You can get the owner's manual at
http://media.browning.com/pdf/om/bar_98355_om_s.pdf
It describes how to get into the gas system pretty much as Scorch says from his second line, there being no trigger guard pins or screws.
As I recall - I once lived next door to the town repair gunsmith and watched him at work some - the BAR has no exposed pins or screws and the action comes apart by taking off the buttplate, then unscrewing the big stock bolt to get the buttstock off. Everything then comes out the back of the receiver.
Getting that slick side look required some gimmicks, I recall it as a rather complicated gun that he did not like to work on. I would not take it any further apart than the above manual shows without instructions. There are
disassembly books available from the NRA and Krause, maybe one of them shows the BAR.
In the meanwhile, clean the bore carefully from the muzzle, or with a Boresnake, clean up the gas system as in the manual, and it will serve for a long time.