So, I know what the deference is but WHEN is one used instead of the other? Is it strictly because of indexing a muzzle device or
some other reason[s]?

Shims, i.e. thin washers, for timing a muzzle device.

Crush washer when timing isn’t important.

Shims for QD muzzle devices for suppressors.

I always use a jamb nut on muzzle devices.

The Gem-tech Halo attaches to the barrel using the flash hider.  The flash hider must be bore-concentric, and Gem-tech specifies that a peel washer must be used for this.

Split washers are only needed/used to make retro AR15s more period correct, they actually don't really even prevent loosening as a determined by a NASA study.

Crush washers are used for timing certain flash suppressors that require it... Side note: The best crush washers out there are the JP Enterprises double crush washers.

Shims are only needed for suppressor mounts... Side note: The best shims out there are the Precision Armament Accu-Washers.

That information has been in the Navy's aircraft design handbook for years . . .

The thing about the NASA study? Or did they do their own study on split lock washers? I'm also under the impression that toothed lock washers (like what is used for the pistol grip screw) don't actually work either.

As far as I know, there is not "study" on these, but NASA's Fastener Design Manual (NASA REF PUB 1228, page 9) states that helical spring washers (aka split ring washers) are useless for locking.  I believe that the NASA handbook largely a copy-and-paste version of one of the military design handbook series.

The toothed lock-washer does provide some locking as during torquing, the teeth deform the surface of the material and therefore, do not go completely flat. However, due to the damage done the the surfaces they can initiate cracks or instigate corrosion.  In the grip of an AR, the plastic will give under the teeth, leaving the teeth at and angle making them work reasonably well in the case.

By the way, when torquing a muzzle device on a barrel the primary holding force on the thing on is the thread friction, regardless of whether you use a split washer, a crush washer, or a shim stack.

Interesting, I remember reading that there was a NASA study. I don't remember where I read that though, it wasn't on this forum.

Not to hijack the thread, but where is a good place to buy peel washers these days?

Down Range Products

It’s a machine shop in Shawnee, KS

It's actually a standard stainless steel, 3/4 inch OD x 1/2 inch ID, laminated shim washer . . .

Let's see. NASA oversaw the contractor who blew up the Challenger shuttle in '86. Never had a split-washer not work as intended.

That's true, but split washers only work as well as flat washers even in my own experience.

I would respectfully disagree with shims being only for suppressor mounts.

On a precision match barrel (sub MOA, especially sub 1/2 MOA)) using a timed muzzle device, you don’t want to distort the barrel by over torquing to get the device timed properly.  Use shims to time it with minimal torque.  Over torquing a muzzle device may be meaningless on a rack grade barrel shooting bulk ammo.  Doing it on a barrel that shoots sub 1/2 MOA can open up groups substantially.

Yes, I can agree with that. That's why when I do use crush washers I don't use the standard "mil-spec" style crush washers, so that I can avoid the possible excessive torque on the end of the barrel.

I would install the Vortex on everything if I could.