Took my 17 out today to fire off some rounds through it and I was getting nasty cheek slap from it.  The riser was down as I just have irons back on it for now.  It seemed no matter where I positioned by head on the stock I kept getting cheek slap.  This is the first time it has done this.  Previously I had no issues with it.  The riser seems kind of wobbly but I never really paid attention to that before this so I can't say if it was always like that or if it is new.

Anyone else have this issue and/or have any remedies for it?  I figure if it continues to be an issue I can throw an ACR stock on it and see if that helps.

Sounds like your SCAR is a bit overgassed for the ammo that you are shooting thru it. You are likely feeling the BCG slapping into the rear receiver plate, which is sort of a secondary recoil when shooting the 17S since the BCG is quite heavy. Were you shooting different ammo the previous time that maybe was a bit lower powered?

Many SCAR 17S' are overgassed from the factory. A simple test is to put the gas valve on the suppressed setting, without a suppressor attached, and see if the bolt still locks open on the last round in the mag. If it does you should decrease the gas control screw (GCS) size down a bit. Even if it doesn't you could still tune it a bit to control the BCG cycle speed.

Stock GCS size in the 16" 17S barrel is usually a 1.5mm. If you are shooting full power/hot ammo you could probably go down to a 1.4mm or 1.35mm and it will cycle much more smoothly.

Midwest gun works sells various size GCS:
https://www.midwestgunworks.com/page/mgwi/prod/FNSGCL

Nope, ammo is the same.  I've pretty much put all Hirtenberger 147gr ammunition through it.  If I put my cheek too far back on the riser it almost feels like it is getting pinched between the riser and the telescoping part.  If I move it up on the riser then it just feels like I'm getting slapped.

Like I said it's weird because it hasn't done this until now.  I've put 200 or so rounds through it and it's been great, now all of a sudden it just seems to be doing this.

I've put some Federal through it, too, but mostly everything has been the Hirtenberger ammo and I haven't noticed a difference between the two.

That said I do believe the Hirtenberger ammo is pretty hot from what I have read.

FWIW I've shot hirtenberger almost exclusively through mine and have never noticed this, I've also downjetted to 1.35 (never noticed it with the factory 1.5)

That feeling is your BCG hitting the rear section of the stock. It is not your stock. Lol.

Jet it down to 1.3-1.35mm and you’ll be good to go.

Everyone needs to watch PMMs yt video explaining why the SCARs are jetted from the factory the way they are…

link?

https://www.youtube.com/watch?v=9Zi52codWEY&t=1332s