I have an M4 with a 7" free float rail.  I would like to put a longer rail on.  I am going to be removing my stock front sight base because I am getting an aimpoint.   What is involved in putting on a longer rail?  Any help is appreciated...

Just remove your FSB, remove your current FF rail, install a low profile gas block, and install the longer free float tube.  There are several threads that have pics of varying FF tube lengths on barrels.

Why are you removing your front sight gas block for an Aimpoint?

Srsly.

I'd recommend keeping a front sight of some kind involved.  Aimpoints are pretty tough, but I'd prefer to have a backup method of aiming my rifle in case I run out of batteries.

But yes, remove the front sight (or grind it down), install a low-profile gas block in its place, and you can run whatever length of handguard your heart desires.

I'm removing the FSB so I can put in the longer FF rail.  I will put on flip BUIS after I install the longer rail.  Kinda new to the site.  Can someone link the forum talking about changing out the rail and putting on the low pro FSB?

would the low pro gas block go in front of the rail or run underneath it?  i.e. do I have to put in a new gas tube or can I keep the same short one?

Your gas system length will remain the same.  Gas system length is fixed by the location of the port in the barrel.  The new gas block will be under the rail, assuming you install a longer handguard.

Awesome!  Thanks.  I have another stock M4 that I am upgrading and will put the short rail on that one...

You stated in your OP that you were removing your FSB because you are getting an Aimpoint. Which is it?

which is it???  It's both...I'm going to remove the FSB so i can put in a longer FF rail...then I am going to put in a flip up front site...Any better suggestions?  Am I doing steps that aren't necessary?

There are some who will argue that a flip-up front sight is not a necessity when using a sight.  Some people like to co-witness their sights so that if their red dot goes down, the irons are right there and ready to rock.  They also tend not to find that the image of the front sight in the image of the red dot particularly bothersome.

I'm not one of them, myself - I've got flip-ups, but it might be worth trying your Aimpoint with the factory sight block in place to see whether or not it bothers you.  Might save you some cash.

Each rail manufacturer is a little different with the install process.  What rail are you using now and what rail do you plan to install if you go that route?

right now I have an MI...Not sure which one I would go with.  Suggestions everyone?

For any rail you will need to switch the FSB to a gas block which requires removing the flash hider.  If it is not pinned it is not difficult.  The small YHM low profile gas block is inexpensive and will fit under most rails.  If your barrel is not parked under the FSB then you may want a gas block that covers the "scars" like a LaRue LT202 or the Daniel Defense clamp on.  Again, they fit under most rails.

Next is select the rail.  The easiest is stick with the same model MI in a longer version if you are happy.  You wouldn't have to mess with the barrel nut, just remove the rail portion and install the longer unit.  You could sell the new hardware and old rail as a unit.

If you have the MI version that uses the standard barrel nut you could switch to a Troy MRF series without removing the barrel nut; just swap rails.

Most everything else will require removing the barrel nut.  If you can do that the choices are unlimited.  Some common ones are DD lites, DD Omega's or LaRues - high quality, high price.  YHM Diamonds - decent quality, great price.  Or some type of non quad like a Troy Extreme or YHM customizable.

If you have to switch the barrel nut you will need the tools (vice clamp, clamp, nut wrenches) so it's more involved.  The DD  Omega is usually an easy install, but since you don't have a Delta ring you would still have to remove the barrel nut to install one.

Most of the manufacturers will have installation instructions on line or if you pick a brand you can ask and someone will give you more specifics.  Good luck.

There are a huge number of options, dependent upon your tastes and your budget.  I've got a YHM rail on mine, and I'm extremely pleased with it.  It's solid as a rock, and the lockup mechanism prevents unwanted rotation.  It's a nice rail, and they're cheap!