They're most commonly installed with the concave portion towards the FH. Although Alexander's post above indicates they were designed to be installed in the reverse.
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I just face the front of the barrel's torque shoulder to obtain alignment.
It's not worth the trouble to make a spacer and surface grind it flat to a specific dimension to get it to index unless there is some other reason why the barrel can't be removed from the receiver and installed in the lathe.
I have made such spacers when necessary though, such as the case of using a KAC QD mount on a 14.5" barrel and trying to achieve a length over 16" with the KAC mount permanently attached.
Did you miss page one of this thread?
This picture was posted there:
http://ar15barrels.com/tech/crush-wa...stallation.jpg
I just removed my Vortex and reinstalled an A2 hider. I really don't need the improved flash hiding of the Vortex, and on my barrel it bottomed out on the shoulder instead of the end of the barrel like it's supposed to. I also like the fact that the A2 is shorter in OAL than the Vortex, which helps a 16" barrel not look quite so long with carbine handguards.
First I tried a crush washer and it was a PITA to get it aligned. It seemed to take way too much torque to get it to crush, but I did find that if I backed off the hider I could get it to crush more on the next torque. I used aluminum DPMS barrel clamps in a 4" vice with a thin piece of rubber separating the barrel from the clamps. Even with the vice tightened way down, the barrel would still slip while I tried to tighten the A2 hider.
So then I tried a peel washer. I heated it up with one of those big butane lighters for charcoal BBQ's and the base washer separated from the shim stack, with all the shims still together! I was able to then peel off layers from the shim stack and combine the remainder with the base washer, but after installation I decided to try again.
I heated up the second peel washer with the same lighter while holding one end with pliers to act like a heat sink. This was to make sure only one side de-laminated. Then I peeled enough to get the hider timed correctly. It's tight enough that the barrel just starts to slip in the clamps.
I didn't use a torque wrench, but I did use a 9" long 3/4" open end wrench and must have put about 30-40 pounds of force on it, which would generate about the right torque.
Here's my big question - how do you install an A2 hider without damaging the flats and rounding them off a bit at the edges? I touched it up with cold bluing and it looks OK.
Also, I found out the hard way that the flats on the A2 hider don't line up with the vents. After getting the flats lined up perfectly with the receiver I checked the vent slots and they were way off! So I had to re-align to get the slots lined up right. A DPMS A2 hider I got from Midway was worse than the generic one I bought from PKFirearms at a gunshow.
regarding the use of crush washers, one hard lesson learned by yours truly was not to torque the hell out of my FH. It seems that the washer crushed and tightened the internal diameter of the CW such that it would not fit back over the threads of the barrel. ...even if i tried twisting it off.
Those are LOTS of fun to remove.
I generally clamp them in the suface grinder and carefully grind through one side.
Then I grind on the other side until I can split the washer open slightly and slide it off.
This is one job that would drive the home-builder absolutely nuts. ;)