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Discussion Starter #1
Do I need to remove my front fixed site to use a red dot? I have purchased a free float hand guard and a gas block already? And can I use a flip up front site with my factory flip up rear? I would really like to free float so what are the do’s and dont’s? I am I have the round pins and they only go out one way, is there also a pin for gas block tube? Would appreciate full directions if possible? Thanks so much
 

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The answer to your first question is no, the front can stay. I can’t answer the others since I haven’t been down that road.
 

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Well, if you are going to install a new gas block and a free-float hand guard, the fixed front sight tower is going to come off anyway, no?

The AR 556 does not use a mil-spec barrel nut. The barrel nut is threaded for the Delta ring. This could be a problem if you purchased a free-floated hand guard that mounts on a mil-spec barrel nut. If you bought a hand guard that mounts on a proprietary barrel nut, it will be included with the guard, and you will have to swap barrel nuts anyway. Also be aware that the AR 556 front sight tower/gas block is proprietary and is fixed onto the barrel using taper pins that reside above the rifle barrel and not below the barrel as with mil-spec front sight towers. Hopefully, whatever gas block you have purchased will mount with pins above the barrel because the barrel has to have flats milled onto it to accept the pins. If you want to mount a front sight block with pins below the barrel, you will need to have some work done to the barrel. Most people I know of who want to convert an AR 556 to a low gas block with a free-floating hand guard simply cut the sight tower portion off of the front sight tower/gas block assembly. In answer to one of your other questions, there is a small roll pin that secures the gas tube into the gas block that will need to be removed and it will probably be better to acquire a new roll pin of the same size rather than reuse the old one.

Putting aside the had guard issue, a fixed front sight tower will not interfere with the use of a red dot sight mounted on an appropriate height riser on the Picatinny rail milled onto the upper receiver top. The AR front sight tower and flip-up rear sights (or removable carry handle iron sights) typically position the sight axis 1.41 inches above the top of the Picatinny rail on the receiver. That rail top is a little over 1.2" above the center of the barrel bore, so your sight axis winds up a little more than 2.6" (usually around 2.625") above bore. There are many flip-up front sights that are made to position the front sight post the same distance above a Picatinny rail either milled into the front top of a free-float hand guard, or mounted on a Picatinny rail segment mounted on the top of an M-LOK or Key-Mod hand guard.

Red dot sights intended for use on AR rifles and carbines will typically come with risers to provide either an "exact co-witness" or a "lower 1/3 co-witness" with back up sights. An exact co-witness riser will put the center of the optic about 1.41" above the top of the Picatinny rail. A lower 1/3 co-witness riser will position it a little higher, around 1.63-1.65" above the top of the rail. With an exact co-witness riser, with your back-up sights deployed and your eye placed to position the dot right in the center of the circular field of view of the optic, the dot will appear right at the top of your front sight post, assuming that both the RDS and the back-up sights are set to the same zero range. With a lower 1/3 co-witness, you can still align your back-up sights looking through the optic, but both the back-up sight picture and the red dot will now be situated in the lower 1/3 portion of the circular field of view.

Some people prefer an exact co-witness and some prefer a lower 1/3 co-witness. A few RDS optics come with risers or a spacer and riser that will allow either. With a fixed front sight tower, I prefer a lower 1/3 co-witness since the dot sits a little above and clear of the front sight post. I find that the fixed front sight post can actually provide a quicker acquisition of the red dot itself. With flip-up front and rear back-up sights however, you are not going to have the back-up sights deployed when using the RDS. In that case, I think the exact co-witness is a little better since it reduces the sight axis/bore axis offset slightly (around 1/4").

Be aware that if you ever plan to use a magnifier in conjunction with a RDS, some can be adjusted upward or downward to accommodate for a difference in the height of the RDS and some can't. Those of fixed height seem to be more commonly set for a RDS on a lower 1/3 co-witness in my limited experience with magnifiers.
 

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David Finley, I went through a similar drill when I first bought my AR556. I tried mounting a red dot on the factory rail …. two issues. The front sight was in the way so I bought a low profile gas block. That solved the front sight issue but I found I could not position my head low enough to get a decent cheek weld. That's when I put the front sight and gas block assembly back on and went with a set of 1" tall UGT scope rings. That combination worked well but the dot was just too large (3 MOA) at 100 yards so I replaced the 1" tube Millet red dot with a Nikon 3x scope. No front sight interference plus it solved the cheek weld problem.

I am very happy with the finished product:

 

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You usually won't see the front sight tower with a scope set to 3X magnification or greater, although you might see a ghostly image of it with some 3X scopes.

You probably will see some image of it at 2X magnification and will definitely see at at lesser magnification. This could become an issue if you ever plan to use one of the low variable power optics that have a low power of 1 - 1.5X.
 

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pblanc, With my Nikon 3X scope mounted on the rail with medium height rings, the lower portion of the scope was totally blocked by the front sight. The 1" Red Dot was even worse where most of the view was blocked and the dot was on the sight. When I used UGT 1" tall rings, not a problem …. I can see over the front sight just fine with either a 0X red dot or a 3X scope.
 

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Interesting.

Although I don't have it mounted right at the moment, I have used a Nikon P-223 3X32 carbine scope on a Ruger AR 556 mounted on Nikon "P rings". Those are fairly high rings that mount the center tube about 1.4" above the Picatinny rail, right around the same height as the front sight post but lower than the front post "wings". I did not see the front sight post at all when looking through the scope, so long as I closed my non-shooting eye.

I usually set scopes up for a "nose to the charging handle" head position, if that makes a difference.

On the other hand, I tried a relatively inexpensive Center Point 3-9x40 scope on the AR556, mounted on a slightly higher (1.5") cantilevered one-piece scope mount. With that scope I did see a ghostly image of the front post with the scope set at 3X, but it disappeared at any higher magnification setting, and really didn't interfere much with using the scope at 3X.
 
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