So I just ordered a new middy from Spike's Tactical with a 9" SAR rail (yay). They gave me the option of front sight post or no front sight post. I went ahead and said keep the front sight post, but I have a few days of wiggle room to change it if I want. So my question is this: is there any tactical advantage/disadvantage to keeping the front sight post if I'm primarily going to be using an optics system (which for me consists of an Eotech 516 and 3x Eotech magnifier)?

The one advantage I can think of in favor of keeping the front sight post is if I need to deploy my back-up sight system quickly, I only have to flip up one sight (the rear one). One disadvantage I think of would simply be the front sight post obscuring a small portion of the Eotech sight picture. Any thoughts?

The front sight assembly is annoying...you learn to ignore it but that is on issue weapons were I have no choice.

I'd ditch it for the flip-up.

Troy Industries "Flipper" all the way.

the only advantage with a fixed/pinned fsb is strength

Which is reason enough for me.

In my opinion I like fixed sights (front and rear) for a fighting gun. If my optic fails, not having to flip up the sights means one less thing I could fuck up when I don't need to fuck something up.

Just my $0.02.

Something else I thought of: if I end up ditching the front sight post, should I get a longer rail or would the 9" rail be sufficient for a good sight radius? I'm really concerned about weight, which is why I ask because if I need to get a longer rail then that adds a bit more weight.

not to thread jack but what if you do the fsp and then decide you want to change. Can you switch it easily or inexpensively?

9" is probably plenty,especially since you want to run optics,  but that's a personal preference.  FWIW, going to the 12" is will only add about 4oz to the weight.

Easily?  Fairly.  You can chop down the FSB or take it off.  If you take it off and go to a different gas block it starts to cost.

I would go with both rear and front flip sights; but would mount the front sight on the barrel instead of the rail. When you apply pressure onto the hand guard (say in a roll over position or when using a car or wall for stability) your changing how your front sight (if it were attached to the rail on your hand guard) is in relation with your barrel...thus changing your sight picture and then of course actual shot placement. If the front sight is on your barrel, it doesn't matter how your hand guard is, your sight is ALWAYS aligned with your barrel.

Your right - it does take about a second more to flip up both sights if you went that way. That's not something I would worry myself with, but may be of concern to you.

Tactical advantage: Sturdy sight, don't have to flip up if optics go down.
Disadvantage: Could get caught on things like brush (if you use it outdoors like that), gets in the way of the optic reticle.

You could always get an open top front sight, which would eliminate it interfering with the optics.

fsp has never bothered me.