Ok, so I'm planning my Grendel build.  What are some recommendations for a very rigid/strong, yet affordable rail for a precision build?  I like the cost factor of the Faxon, but I haven't seen many reviews.  Something that doesn't require timing or adhesives would be even better...

As far as mounting systems go I think BCM takes the cake. They are slim and light though, not sure if you want a wide flat bottom like mentioned above or if you want to keep it light.

Another +1 for the Seekins V3.

Have a 15in on my 18" Grendel, love it.

Sturdy and relatively light. I like the flat bottom for shooting out of a blind.

I don't think enough can be said how rigid it is once installed on the included barrel nut.

SY

SLR is great. No timing or adhesives. Have 2 on M5 builds, and 1 on a SPR build.

Really hard to beat the prices on the ALG line, but you do have to time it.

I have seekins, DD M4A1 RIS II, DD MK18 RIS II, BCM KMR (the regular KMR not the alpha), nordic defense, ALG, Faxon Carbon Fiber, Mega MKM (on a MATEN), PRI (mod H), and have had/will again have a KAC FF RAS (mod 1).  Wife has something from gungoddess.com; i'm honestly not sure of the brand--it's purple and has some sweet designs in it so she wanted it.

All are solid as hell, zero issues with any of them.  The MATEN has its own niggles (the bolt catch refuses to work and I've tried two different ones both labeled "for 308" from DPMS; it barely comes up high enough to contact the bolt face and rarely keeps the bolt held back for more than a couple seconds) but as far as the handguards go, really just about any quality unit will be fine.

If you're on a budget and don't need a flat bottom, the ALG and Nordic Defense are both extremely difficult to beat for the price.  The faxon is quite lightweight and very solid; however, (and this is merely a personal observation only, not at all a rip or degradation of faxon AT ALL) it *IS* quite pricey.  Yes it's carbon fiber and all that, but there are definitely less expensive CF handguards out there to be sure.  It *IS* quite simple to install and as mentioned is extremely solid.  Looks great too, so if you wanna splurge it's hard to go wrong with it.  

I'd probably say out of all of them, I'm probably fondest of the ALG for the value.  Good, simple, solid handguard, free-floated, easy to mount (people who harumph about the mounting/shim system online don't read the directions ), and can easily be had for 100-160 or so, depending on length and desired color.

Seekins are great if you're suppressed and want the can under the handguard, so there *IS* that consideration as well.

ALG EMR of your flavor.

Since AA has been there from the beginning, why not one of their Grendel uppers with proven barrel, handguard, bolt, etc.?

Or do you want the satisfaction of building it from parts?

Yeah.  I kind of want the building experience.  I've slapped together lots of lowers, but I finally have the tools to start assembling uppers.

Aero Precision's Quantum line in VERY affordable and solid. There is some timing involved but they do come with shims, it's not very hard to mess with.

If you mate it to their enhanced upper, there's no timing at all

Requires timing but slim, very stiff and adaptable for accessories
https://www.tacticalindustries.us/product-p/pxt-15-ffssr.htm

Geissele mk4