On an 1100, unscrew the magazine cap, pull the forearm, pull the barrel (sometimes you need to let the bolt fly home a time or to to pop it loose).
Be sure to remove and retain the O ring. Remove the piston rings, then with the bolt in battery pull the charging handle straight out (it's held in with a detent ball and are tight when new, but will loosen up to the point of flying out during use when the gun has about 25,000 rounds thru it ;) )
Then, you push up on the loading gate and depress the ctg. stop to clear the bolt carrier on the way out.
If this is an 11-87 and not an 1100, then I have no clue.