I'm rebarreling my FAL with a new DSA 21" and was wondering if the standard Belgian flash hider just screws on or is it supposed to be timed. The photos I see they seem to be timed where the wrench flats are vertical and the 4 rows of ports are at 11:00,1:00 and 5:00, 7:00 roughly. My current 15.5" barrel has a pinned welded Smith 4 prong, so I don't have a reference.

The ports should be in an x-pattern. If it has the bayonet notch, it should be timed so the notch sits at the six-o'clock position. I read somewhere there is actually some sort of washer to do this, but most people use a paperclip as a crush washer. I used a piece of household copper wire. Wrap it around a 5.56mm case bottom and it should be the correct diameter.

It's supposed to be timed.  The factory method is to ream out material from inside until it times correctly (except IMBEL, which seems to have just continued to apply torque until it timed correctly, sometimes with detrimental effects on the barrel).

The timing washers apparently are not compatible with the combo devices, only with the long flash suppressors.

I've tried copper wire, paperclips, and the wire use to make coat hangers, and none of it has worked for me.  The former two, if I could get it to time correctly, ended up getting destroyed pretty quickly, causing it to lose its timing.  I'd probably have to try and find washers with the correct inner and outer dimensions that are thin like shims, or have someone make them, I think; that, or have a machinist work on the combo device until it times correctly.

The barrel and combo device are DSA, so hopefully since they're both from the same manufacturer I'll have good luck.
thanks for the replies.