COLLECTING old and unusual cartridges may seem to be an odd pastime, but it's becoming ever more popular as the prices of classic firearms climb out of the reach of many collectors.

Consider: A 45-70 caliber Model 1886 Winchester rifle in so-so condition sells for more than a thousand dollars nowadays, but you can still get one of the original cartridges for about a buck. A fairly respectable collection can be assembled for $100 or so, if you exclude rarities.

Freebies are possible, too. Uncle Charles isn't likely to part with the P-38 pistol he brought back from the Big War, but he might be only too happy to get rid of that "old box of shells" cluttering up the basement workshop.

There's an investment angle as well.

When I lived in Brooklyn in 1958, I sent five bucks off to an outfit called "Ye Olde Hunter," located in an exotic town in Virginia that I'd never heard of: Alexandria.

I received a collection of 75 goodies, including the scarce 11mm Gras cartridge that WWI French gunners fired at German observation balloons and a tiny "Velo-dog" cartridge, designed for use by turn-of-the-century bicyclists to ward off canine pests! That same mini-collection would retail today for around $150 -- not bad, even allowing for the devaluation of the dollar.

Cartridges can be fascinating and colorful. They range from the teensy Kolibri pistol variety that will stick to a damp finger like a grain of rice to one-inch Gatling shells that are paperweight size or larger. Especially interesting are rainbow-hued paper shotgun shells and military cartridges with color-coded bullet tips: tracer, incendiary, armor-piercing, etc.

If you'd like to see what it's all about, the second annual Northern Virginia Cartridge Show might be a good place to start. You can buy or browse, and legendary collector-dealers like Arizona's Bill Woodin and New Hampshire's Jim Tillinghast will be on hand to answer questions about those shells you got from Uncle Charlie.

The show's at the Dumfries Holiday Inn at I-95 exit 51 (Rte. 234). It's open from 8 till whenever this Friday and 8 to 4 Saturday. Admission is $2.

For more information, call the hotel at 703/221-114 or Bill Vanderpool at 703/323-0736.