Iron sight at the end of free float rail?

A lot of ar-15s now come with low profile gas blocks and long picatinny rails. Isn't a sight mounted at the end of that rail going to have the same problems that "bridging the gap" or mounting an optic on the hand guard would have? Yet I see it all the time now. Am I missing something? Meant only for close-in?

ericoak said:

A lot of ar-15s now come with low profile gas blocks and long picatinny rails. Isn't a sight mounted at the end of that rail going to have the same problems that "bridging the gap" or mounting an optic on the hand guard would have? Yet I see it all the time now. Am I missing something? Meant only for close-in?

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Well for sure the sight at the end of a Picatinny rail is going to deflect along with any pressure applied to the handguard. Using Iron sights, you must wonder how much of a difference this will make to most people.

I have always figured that bridging the gap could compound your issues with movement on the sight since some of it is not moving and is torquing at the gap. I would avoid if possible but I would not totally rule it out.

I guess for me it depends on what you are doing. Target gun with Iron sights? Better put the sight on the barrel. Normal shooting... not a big enough deal to really worry about. Most of these sights are BUIS, with the first two being Back Up. For back up sights, its not an issue. However I think your concern is valid and some are not thinking about it.

I thought the bridging the gap issue with optics is that you'll actually crack the housing if you get too much shift.

I am with IMBLITZVT; these are backup sights. IMHO, if they're minute of man to 100m, you're in good shape.

erwos said:

I thought the bridging the gap issue with optics is that you'll actually crack the housing if you get too much shift.

I am with IMBLITZVT; these are backup sights. IMHO, if they're minute of man to 100m, you're in good shape.

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^^^This. It's no so much a sight misalignment (all ff rails are not created equal) , but damage done to optics bridged between a rigid receiver and a whipping barrel and the prolonged effects shooting.

A hard strike or drop that impacts the rail will likely knock the backup 9r primary sight alignment out by several feet. I have done it and I show people pics in my class. Sometimes the rail gives and returns to zero, but sometimes it bends or cracks.

Free floated rails are also more prone to sight deflection if you are using a FVG, sling mounted to the rail, or bipod, and apply any force on it.

I have an 2 ARs setup this way, but they are inferior to a good front sight that's mounted to the barrel with taper pins (clamp on, and set screw are not reliable).

I usually buy good enough quality items that I am not super worried about cracking anything on the optics with bridging. However I could see that happening with lesser quality items. Same with the rail, I can see poor quality rails having much more of an issue. As always, you get what you pay for. You but a NC star rail or optics on, the recoil might break it! A drop can screw up anything. Even with pinned FSB you can twist it, bend the barrel. I had a bent barrel on a Garand and having a FSB attached did not help keep it on target. 7ft off with the sights full adjusted. However I feel like most of the time, these things are in the margin of error for most shooters. Most can not or do not shoot accurately enough for this to matter.

On my Shrike, they have the Front sight on the barrel. Well great in theory except that it a 3-4" dagger sticking perpendicular to the barrel. Screw that, I took the OEM sights off and replaced with flip sights. Hell the rear sight is on the top cover!!! However you want to think of a bad way to mount something. Think of a FAL where the Optics on the sheet metal top cover. Or those after market railed AK top covers. The amount that shifts and bends makes sights on an AR rail look good!

IMBLITZVT said:

I usually buy good enough quality items that I am not super worried about cracking anything on the optics with bridging.

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Part of the issue here is the laws of thermodynamics. Materials contract and expand at different rates, and your gun does not heat up evenly. You could conceivably induce the sort of failure I discussed by doing nothing more than shooting at a good clip for a sustained period of time, especially in full auto.

I am also skeptical that there is a magic handguard that is somehow impervious to getting bent on a really hard bounce. Aluminum is strong, but it's not that strong - especially when you're trying to use a very light-weight handguard system. If your optics are mounted tightly, you don't need much shift to induce a failure mode.

Bridging the gap is not a good idea for high-end optics. Don't do it.

Thanks for the answers guys. Figured they would work for a back up roll, but not a main sighting option and that seems to be the case.

