So I'm in the process of assembling the parts for my super Grendel build and I need your advice.  The handguard will be a Midwest Industries Gen3 FF Mlok lightweight but I'm not really sure what length to get.  I've seen them with 13", 15" and even 18" tubes.  I'm leaning towards the 15" because I don't really like the end of the handguard being right behind the muzzle device.  What would you guys choose?

This is going to be a SAAMI 6.5 Grendel, 20" Faxon barrel, medium-light profile for all-day carry, and are melonited inside and out for all-weather durability. Rifling is of the respected 5R type, and 1:8 twist will handle the heaviest 6.5mm bullets at range,  Sharp Bros. The Jack lower receiver, LaRue MBT trigger, FailZero bolt carrier, haven't decided on a particular bolt or muzzle device and haven't decided on a stock yet.  My Vortex HSLR 4-16x44 and it's LaRue mount will move from the Loki to this one. Should be an awesome rifle that is still light enough to carry hunting but capable of excellent accuracy at the range.  I may end up panhandling to afford finishing it but it will get done one way or another.  I've named it "Gren Reaper" heh heh...

Gas system length..? At least long enough to cover the gas block.

at the end of the day, it's 100% personal preference.

It has a rifle length gas system.

I know it's personal opinion, I want your opinions.

For all day carry I'd look at a carbon fiber barrel light upper and lower and go shorter on the handguard. I'd look at the JP enhanced bolt carrier and capture spring.  I'd shave ounces every where I could. And as far as the 18 1/2" vs 20" barrel I personally don't think your gaining 75 yards more range. Brownells had a skeletonized upper lower that was sharp as hell and I can't remember the brand name for the life of me.
I guess memory is the first to go.

Now if I wanted a range queen and a wheat field gun I'd go billet upper and lower with 24" barrel and as long a hand guard as I could. PRS stock heavy optic etc keeping some weight in the rear.
I would also check out the LuthAR stock fairly light weight and it just keeps growing on me.

18 inch free float M lok.

Light, durable, easy to add or remove accessories.

I thought about that but I'll have more than enough room for accessories with a 15" rail not to mention being lighter.  I looked at a bunch of pictures of 20" AR's with 15" and 18" rails last night and the 18" just seemed too long.  Probably be good with a longer barrel but I think I'm going with the 15".  

Thanks!

Couldn't afford a carbon wrapped barrel if I was forced to do it.  The barrel is already paid for in a group by from Faxon so just waiting for it to arrive.  Supposed to be between the 4th and 11th of next month.  There was no way I was going to be able to afford a high quality barrel at normal price, I just got lucky and came across the group buy that was ending soon.  I've always wanted one of those The Jack lowers, hopefully I can swing it and all the other parts I need or I'll be using the spare Spike's I have in the safe.

I have a 15 inch guard on my 20 inch 6.8 spc.

Will do! Thanks for the group buy on the other site, there's no way I could have afforded this project without it.

Gotta pic?

Mine is actually a 13 inch handguard.  I rechecked.

Ah, ok, thought it looked off when I first saw it.  I think the 15" will be perfect.

I would go with the 15 if I had it to do over.