Mine did that twice to me.
My solution was to tap the barrel with a rubber mallet, It fixed it straight away and I've not had the problem return for 4 months now.
It is a common problem with all the 80- mid 90 BM's.
The pin doesn't have to be broken to cause the problem, 16 years of dust is enough to cause the cast locking parts to freeze up from time to time.
DO NOT USE CRC OR WD-40 AS THESE WILL CAUSE THE BARREL TO DIE A SLOW DEATH, ONLY USE GRAPHITE POWDER ON THE BARREL!!!
BMW parts professional
Previous rides:
BMW
E12 M535i (go the dogleg!)
E28 M535i (MMM M88!)
E34 535i M
MAZDA:
RX7 S-IV T04 (440 hp ATW)
RX7 S-I 13b bridge port
RX4 13b peripheral port