IMBLITZVT said:

I usually buy good enough quality items that I am not super worried about cracking anything on the optics with bridging. However I could see that happening with lesser quality items. Same with the rail, I can see poor quality rails having much more of an issue. As always, you get what you pay for. You but a NC star rail or optics on, the recoil might break it! A drop can screw up anything. Even with pinned FSB you can twist it, bend the barrel. I had a bent barrel on a Garand and having a FSB attached did not help keep it on target. 7ft off with the sights full adjusted. However I feel like most of the time, these things are in the margin of error for most shooters. Most can not or do not shoot accurately enough for this to matter.

On my Shrike, they have the Front sight on the barrel. Well great in theory except that it a 3-4" dagger sticking perpendicular to the barrel. Screw that, I took the OEM sights off and replaced with flip sights. Hell the rear sight is on the top cover!!! However you want to think of a bad way to mount something. Think of a FAL where the Optics on the sheet metal top cover. Or those after market railed AK top covers. The amount that shifts and bends makes sights on an AR rail look good!

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I have worked on AR's with damaged rails from D.D., BCM, Geissele, M.I., and Troy. Unless you have a Co-Witnessed Optic, or carry a Laser Bore Sighter, you wont know after a hard hit/drop. I should have clarified, but my comments have little to do with who makes the rail. It has to do with the attachment method of the rail and how long and thin rails are getting (This is the popular thing now). They are weaker by design because everyone wants "light".

Sights attached to a barrel via taper pins is more durable than sights attached to a floated rail, and sure, you are correct sights attached to a dust cover are not very solid. I have guns with rail and dust cover mounted sights. I know their weaknesses. I was just relaying the weaknesses to inform others that may not know.

erwos said:

Part of the issue here is the laws of thermodynamics. Materials contract and expand at different rates, and your gun does not heat up evenly. You could conceivably induce the sort of failure I discussed by doing nothing more than shooting at a good clip for a sustained period of time, especially in full auto.

I am also skeptical that there is a magic handguard that is somehow impervious to getting bent on a really hard bounce. Aluminum is strong, but it's not that strong - especially when you're trying to use a very light-weight handguard system. If your optics are mounted tightly, you don't need much shift to induce a failure mode.

Bridging the gap is not a good idea for high-end optics. Don't do it.

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Yeah I took Thermo in College. I get it but don't see that happening. The free floating handguards only attach at the chamber/barrel nut area. So you have pretty even distribution of heat in those surfaces. Sure there is a bit of a difference, its going to be minimal and not something your quality mount can't handle. If this was the case, then just heating the receiver or handguard when not bridging could cause a failure as the rail expands.

As far as a bend, sure, any can bend but where? Does the guard itself bend or does the guard/nut connection bend? My thoughts were that the guard itself would probably bend long before the connection at the handguard but I could be wrong. If you are bridging the connection is close to the connection point and probably would not bend as much. However if they do bend at the connection point... then yeah I could see it. However if we are hitting the gun this hard... whats the likelyhood the optics are not taking almost the same impact no matter where mounted?

However we both agree, it would be better to avoid it.

clandestine said:

.... It has to do with the attachment method of the rail and how long and thin rails are getting (This is the popular thing now). They are weaker by design because everyone wants "light"....

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Yeah, I did not really consider those longer thin types. We are kind on opposite sides of the gun world there. I never see long rails because most are on SBRs and MGs. Long guards are like 10"s to me. Also MG people are less likely to use these thin rails, as that barrel gets hot! You want a little space. A large percent are mounting a Vertical grip to get away from the handguard all together. I was more thinking of the heavy duty quad rails. However I am sure you see some of those 20" thin guards. I can see it much more there! I see some of these and wonder how normal usage does not mess them up with only mounting point at the rear.

Seems few appreciate weight more than the MG world...

My two competition ARs are optic-only. If I were to add Iron Sights, they'd be on 45 degree offsets. My house AR has a pinned FSB and a carry handle upper. If either of those two move, I wouldn't be able to use the rifle until it gets replacement parts.

My answer would be Dissipator...

TheGunnyRet said:

My answer would be Dissipator...

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Or RPG...