I'm building an iron-sight fighting carbine.  It's intended purpose is primarily for home defense and urban settings.  I don't intend one engaging targets out much past 100 yards, and even that is probably a stretch.  I'm leaning towards a BM 16" lightweight carbine upper because it's (obviously) lightweight.  I can also eventually have the barrel shortened the 14.5" and have the FS pinned to keep OAL and weight to a bare minimum.

Then there are dissy's.  These obviously offer a longer sight radius.  Since I'm sticking with irons (for the time being, anyway) is the longer sight radius really that much better for man-sized targets at 100 yards?  By going with a dissapator, I'd have to use a heavier M4 barrel (or pay extra $$$ for a barrel job) and I'd completely lose the ability to shorten the OAL.

The mid-lengths are out of the question because Surefire doesn't make an M500 in that size and I'm pretty well set on the Melininum M500A.

Thoughts... comments... suggestions?

Thanks

For home/urban defense the longer sight radius isn't needed.  Its nice for tighter groups (particularly as you start reaching 40 and over), but its not needed.

My other comment is on your choice for the M500.  WHY do you need that light system?  While they look impressive, I find SureFires other options to be far more useful.

is a longer sight radius really helpful?

DOES A BEAR cRAP IN THE WOODS?

A: both questions... HELL YES.

Get the dissipator if you plan to use irons.

If you have no intentions of shooting it past 100 yards, you probably wouldn't appreciate the difference in accuracy.  The irons on a carbine are good far past that distance.

As far as the OAL, I wouldn't consider cutting down a 16" barrel.  If things go our way next September, you'd be better off to thread it and add a brake or flash suppressor (adding about 1") rather than cut down the barrel 1.5" and be forced to choose from a limited number of longer brakes.

I say go with the BM lightweight[8D]

The longer sight radius isn't going to make a poops worth of difference on man-sized targets inside 200 yards.  Go with the lightweight.

The 500 series is an intergal unit.  It has no rings, rails, mounts, or screws and that means less things to come loose, break, or snag.  I don't like remote tape (exposed wire) switches or vertical foregrips either, so the 500 series seems to fit the bill nicely.  I'm not knocking other settups, I just think that 500 is the right one for me. Suggestions are always appreciated, though.

I know that longer sight radiuses allow for more precise shooting but I meant to ask that question from "practical combat" perspective.  Since I have limited experience with a carbine and no experience with a battle rifle, I was curoius if the longer radius offered a substantial advantage. I probably should have phrased it a little differently.

The upper is going onto a pre-ban carbine so I can have a flash-supressor.  The Vortex is somewhat longer than a birdcage so I considered having it permanantly installed to bring the OAL to 16".  I know that limits the effective range a bit, but the gun isn't intended for long-range engagements.  

Thanks again

ANYONE HERE WILL TELL YOU LONGER IS BETTER:)
Jack

If your going to go to an AImpoint in the future anyway my suggestion is to go with a flat top with LMT rear sight.  You get the irons now and can add the Aimpoint in the future.  Yes the Dissipator is better in sight radius but its much heavier.  You seem to be interested in light weight so I dont see the value in a Dissipator just to have a bit better accuracy in the short run.

Next if you want a light why do you want a light so big and hevy as the M500?  You dont like tape switches?  M500 uses tape switches.  You only need a 6V light anyway for the extreme close range you are describing. Have you considered using the off hand thumb on a front sight post mounted 6V Surefire?  It would cost less and weight less too.  This is the only way you can avoid tape switches.

Like so many things...it comes down to friggin' math (geometry actually)

By building a "fighting carbine" there is an inherent suggestion that you will be involved in "fighting". Fighting at close range will generally involve the need to perform what many call a "snap shot" where you "snap" the rifle to your shoulder, "snap" a quick flash sight-picture, and "snap" the trigger to break the shot when the sights look centered.

When you have time for a deliberately aimed shot, sight radius doesn't matter much at 100yds...but it matters a LOT when you make a snap shot at a near-to-mid but unknown range.

Think about it...when you take a flash sight-picture, you are looking for a front sight that is close-enough to being aligned to get a decent center (of mass) hit but still not taking the time for a precision-aligned sight picture.

The closer the front sight is to your eye, the farther it can be off-center and still LOOK like it is "on" (especially as range extends due to the angle between the true center of the target and the line of your flash sight-alignment).

The farther the front sight is from your eye, the closer it HAS to be to the center of the target before it looks like it is "on".

...and there is no real difference in speed.

So, yes...longer IS better...the math proves it ;-)

DevL,

What would you suggest for a light mount?

Is the 500 series really that much heavier than a 6V system with separate mount?

Thanks

Not a WHOLE lot unless you go with something polymer and theoretically less reliable like a G2 but you can ask around, I have not heard of many if any failures of the G2 handheld used as a weapon light.  A seperate post with a question of who has a G2 as a weponlight and has it failed and how many rounds fired would seem to be in order [;)]

Oh yeah almost forgot the mount most people use is the Wilson Combat mount for the G2 if you want to use your off hand thumb.  That light and mount would be MUCH lighter than an M500 and with a 120 lumen lamp would give you all the light you would need for your application.

If you go with the shorter sight radius, check into narrower sight blades.  I have a slimmer one on mine and it feels more a lot less like aiming with the tip of a crayon :)

And that rifle can do <2" at 100 yards with the irons from a bench, so they work fine for me.  
If you are seeking light weight, check into a flattop, they can save 1/4 to 1/2 pound, depending on the sight you use.  Also, the Dissipator is heavier than the shorty, and that weight is up front.  I don't know if BM makes a Dissipator LW.  Send pics when you're done!

The 500 is heavy and limits your future 'upgrade' capability.  If you want light (and useful) try the 650-A04 ($199) which is basically a ruggedized 6P (internal improvments to prevent the batteries from hitting the bulb during recoil).  The other options is the P101 which is Surefire Polymere weapon light (for handguns - but it works fine for carbines too).  Think of it as Surefires version of the M3 (only better).

Both lights I've mentioned are 6v and come with the standard 65 lumen P60 bulb (same as the handheld 6P/G2).  This is fine for inside use, and limited ranges outside (say 50 yards).  Either light can also use the 120 lumen P61 bulb for enhanced range (I don't recommend it indoors if you have light colored walls).

With either option you'll need a bit of weaver/M1913 rail to mount the light.  Here you have lots of options from adding a bit of weaver rail to the standard handguards, to Fobus handguards, to detachable mounts, to free floating systems like the RAS or SIR.

If you are young and have good eyesight, the carbine sight radius is just fine for the situations you describe.  Personally, I shoot much better with a 20" than a 16".  Longer sight radius means more forgiveness as well as better groups as mjn9999 noted.  

At this point I have give up and put optics on all my carbines [;)]

Hehe, same here.  I bought a 16" pencil barrel and an Aimpoint.