View Full Version : Why not to mount an optic over the gap between upper and rail
Cincinnatus
06-03-11, 10:47
First off, here's a little background:
https://www.m4carbine.net/showthread.php?t=70638
Now I do not doubt that mounting an optic over the gap between rail and upper is a bad idea, my question is the exact why of it. In other words, I am seeking to understand the exact technical reasons why this is a bad idea.
One poster in the other thread said it was because of the "different expansion properties" of the rail and the upper. I assume that means heat expansion. Does receiver flex and handguard/rail flex also play into it?
To give an example scenario, if someone were to mount a T-1 in a DD mount across the small gap between the upper and say a SWS rail, would accuracy be affected when the weapon heats up? Would the zero fluctuate?
Just seeking to better understand what's involved here.
With a drop in rail you can have more play in it. With a rail that is a free float rail you might have some extra flex but I don't know if you would be able to notice it. I'd say try it both ways. See if it makes a difference to you or not.
patrick sweeney
06-03-11, 13:36
My SWAG:
We'll start out assuming you have a FF rail that is correctly aligned and properly made so as to be in-line with the upper. If not, you wont get it zeroed anyway, and your problems will not be with heat.
How much difference?
The rail will heat a bit faster, but heat will soak back to the receiver too. The differential expansion of the aluminum would be pretty negligible. if you've heated the barrel enough to start making the rail toasty, the heat effects on the barrel itself are probably a bigger hindrance to accuracy than the expansion differential.
Vibration? The FF rail will vibrate differently than the receiver. however, if you have the mount tight, and the screws/knobs painted in, you won't have a loosening or shifting there.
Short answer; in and of itself, I don't see a problem mounting across the gap. However, the dimensional differences between the myriad of rails and receivers makes it a toss of the dice just getting them to line up properly. So avoid it if you can.
Which is why two of my rifles wear Vltor CASV-M's. I will admit that my work Colt, with Knight railed hand guards has a Eotech (can't change it) sitting atop a LaRue QD Eotech Riser bridging the Gas Ring Gap. I have had no issues with this set up at all, even during high round count automatic fire, with the associated heat.
Watch a High speed camera shot of an AR being fired. You can see the movement between the upper and the rail. This is not a place I would want to be mounting an optic. There are two points where this poses a potential problem.
1: Damage to the optic/Mount.
2: Damage to the rail/barrel nut.
I don't want either of these things happening. Now if you have a monolithic style upper you don't have this problem. But with a standard rail/upper there is no way it is a good idea.
There is way too much thinking going on here. Having said that I think that kwelz probably said it better than I could. However, it has been my experience that some people will tempt fate anyways.
There's a few things to consider. Primary thing IMO is that while the upper receiver generally doesn't move/flex, when you span across onto a hand guard/rail, that possibility does open up, as the handguard/rail can. Chances are that you'd have to be doing something to provoke it (resting on a fence post etc) - but nonetheless the possibility exists, wheras with an optic solely on the receiver, it doesn't. Particularly with a free float rail - in an extreme case, given the rail moves independently of the barrel, an optic mounted on it/spanning the gap would also move with the rail - this can make for a wandering zero depending on what you are resting the rifle on, or pulling on a sling, or holding offhand, etc.
Another point of consideration is that loads of rails do not quite exactly match up with the receiver - they are minutely higher or lower (and in some cases quite a bit more than minutely). Consequently, mounting across the gap can add/subtract elevation to the scope and in more extreme cases at long range make it impossible to to zero due to lack of elevation adjustment. In addition, if you were using a one piece mount, say, with such a rail it could be awkward to get it lock up properly across the gap, opening up the possibility for damage etc.
All that said, chances are, in reality, these factors will not be hugely noticable - at least at shorter ranges. But as range increases, or if you are forced to exert excess pressure on your rail to get into a good position etc, they can make a difference, hence why it is generally thought of as a bad idea. Particularly if you do this with a freefloat rail - you are basically giving up the advantage of that freefloat and might as well have a regular handguard/non FF rail at that point.
Cincinnatus
06-06-11, 10:21
Thanks all for the input. I knew it was/is a bad idea to do this--I just wanted a better understanding of exactly WHY this was a bad idea.
