So, I recently just removed my Scar 17S PWS Brake.  I ordered a new FNH Euro 3 Prong Flash Hider.  What is the proper way to install this flash hider?  Do I need a 7/8" crow foot to attach to my torque wrench?  What is the correct torque weight to install this hider?  Do you need to time these flash hiders or just apply Rocksett to barrel threads and torque it down.  I look foward to everyone's input. Thanks.

I just snug my AAC flash hiders down with rockset. I've never used a torque wrench to install flash hiders. As for timing it? For the AAC flash hiders never did. My OCD kicks in looking at them on my guns crooked. I need to invest in a torque wrench to start doing my gun work the right way.

Anyone else?

The proper way to do it is to remove the barrel, lock it in the TangoDown vice blocks, and torque down the flash hider.

Unless you're mounting a suppressor, I doubt rocksett is necessary. I suppose it can't hurt if you have it lying around, as it's not difficult to remove a muzzle device with it on.

I haven't seen official torque specs, though I'd keep it in the 25 ft-lb range. I recall Steve or someone from ADCO stating somewhere on this site that muzzle devices in general ought not be over-torqued (though some have reported here tha factory FN flash hiders are often torqued pretty tight). If you want factory torque specs, you could always call FN and see what they say.

According to Surefire, too much torque can take a little accuracy out of your shots.
I would do what the manufacture recommends, and make sure you don't go over.

Why does everyone get rid of the PWS?  It reduces the felt recoil of the gun about 50% and it's a decent flash suppressor.  I don't see a point in it unless you need to attach a suppressor.  Sorry just my rant.  I've shot mine with a Surefire Warcomp 7.62, Surefire brake and I went back to the PWS as it's the best of the three (I'm not getting a suppressor anytime soon).

Anyways on to your questions:  Always clean and degrease the threads first. I also like to test fit everything and then torque everything down. Check alignment of everything, mark the position with a lead pencil or paint marker, then un-torque, apply locking agent, then re-torque, check alignment with the bore.  (I even use a laser level to make sure everything is timed in the proper position). I know it's a sickness.

1. The Scar barrel has a very narrow shoulder, that is why FN uses a locknut with the factory brake (PWS).  So the muzzle device is concentric with the bore to avoid a bullet strike.  You can install a muzzle device two ways:
   
   a. Use the factory FN locknut; screw the locknut all the way down, then screw the FH on all the way down and back the flash hider off until it times right.  Then bring the locknut against the FH, torque with opposing wrenches (I hold the factory locknut in place with a wrench and use a torque wrench on the muzzle device. Torque with the lock ring 20-30 ft/lbs (torque over 30 ft/lbs can squeeze your barrel and affect accuracy/safety).  I torque to the minimum of 20 ft/lbs
   
   b. The second option is to purchase a shoulder ring, here:   http://www.parkermountainmachine.com/store/p113/KBA_Custom_SCAR_17_Shoulder_Ring.html  (Install instructions are on their website)
   
Place the shoulder ring on the barrel chamfer facing away from the barrel, hand tighten the device and check for proper alignment. If you need shims, they go on between the shoulder ring and muzzle device. When installing shims point the muzzle toward the sky/ceiling to get the shims to sit concentric, hand tighten, then back to the vice to torque.  Most muzzle devices in my experience need to have their top/12 o'clock line up at about 11 o'clock position on the gun, if you're looking from the muzzle end, to achieve proper torque.  Recommended barrel torque on a normal barrel is 20-30 ft/lbs.  On the Scar with the shoulder being so small and shoulder ring installed, MAX TORQUE is 25 ft/lbs.  I always try to torque at the minimum of 20 ft/lbs, as you can squeeze your barrel and affect accuracy with too high a torque (My PWS was torqued on too tight from the factory, the gun was more accurate after the brake was properly torqued).

NOTE:  On some muzzle devices you will need to use the locknut instead of the shoulder ring. The PWS brake is one of them.  I was going to use the shoulder ring with the PWS as it makes for a cleaner look, but the threads on the PWS bottoms out with a big gap between the shoulder ring  and the PWS.  I would have to use another shoulder spacer on top of the shoulder ring, which would look ridiculous.  
             
The reverse is true for the Surefire muzzle devices, where the shoulder ring might be the better choice if attaching a suppressor.  The locknut is large and might interfere with the suppressor attachment.  The shoulder ring also allows maximum thread engagement by the FH or brake.  Good to have if you're hanging a suppressor off the end of the barrel to keep everything concentric. 

You would time your flash hider according to the instructions included.  Most of the three prong flash hiders are timed in a specific orientation for aesthetics, and dust signature. If the orientation isn't specified I would time the FH with one prong at 6 o'clock. Leaving a single prong at the 6 o'clock can help when shooting prone in a dusty area.  Gases diverted at the 12 o'clock between the two other prongs can help slightly with muzzle rise. 

LASTLY/SAFETY:  I check the muzzle device alignment visually, by looking down the bore at the chamber end and check alignment with a Geissele Suppressor Alignment Rod.  Here:   https://geissele.com/17-7-62mm-suppressor-alignment-gage.html

DISCLAIMER: I am a armorer for several other weapons, but not a FN Scar 17 armorer.  These are just my experiences with working with other weapons and with my personal Scar 17.  If in doubt about your skill, you should have a gunsmith or armorer install your muzzle device.

Hope this helps, sorry I know, I went deep...LOL

I personally think the PWS FH works decently well, but is ugly as hell.  The FNH European FH does not require a lock nut or shoulder ring.  Also, the locking agent should be Rocksett.  I wouldn't recommend anything else.

So does it go back over the barrel like the AAC?  I'm not familiar with that flash suppressor.  If it's flush with the shoulder than you would still need this if you want to time the prongs in a specific orientation with shims.  The thin barrel shoulder will not support shims, unless you don't care about the orientation of the prongs (Or the FH uses an internal crush washer to time). Like I said, I'm not familiar with that flash suppressor, I believe it has been discontinued by FN. 

Yes, it overlaps the barrel shoulder.  There is no prescribed orientation of the prongs from FNH.  They literally just torque it on.  However, for safety sake, you should email Marvin Pitts at Nefarious Arms or Jarod at PMM.  They can tell you definitively, but I am pretty sure I am right.