Have a g&p mk18 mod0 and I noticed that the RIS with the tactical foregrip installed has a bit of a rotary play. It's not rock-solid as my other m4 with a RAS.
Here is what I found...
RIS is secured by a tab of the upper handguard against the front handguard cap. In my case I don't see one. There is just a flat head screw that after tightening, it has no effect.
After some online research I got to remove the upper hand guard and there's the tab...
The cross-section of the tab itself, it's concave and same shape of the the front handguard cap as it would be resting on it.
After re-installing correctly:
The little rotary play is still there. That play comes from the handguard cap which is secured by the outer barrel and the gas tube.
Is this normal?
The difference with the RAS, it has a tab on the other end (upper receiver), that when you tighten the screw (usually allen), it grabs/clings with barrel nut; and that tab has a cut-out which passes the gas tube, thus preventing it from rotary play once tightened.