Ronnie Milsap, the Bellamy Brothers and Louise Mandrell are part of the line-up at a good old-fashioned country music jamboree his weekend at Bull Run Regional Park. On Saturday at 10 country games, contests and demonstrations begin, including a hog- calling and watermelon-seed-spitting contest, wheelbarrow and frog jump races (frogs are provided) and a greased-pole- climbing contest. When you're not spitting seeds or calling hogs, you can gander at the yarn-spinning, blacksmithing, dulcimer- building and quilting demonstrations that will be set up around the stage area, too. At 12:30 the Kendalls start to swing, followed by a star-studded country music afternoon. Picnickers are encouraged to bring lawn chairs and blankets to sit on while enjoying the jamboree; if you don't want to fuss with food, sandwiches and soft drinks will be sold. The park will be opened only to jamboree ticket holders, those using the public shooting center and campers that day. Tickets, $10 in advance or $12.50 at the gate, are available at all Virginia regional parks or 7-Eleven stores in Northern Virginia; free for kids under six. To get to Bull Run park, take I-66 west to the Centreville exit and follow signs to the park. Call 278-8880 weekdays, 631-0550 on the weekend.