Thanks! Appreciate the info!

@Greyswandir

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You can use up to an 8.8" rail on a 14.5" carbine length barrel, 10.8" rail on a 16.5" mid length, and 14.1" on a 20" rifle length

That is very helpful thankyou!

I'm one of the few guys in the room who tends to prefer a shorter rail.  The big reason is access.  Trying to work on or adjust settings on a gas block that's buried under a handguard is a PITA.  Also, I never cared for the manlicher style stocks.  And I prefer a lighter and thinner profile.  There's also some weight savings.  

So for me, I tend to go 9" with a middy gas.  I will say 7" is a bit short, and I'd probably stick with 9" over carbine gas anyway. But in my experience, I just don't run my support hand much further than the 9" stock anyway.  I also run accessories very lean.  And when I do run something like a flashlight, I use one of those add-on M-LOK to rail bolt-ons,  that extend out and forward a bit, to get the light farther out and  away from glare inducing stock.  Which also extends an inch or so out past the end of the rail.  Try that with a longer rail, and the light starts getting uncomfortable close to the muzzle blast, is my take at least.

So for me, while everyone wants the tactical longer rails, I tend to run the 9" rail and a middy gas, for a nice low profile light and easy build, that I can access the gas system if I want to.

Here's one example.  

Can't be much longer than 10.5".

This is a 10.5" Mk16 with a bayonet mounted in front of it, 16" mid-length gas barrel.  As you can see, an additional 2.5" would be way too long to permit the use of a bayonet without some kind of rail-mounted adapter like a couple of companies have made.

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My CWAR. Though some purerest discount it because the rail is ALG vs G. The rail is privilege so there is that.

A 15 on a 16 keeps you from bracing on things with the barrel and throwing off you POI.

I will say this... if you are deciding between a BCM MCMR 13 and 15, consider the MI 14" Combat rail.

Compairing the MCMR 13 to MI Combat 14, they are the exact same weight, once you add the included QD socket on the MCMR, but on Combat 14 you get a flat rail front, more rail length, better anti rotation tabs, and a better, blacker finish on the Combat rail for the exact same price (or about $3 less on average) and exact same weight (due to the integrated QD on the Combat rail).

Since the top of the MCMR is not as long as the sides, you get virtually all the sight radius and bipod mouting length on the MI Combat 14 rail as the MCMR 15. I own 3 of the BCM rails but after my first MI Combat 14 rail, it will be my choice for any rifle I was going to put an MCMR 13 or 15 on.

If you were considering V7 titanium hardware to save weight on your MCMR, MI even has a titanium barrel nut and screw kit that cuts 0.6oz and makes the installed weight of the Combat Rail 14 just 9.9oz with QD points.

Thanks for the info I will check it out.