions are pictured and described in the book Confusing Collectible -- A Guide to the Identification of contemporary Objects, by Dorothy Hammond, available for $19.95 plus 75 cents postage from Wallace-Homestead Book Co., 1912 Grand Avenue, Des Moines, Iowa 50305. Q. I have an antique curio cabinet. On three sides, glass is curved at the top and bottom. In cleaning the cabinet, I accidentally broke the glass on one side and would love to have it replaced. Would you know of any place that can help? A. One place is Squaw Alley Antiques, Gifts and Hardware, 106 Water Street, Naperville, Illinois 60540. Insert brown wrapping paper in the bottom edge where the glass fits and then trace the curved edge with a pencil. Send the tracing along with the height and width measurements of the frame that held the damaged glass to Squaw Valley, and enclose an addressed, stamped envelope for information on whether the glass needed is a standard size or has to be made to order.

Squaw Alley offers hard-to-fit services and replacement parts, polishes, cleaners, supplies and specialties for antiques and carries pretty gift items. Among its offerings is an antique iron ice skate holding a dried floral arrangement at $9 plus $3 postage -- but the supply is limited. Its catalogue is $1. Q. Do you know where I can get blue glass liner replacements for 65-year-old sterling silver salt cups? A. Write to Hess Repairs, 200 Park Avenue South, New York 10003 and enclose an addressed, stamped envelope for information about the blue glass salt-cup liners they supply. Q. I have a large collection of pattern glass that I would like to sell. Could you give me the name of someone who would be interested in buying such pieces? A. Write to Shirley McGill, Shirley McGill Antiques, 717 East State Street, Geneva, Illinois 60134 and enclose a description or pictures of the pattern glass pieces you wish to sell along with an addressed, stamped envelope for an offer or reply.