The farthest point from the rear sight will give you the longest sight radius and easier to shoot accurately, the tiny bit the barrel moves with heat, will not be noticeable.

 

I concur. Long sight radius.

 

If you have a carbine length fore end then the gas block is best. If you have a 12-15 inch free float tube covering the gas block then the sight mounted on the fore end is better. This is the same thing RicMic said but using different words.

 

Just remember to not use a plastic (non metallic) flip up on the gas block as the heat will do a number to your front site.

 

Wouldn't the sight on the gas block always be more accurate? As the barrel flexes with heat and our pressure, the sight would stay aligned with the rear sight for point of aim

 

In theory, you are right that since the front sight mounted on the barrel is moving with the barrel, but it would be hard to prove in the real world. The sight radius thing really easy to prove.

 

My barrel is an AR-Performance 16" scout profile with a mid-gas port, so I'm now thinking that I could uses a flip-up sight gas block combo, (metal) with mid-length fore-stock, and still have a longer sight radius than the carbine configuration. Kind of the best of both worlds. I did some calculations and found that if the barrel moves .005 of an inch from heat, (and that's assuming the barrel isn't just simply expanding proportionally), at 100 yards the point of impact would change 1 1/8". Quite honestly, with my old eyes, I probably wouldn't make a practice of 100 yard shots with the iron sights anyway. Am I thinking right on this?

 

Unless you have a very long rail the flip-up front sight/gas block combo is a good option.

http://www.joeboboutfitters.com/Sta...ng+goods+outdoor+recreation+hunting&Click=4368&gclid=CLaW2OO33bICFURxQgodmgQAmg

http://www.weaponevolution.com/forum/showthread.php?2669-Folding-Front-Sight-Bases

 

Back in the days before scopes the most rifles had very long barrels. This was simply to get the sights as far apart as possible. The Kentucky long rifle is a good example.
This principle applies to your modern AR. The farther apart you can get those iron sights the better. Even though my 6.8's all have 16" barrels and they all have short and mid-length gas systems, yet I all have a full length rail on all of them precisely for this reason, to get that front sight out as far out as possible.

 

You are exactly right, look at the buffalo hunters rifles, many of them with tangent sights had over a three FOOT sighting radius and they were phenominal shots, even by todays standards.

 

As forward as possible, and that's typically on the gas block.

 

I think it's a given that different parts of the weapon will be heating/expanding and cooling/contracting at different rates and therefore nothing stays "aligned," so the longest sight radius is the best. However, I thought it would be interesting to run some back-of-the-envelope numbers to see how heating might affect the point of impact. Caution -- I am not a mechanical engineer (IANAME). So here goes, rounding all the way into the ballpark.

The coefficient of expansion of generic 4140 steel is about 6.5x10-6 in/in per degree. Assuming an outside barrel diameter of 0.7 inches and a 50 degree temperature rise, the barrel expands (6.5x10-6 x 0.7 x 50 =) 0.00023 inches. Since we only care about the expansion above the center line of the barrel, divide that by 2, which equals 0.00012 inches. Assuming a sight radius of 20 inches, that gives a tangent of (0.00012/20 =) 0.000006 (which is an angle of 0.00034 degrees). At 100 yards, that's equivalent to (100 yds x 36 inches/yd x 0.000006 =) 0.022 inches, or 0.021 MOA.

Later edit: Just to be clear, this calculation relates to how much a front sight mounted on a gas block or a sight tower would change its position when the barrel heats up, and what that translates into at 100 yds.

 

I'm glad someone did that and it wasn't me!

 

Thanks for the "JoeBob" link. Great prices. I just purchased my gas block/sight tower combo along with my forestock and several other items. Extremely fast shipping, great vendor !!