I hesitate to ask, because I know I'll get several different answers.  So I'll explain what I'm thinking, and you guys can set me straight.  I have two AR's for hunting.  Both are flattops, one carbine, one 24" barrel, both are bull barrels, both shoot 1/2 MOA.  I want one for defensive/tactical shooting, i.e. farting around.  I was initially thinking standard A2 upper with M4 barrel and free float hand guard, scope on top.  My thinking was to keep it as simple as possible.  I have never shot an A2 and wonder if that additional height is awkward, especially when looking through optics?  Second question, if I go with a flattop, how difficult is it to use optics with a standard front sight?  Is it better to go with a flip up front sight system?  Also, I notice that most of the pics here have AR's with standard handguards.  My understanding is that free float is much more accurate say in the 100-300 yard range.  Also, if I've left out anything else, feel free to mention it.  Yeah, I know the answer is to buy/build one of each, but for the sake of conversation, let's focus on my next one mentioned above.  Thanks for the advice.

Blake

For home defense, I would go with a 16" barrel and open sights.
Possibly with a Tritium insert on front post.
For GP use, a good 20" barrel is the best, It will get you out to 300m.
If you want optics, go with a flat top upper, use flip up rear and standard front sight, the front sight will dissapear with any scope over 4 power.
If the scope fails, take it off and use your flip up.(plus you can take it all off and attach a regular carry handle)
If you use a low or no power optic, then you can co wittness your optics and sights as needed.
Free floated are OK, but not really needed on a GP rifle, they are better suited to varmint/target shooting.

For the person who may be considering cost......ie. no preban lower.  I would say 16" upper w/ flat top.  Rear BUIS and an Aimpoint M2 or Eotech 551(or 552)  That is all one really needs for very fast weapon.

Preban?  Then same as above with Vltor or Magpul stock, Any RAS variant (RAS II or FF RAS preferable) or SIR variant.  Mounted light and forgrip.  Aimpoint/EOTECH and BUIS Single point sling

Most of the above goodies can be had for the postban version as well but cost efficiency was the issue.

Heck I could go on from there I guess!

For a defensive/tactical setup, Shoooting out to 300 yards is not really that necessary.  Most combat engagements will take place under 100 yards.  Free floating does help a little, but the best free float systems that provide good cooling for fighting/rapid fire are the high end rail systems which are costly.  Like I said, it's helpful but not entirely necessary.  If you want to spend the money, though, definitely get a high end rail system for the versatility.

that being said, a carbine is the best setup IMHO.  It is easier to use and handier in urban and other crowded environments.  In the great majority of combat engagements, 150 yards is about as far as your fighting is going to reach.  Less than 100 yards is more realistic.  

A midlength upper is probably the most veratile.  You have a longer sight radius than traditional carbines, which helps if you are using irons.  The gs system is also slightly more reliable, which always helps.

I will second the flat top upper.  It is more versatile.  Flip up front sights are a matter of preference, I prefer as little to flip up and manipulate as possible.  With a red dot type sight, I prefer to cowitness with the irons, so I keep the front sight standard.  With an ACOG or any other 4x or greater optic, the front sight is not noticeable.  

As far as magnigified optics go, the ACOGs are currently the only viable solution for a tactical gun.  Traditional scopes don't really belong on a high speed type fighting rifle.  They just aren't versatile enough.  If you have to make distance shots, a SPR type or other dedicated setups are the way to go.  At 100 yards (150 yards at the most) ACOGs, Aimpoints, and EOTechs are really the best choices.  Which one depends on taste (and funds).  

A chrome lined barrel over any other type is also preferable for a fighting rifle.  It increases life, durabilty, and reliability.

It seems you already have two long range/accurized ARs that are made for distance, sub MOA accuracy, and sporting purposes.  A fighting rifle should be looked at with more versatility, durability, reliability, and modularity.  The rest depends on personal taste and available funds.

