Thoughts on Optic Bias

[win&legend]

I often hear people say the bias of the optic does not affect the accuracy, even including the optics manufacturers. This is only partially true. It does not affect technical precision or accuracy, but it does affect practical accuracy. Below is an illustration which clearly shows the reality of the physics of the placement. For reference, I am defining Technical accuracy as something that is independent of the user / operator's ability such as bore precision, cartridge consistency etc. where piratical accuracy is what affects how a human interacts with and takes advantage of the technical abilities of the fire arm.

Arc of Motion of Optics based on Mounting Position.jpg

I did some searching, spent a couple of hours really, looking for photos of Army Special Forces and other various elite units, I noticed there was a common but not universal theme, almost all of them had a rear biased mounting position of their optics, not forward / scout, which to me suggests they are geared towards close quarters. Speed and endurance are the two advantages to a rearward mounting position. The disadvantage is precision as the same motion of the muzzle results in less detectable movement of the reticle by your eye, making precision shots at longer ranges more difficult. I've used both positions and confirmed this reality, but I'm torn between loosing that longer range precision vs the speed and endurance gains. I'm interested in everyone thoughts and even real world experiences of veterans on the practicality of one other the other and what level of precision one would typically really need. The application is a defensive rifle, the environment is urban country (sprawling urban neighborhoods, a 1950's country suburb, right on the edge of vast open farm fields).

I think it's quite apparent you can't have your cake and eat it too so to speak. There are trade off's with either position. Also for reference, when I say scout position of the optic, I am referring to a forward bias such that the entire optic (an EOTech 512) is centered between the front of the receiver and the back of the gas block, meaning the optic is entirely on the rail and not the reciever. By rearward bias, I mean the hood of the optic is centered over the mag well or just just slightly back of the mag well. Thanks all.

[Eurodriver]

Hell of a 15th post! I'll be watching this with interest. Good stuff.

[sua175]

Since you want the input from a veteran I will give it. I think you are over thinking in your analysis.

First, the majority of guys in SOF units do not mount their optics forward of the upper receiver for two main reasons.

first, it is commonly felt that the actual receiver is a more stable platform than a bolted on rail that has the potential to shift or move and thus affecting the zero of your optics.

Second is that the rail is used for auxiliary equipment such as lights, lasers, leaf sights, so not a lot of rooms for optics, not to mention making the weapon front heavy.

Now when talking about non magnified optics such as a eotech or aim point, I as well as most guys prefer to mount then as far forward on the receiver as possible.


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[Failure2Stop]

As pointed out above, mounting the optic to the handguard of a rifle that does not have the handguard as an integral part of the receiver reduces the precision of the optic for a few reasons, and can show POI shift with different pressure applied to the handguard.
Most folks place optics on the upper receiver because it is the most rigid place to put them.
Aimpoints and EoTechs tended to migrate forward to the front of the upper receiver, and even further out with cantilevered mounts, to permit getting a magnifier behind them.

[TAZ]

I am no High Speed Low Drag kind of guy so YMMV on my opinion. Folks mount optics to receivers many cause they are the most rigid and stable items available. On bolt rifles you had the barrel or the receiver. Between those two the receiver is the obvious choice. Plus you don't get the goofy harmonics issues with hanging odd weights of a barrel. Jump forward to the AR platform and you have the receiver and the snap on hand guards. Not much choice there. Heck even most free float tubes aren't designed for precision optics mounting. Id bet if you asked some high dollar hand guard engineers they's tell you you're better off mounting to the receiver. Far more rigid, far better control over concentricity to the bore and far more consistency. And in long range shooting consistency is king.

WRT shooting, Ive shot Scout rifles, Red dots and decision optics (standard mounting) and my experiences are the opposite. The further out the scope the faster I was on target, but I was far more accurate with standard optics mounting locations. Its not a fair comparison, as the scout was a 4x and the precision was 10x or 22x. Definitely NOT apples to apples. Personally, at range I had no issue with noticing my heart beat move the reticle about. I also wonder what the physics behind making something like a 22x scout the scope would be.

As for your dilemma of a defensive rifle in urban sprawl; the question to be answered is what are your goals. Minute of man or making head shots at ranges past Ft. Dix? If you're talking plane jane defense then a red dot with a magnifier for target ID or a low power variable should work fine. Best thing to do IMO is figure out what you will realistically need to accomplish using todays equipment and then go about making that happen. Understand that you'll make compromises to attain your goals. You won't have a sniper rifle be effective at CQB nor will you have an SBR be a sniper rifle. Also understand that technology changes and in a few years you may be able to make changes that will shrink the compromise window a bit.

[win&legend]

Thanks for the input everyone. One point I did not mention but had pondered was POI shift, but the Samson rail I have on there is very secure, it would take one heck of a knock to shift it, but it's certainly in the realm of possible. What I liked about the optic being mounted on the rail was how naturally I felt the rifle pointed (in terms of reticle movement relative to my arm movement). I agree, it did seem faster on target, but also seemed more accurate because I could detect muzzle movement more easily (see image above) having not so great eye sight. Going back to the POI shift, I did encounter one very strange problem during a V-tac drill, I couldn't hit a thing until I realized I was flexing the barrel or rail, I couldn't tell which was flexing but I'd guess the rail given the barrel is a reasonably hefty gov profile, so that may very well have been the issue (I originally suspected a loose barrel nut but found that wasn't the cause).