Now I have a better grasp of the dynamics involved; I always learn something everytime I come to M4C. ;)
Forgive me for resurrecting an old thread. I did not want to start another due to this being a hot topic. I've read several threads on bridging the gap. I pulled the scope of my Mod Holland and put it on the weapon below. Of course, the mod holland has a swan sleeve. I'm certainly breaking the internet rules by bridging the gap with the 2 piece mount. Is there definitive evidence or a history of damaged receivers/rails/scopes/mounts due to this? I've shot some excellent groups with this setup. The photo below is from earlier today. Just under 1" at 200 yards. I love this setup the way it is. If someone can provide more information than "just because", I would appreciate it.. I know the theory, but need the evidence. I appreciate your help.
http://i69.photobucket.com/albums/i75/thebestofindica/Guns/20151206_134939.jpg
http://i69.photobucket.com/albums/i75/thebestofindica/20151206_201025.jpg
So all of the reasons above just weren't good enough? Gotcha.
I guess not. Maybe I missed all of the confirmed cases of damage. Thanks for the contribution.
It's as simple as the fact that two independent mounting surfaces apply two different tensors onto the optic mount during recoil. You'll see unusual wear, possible damage and zero shift.
ETA: Damn, I need to check post dates before jumping in. I should have been tipped off by such a worn out subject.
I didn't start a new thread because there are plenty of threads out there regarding this topic. I am looking for hard evidence of damage and it seems that nothing can be provided. I can move the mounts back, but I like to shoot nose to charging handle. This gentleman doesn't give a shit apparently.
http://img.photobucket.com/albums/1003/SturmGrenadier/MK%2012%20Mod%201/Image8.jpg
LMT MRP solves the issue.
Sent from my iPhone using Tapatalk
Keep shooting and supply us some hard data.
Mysteryman
12-07-15, 16:52
Forgive me for resurrecting an old thread. I did not want to start another due to this being a hot topic. I've read several threads on bridging the gap. I pulled the scope of my Mod Holland and put it on the weapon below. Of course, the mod holland has a swan sleeve. I'm certainly breaking the internet rules by bridging the gap with the 2 piece mount. Is there definitive evidence or a history of damaged receivers/rails/scopes/mounts due to this? I've shot some excellent groups with this setup. The photo below is from earlier today. Just under 1" at 200 yards. I love this setup the way it is. If someone can provide more information than "just because", I would appreciate it.. I know the theory, but need the evidence. I appreciate your help.
Three round groups are not conclusive. Ten round groups are a much better indicator of rifle/ammo/shooter performance.
MM
I agree. I will shoot 10 rd groups this weekend if weather cooperates. I did call an individual that is very well respected on several forums to get his opinion. He built me a rifle not long ago. His input was that if the rail to receiver interface is good, he wouldn't worry about it. I could always buy a recce rail and solve the issue as well, but it increases height over bore.
Digital_Damage
12-07-15, 20:14
Did a significant amount of testing on this subject last year.
The conclusion...
is most one piece mounts bridging the gap even properly torqued will exhibit shift over time.
Hell, even a properly torqued mount not pushed forward on the receiver before final torque will exhibit shift.
Interesting. I don't recall ever seeing a loss of zero on any of my rifles with mounts on receiver only. I'm not crazy about the cantilever mounts. If the 1-piece mounts show a POI shift, does this mean there is most likely some shift with recce rail or swan sleeve as well? Thanks for the input.
Digital_Damage
12-07-15, 21:47
Interesting. I don't recall ever seeing a loss of zero on any of my rifles with mounts on receiver only. I'm not crazy about the cantilever mounts. If the 1-piece mounts show a POI shift, does this mean there is most likely some shift with recce rail or swan sleeve as well? Thanks for the input.
You will not see much shift with a 4 bolt Swan. The issue with one piece is mostly due to two bolts (one on each side) and the rail flexing under recoil.
Mysteryman
12-11-15, 19:13
Interesting. I don't recall ever seeing a loss of zero on any of my rifles with mounts on receiver only. I'm not crazy about the cantilever mounts. If the 1-piece mounts show a POI shift, does this mean there is most likely some shift with recce rail or swan sleeve as well? Thanks for the input.
I believe Digital Damage is referring to one piece mounts that "bridge the gap" from receiver to forend.
MM
Why chance it? Just use a cantilever mount if that fits you better.
But handguards will all flex to some degree relative to the receiver.
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