Exactly my sentiments for a SHTF rifle.
But,what about a rifle intended solely for home defense? For me personally, in my urban environment, engaging a target at 50 yards may not be considered self defense.
I think a carbine in the home defense role is a better idea than a pistol because the rifle doesn't require the same amount of skill as the pistol to achieve the desired result, but I question the necessity of considering long range performance in this role.
Now, if it is to be the only rifle, then compromise has to be made, but if it is one of many, I would think that a light weight carbine would be ideal. Flat top if you intend to use optics, carry handle otherwise.

Well it all depends on how much money you want to spend. From your post it sounds like you want a kick around gun that can be used to defend the home. In this roll you will want a light weight carbine that handles fast and easy. So here is my recommendation.

If you are on a budget go POST BAN.
[img]http://www.bushmaster.com/shopping/weapons/Images/pcwa2x16sl.jpg[/img]
Barrel - 16 inch light weight barrel made by either Bushmaster or Colt.
[img]http://www.bushmaster.com/shopping/barrel-assemblies/Images/abbl-16sla.jpg[/img]
Upper - This is up to your preference. For what you want it do to I would go with the Flat top.
[img]http://www.bushmaster.com/shopping/uppers/Images/vur.jpg[/img]
Or the A1 (C7) style upper.
[img]http://www.bushmaster.com/shopping/uppers/Images/a1ur.jpg[/img]
Stock – Being a post ban I would go with either the ACE stock.
[img]http://www.bushmaster.com/shopping/buttstocks/Images/Ace-on-Rifle2.jpg[/img]

Or the Vltor A1 length rifle stock.
[img]http://www.vltor.com/images/rifle-standard-modstock.jpg[/img]

Both are light and comfortable. The Vltor is more comfortable to me and gives you the opportunity for modular panels and expansion with time.

Front hand guards - Personally I would not go with a rail system on this rifle. I would just use the regular carbine hand guards unless you want a light. If you just have to have a front rail system the FF RAS, RAS II and SIR #50 are my personal favorites. Selection is dependant on preference and if the rifle is a flat top or an A2/A1.

Weapon light - If you want a light I would go with the Surefire M500A. This light is bright and does not add a lot of weight to the rifle.

If you have lots of money and a collapsible stock is important to you go PRE BAN.
[img]http://www.colt.com/law/images/ar15a2.jpg[/img]
The only thing I would change is the Stock. I would recommend the Vltor carbine.
[img]http://www.vltor.com/images/car4.jpg[/img]

Or the MSS M93 stock. Of the two I like the Vltor but that is just me.

Optics recommendation is the same for either rifle. In the following order.
ACOG TA-31. This scope works well mounted on the carry handle.
[img]http://www.trijicon-inc.com/parts/A2C1.jpg[/img]
Compact ACOG TA-47-2 (2 X 20MM) This scope works well mounted on the carry handle also.
[img]http://www.trijicon-inc.com/parts/8A69.jpg[/img]
Aimpoint M2 or ML2. You can mount this on the flat top or in a goose neck off of the carry handle.
[img]http://www.aimpoint.com/galleri/vapen-140.jpg[/img]
The goose neck puts it out further forward and makes for a good sight picture and allows for co-witness.
Trijicon Reflex II. You can mount this on the flat top.
[img]http://www.swfa.com/riflescopes/trijicon/trijreflex.gif[/img]
Or in a goose neck off of the carry handle. The goose neck puts it out further forward and makes for a good sight picture and allows for co-witness. Our SWAT team uses the Reflex on the goose necks and love them.
[img]http://www.trijicon-inc.com/parts/2AD4.jpg[/img]
EOtech. I do not like these but many people that I have respect for do. If you like them go for it.
[img]http://www.swfa.com/riflescopes/eotech/durability.gif[/img]
[img]http://www.swfa.com/riflescopes/eotech/tacmount3.gif[/img]
[img]http://www.swfa.com/riflescopes/eotech/tacmount1.gif[/img]

Over all I think you would be most happy with the ACOG.