BLACK FLAG started out as a band of musical outlaws. Playing to a renegade crowd of surfers and punks in the late '70s, the group quickly developed a noisy, aggressive approach that set the hard-and-fast rules for California hardcore.

But now that the possibilities of punk have been thrashed over by dozens and dozens of the band's successors, it takes more than mere musical muscle to maintain the angry distance for which Black Flag still strives. What a band needs is a sense of purpose, which Black Flag often has. Trouble is, there's often more purpose than there is sense.

"The Process of Weeding Out," for example, is a bitter response to the "rock porn" furor. Noting that the band's records have been refused by some retailers "because of objectionable . . . lyrical content," the band has assembled an entirely instrumental album. However, guitarist Greg Ginn argues in his liner notes that anyone who listens "intuitively" will understand the album as clearly as if it had words. Maybe so; but though Ginn's unfashionable fixation on the sounds of '60s metal comes through loud and clear in these rambling jams, it's not hard to miss the contrast and focus Henry Rollins' vocals usually provide.

It's all a matter of context, which is why the band's live album, "Who's Got the 10 1/2?," sounds both leaner and meaner. The rock porn censors are again the butt of the band's jokes, but the guts of the album lie with Ginn's squalling, fragmented guitar lines. Cleverly updating the ragged riffing of Johnny Thunder, Ginn's playing simultaneously boosts the rhythm section's drive while providing gritty counterpoint to Rollins' vocals. As a result, even the juvenile jokes in "Gimme, Gimme, Gimme" seem somehow more menacing than they might.

BLACK FLAG -- "The Process of Weeding Out" (SST037), "Who's Got the 10 1/2?" (SST060); appearing Saturday at St. Augustine Hall, 15th and V streets NW.