I did a search and have gathered enough info to narrow down my choices but I still have some questions.

SMG Lee's photos of the MI #20 were helpful but it raised an additional question I was hoping someone could help:

1). It appears the Larue uses a metal plate that hugs the upper receiver to prevent HG rotation. How snug does this remain after subjected to harsh treatment and vibration? I.e. throwing the rifle onto the floor of a Bradley while driving around. Does the alum upper get worn out slightly (just a few thousandths appears to be enough to allow play in the HG)?

The Daniel Defense appears to use something similar but instead of a plate, it uses 2 pins? Is that correct?

2). The Troy MRF-C, Samson MRF-C, and Midwest #20 (MCTAR 20) appear to be of similar designs and has piqued my interest due to its 2 piece design. However, I am unfamiliar with how the locking piece is retained against the steel barrel nut. A few people mentioned that there are pins that lock up against the barrel nut half-moons. Are these pins made of steel, aluminum, or ?? It appears from the photographs that the Midwest locking piece is made of the same anodized aluminum with the teflon coating. Again, my concern would be wearing out the pins from rubbing against the steel barrel nut lugs in a harsh environment.

ETA: Midwest uses steel pins (through an aluminum upper cap?)

These may be invalid or stupid questions, but thanks in advance for your patience.

The LaRue has the two pins in addition to the locking plate.  As does PRI, KAC and most other quality FF systems.

What the locking strap on the LaRue does is lock the locking nut in place, so it cannot vibrate loose under nearly all conditions.  It really isn't designed to use the receiver to prevent rotation, although the design does make this possible.

I just looked over the BIY forum procedures and you're right. The locking plate bolts through the barrel nut and into the handguard. So essentially this system is bulletproof?

The "keeper" (anti-roll device) actually lock both on the back end of the barrel nut and the knurled ring preventing the ring to rotate loose.

The keeper also have tabs that contacts on the sides of the upper receiver preventing it from rotating.

In my opinion and I have the LaRue 9.0, it is currently the design/ brand that is most resistant to getting loose. YMMV

I also have two Samson MRFC.


Step by Step Instructions on Installing LaRue Handguards link

Here's the way I see it:

As long as the locking ring stays tight, the rail system isn't going anywhere, anytime.  The strap on the LaRue ensures that the locking ring CANNOT come loose.  The pins keep it indexed to the barrel nut, so the rail system itself isn't going anywhere, unless the barrrel nut comes loose.  At this point, the gas tube will limit movement, but you may affect rifle operation when you start putting lateral force on the tube, causing bolt carrier key to gas tube interference.

The LaRue is as bullet proof as a conventional railed FF system can get.  Now introduce the ARMS SIR, the VLTOR system and even monolithic rail systems and the discussion gets interesting.

Can you give the dime tour / cliff's notes discussion on the ARMS SIR and Vltor? I've seen those and the ARMS weighs a ton. I wonder what makes it worth the extra weight.

The KAC RAS-II never seems to get any respect.  The way the hump grabs hold of the top of the upper receiver would seem to alleviate alot of the stress rails otherwise put on the barrel nut.  It requires no dismantling or specialized tools to install or remove.  Works great with an Aimpoint.

The only con I see is that it can be difficult to use an Eotech with it.