After having tested the scout position out with a plate carrier and armor I found what was mentioned about front heavy to be the case. Technically the rifle isn't front heavy, it's balance point is right at the front of the mag well which is about mid length on the overall rifle, I can literally balance it on a fine edge, but the reality is that we want more of the weight to be concentrated in the rear which facilitates ease of use (like a bull pup), especially when it comes to endurance. I had read an army study (1997) proving at the very least that keeping the mass of the rifle closer in to the body (they were experimenting with shorter butt stocks) facilitated less muzzle movement, this was more dramatic the longer the time frame as fatigue significantly increased the muzzle movement off target.

I can confirm that after having run some box drills, that my support arm (shoulder and back) became fatigued very quickly having to drive that extra 3/4 lbs that far out, which is much more cumbersome and difficult when wearing armor, even with a good plate carrier like a Crye JPC and 5 lb ceramics. As I became more exhausted it just became that much more difficult to keep the rifle on target regardless of how naturally it did or did not point. All of the sudden I didn't care for the scout position so much and was wishing for something less "cumbersome"! It's quite a amazing at how often an idea may seem plausible and practical in the mind's eye yet the application of it so often proves the fiction of it. Here's a photo of SF training Hungarian troops if memory serves correctly. The second man on the line has a flat top receiver and his 516 isn't pushed as far forward on the receiver as he can get it, in fact his is about mid receiver, similar to how many Russian optics are mounted on AKs (having had a 74M, there are lots of side mount rails that allow mounting of optics anywhere over the dust cover, but still most I see are mid or rear receiver). With the AK weight is more of an issue, so it makes absolute sense there, but this appears to be the case with the AR as well which was my primary reason for switching. It's lighter, but not that much lighter that I can get away with certain things and as mentioned there are other caveats like POI shift from the rail.

US-Special-Forces-4.jpg

"That is the trouble with many inventors; they lack patience. They lack the willingness to work a thing out slowly and clearly and sharply in their mind, so that they can actually "feel it work". They want to try their first idea right off; and the result is they use up lots of money and lots of good material, only to find eventually that they are working in the wrong direction. We all make mistakes, and it is better to make them before we begin." Nikola Tesla

[mkmckinley]

Don't take what one guy does as gospel. I've seen some "cool guys" with some pretty jacked up weapon configurations. Maybe he's just trying that mounting position out for shits and grins when that picture was taken.

Mounting toward the front of the receiver is the best compromise between weapon balance, fields of view both through the optic and around it, and use of rail estate for different add-ons. Also consider that precision shots can be aided by placing the red dot on top of the front sight post. Finally, some RDS mounts overhand the front of the receiver slightly. This can help keep the handguard in place on some handguard designs which keeps the IR laser zeroed.

[win&legend]

Don't take what one guy does as gospel. I've seen some "cool guys" with some pretty jacked up weapon configurations. Maybe he's just trying that mounting position out for shits and grins when that picture was taken.

Mounting toward the front of the receiver is the best compromise between weapon balance, fields of view both through the optic and around it, and use of rail estate for different add-ons. Also consider that precision shots can be aided by placing the red dot on top of the front sight post. Finally, some RDS mounts overhand the front of the receiver slightly. This can help keep the hand guard in place on some hand guard designs which keeps the IR laser zeroed.

Agreed, because one person does it does not mean it works, on the contrary, because everyone does does not mean it works either, or that there isn't a better way. That is not however a lone instance, I've done several searches with simple combinations of nouns like Army Special Forces or Marine Special Operations and the majority of photos taken where EOTechs are used have the optics placed about mid receiver. There are some who do place the optics all the way to the front of the receiver even if they don't have a magnifier on there, but if I had to guess, from my own training under Marine instructors, hence why I"m doing V-tac drills etc. is that fatigue for me is a big issue over time. I could imagine the effects stress have on the body when it's not just a clock your trying to beat.

I'll have to experiment and see what I actually shoot better with, I've used the forward receiver position before and it was good, which is why I decided to push the concept further by going onto the rail in the scout position (similar to what I did with AK's and the Ultimak rails, but those rails were locked into place to the trunion), but I've yet to try mid receiver positions. Here are some more examples, just a collection of photos showing a common theme (but also the outliers):

HK416.jpgMarine EOTECH 1.jpgMarine Spec Ops.jpgMARSOC Operator 1.jpgserveimage (1).jpg

[win&legend]

Attachment 35750Attachment 35751

One of the photos has some XPS's on the forward most part of the receiver, but XPS's are 1/4 lb lighter in weight than the older 512 series which is what I have and seems to be the most common due to it's low cost and use of widely available AA's (I use Lithium AA's so run time is double a single CR123). Obviously using a magnifier would require a forward receiver position simply due to receiver space available and some of them are rear mounted due to PEQ's etc but others have room in front. It's interesting to see that even when they could mount them more forward. I'd bet my bottom dollar it's due to endurance reasons which is in my opinion the bigger advantage of bull pup designs than their compact size (IMI Tayvor, Styer AUG etc.).

[Inuvik]

This is the Precision Semi-Auto Rifle sub-forum. The Eotechs in all of your sample photos would not generally be considered a precision optic, and the way they are mounted is usually quite different from a variable-powered scope. Based on the description on your first post, I would have expended you to be looking at examples of Mk12 or Recce rifles instead of the carbines you are evidentially looking at as examples. The photo sticky at the top of this sub-forum may be a good reference for you as well.