I’ve been a pretty staunch advocate of the fixed front sight base, even if one plans to run a magnified optic. That’s not to say that I’m absolutely married to the idea of keeping it in all circumstances, because I’m not. I do, however, think that most people are discounting them these days because they see a lot of pictures of rifles without them, they look cool, and there are a lot of folks out there talking about the inherent benefits of low profile gas blocks.
In this post, I simply want to discuss the relationship of optics and iron sights on a rifle. There are really three ways to look at this: 1) Cowitnessing, 2) Sight shadow, 3) Necessity.
Cowitnessing is the practice of getting a sight picture with the irons while looking through the optic. This only works on non-magnified red dot sights that are mounted in alignment with the irons. The intent here is that the user can quickly align the irons in the heat of the moment in case the red dot sight fails. Some people also use cowitnessed iron sights as a quick way to check the zero on their red dot (I do not agree with this practice, for reasons I will get to). Cowitnessing is further divided into two general categories: absolute and lower third. With an absolute, looking through the irons will align with the red dot right in the middle of the optic. Everything is aligned in a straight line. With a lower third, the optic is mounted slightly higher than the sights. This latter configuration provides a slightly less cluttered sight picture for the red dot, but allows the shooter to drop his or her head slightly lower and still get an iron sight picture through the bottom third of the red dot sight. Larue Tactical has a good depiction of the two methods using their mounts.
I mentioned that some folks will check their RDS zero by seeing if it aligns with the iron sights, and that I do not like the practice. The reason I don’t like it is because the sight’s zero can move slightly as the dot moves around the window. I know red dot sights are often sold as being completely parallax free, but it is not true. It is best to zero the irons and zero the red dot separately. When you do that, you also have the option of choosing different zero distances for each system (if it makes sense for you). That might mean a bit more mental work and practice up front, but could offer some more versatility later.
However you choose to do it, you absolutely need to zero your irons. Red dots can and will fail you. Rain, fog, mud, battery failure, and other factors will all disrupt your sight picture through a red dot. To be fair, such occurrences (except for battery failure) will also limit your ability to use the cowitnessed iron sights, so I suggest keeping the RDS on a mount that you can quickly remove, if needed.
As I previously said, cowitnessing only works on sights that have zero magnification. As soon as you introduce lenses that bend light, cowitnessing is no longer an option. As much as it may appear that you can cowitness through a 1x-4x sight set at 1x- you cannot. This is a picture through my TR24 set at 1x behind a the rifle length FSP on the Musket.
“Well, that looks pretty cowitnessed to me!”
I assure you, it doesn’t work. The reason is that the TR24 is still bending light in such a way that the sight appears to have no magnification. The image is still being taken at the objective end of the scope, a full twelve inches in front of my eye, and about eleven inches in front of where a rear sight might be. The way light moves through this arrangement is simply not the same as how an RDS or bare sights work. If you attempted to cowitness (assuming you could fit a rear sight behind the scope), you are really just aligning the rear sight to a picture rather than a physical front sight.
This is where people start recommending ditching front sights with magnified optics. In truth, a physical front (and rear) sight serves no purpose when you are employing a magnified optic. They exist only as backups. Furthermore, as you can see above, the front sight will be somewhat visible through the scope. How visible it is really depends on the field of view of the scope, though. This is the same configuration, but with the scope set on 4x.
From this perspective, you really can’t see the front sight at all. That is because the TR24 has a fairly narrow field of view. This next photo is on the same rifle but with my fixed 4x ELCAN, which has a much wider field of view that better includes the front sight.
This is for illustrative purposes only, and is not definitive. In actual use, the front sight is not that visible through the ELCAN. Your eye focuses and captures light differently than a camera lens, so the blur at the bottom is much less obtrusive in actual use than this picture would have you believe. That said, it is still there and quite detectable if you are looking for it.
Here is another photo I found through an ACOG that is closer to how it appears through the ELCAN.
The important thing to remember is that if you are doing your job and focusing on the target, you will hardly notice the dark blur. The higher the magnification goes, and narrower the field of view goes, the less visible the front sight becomes. At 10x, as with my 2.5-10x scope, I don’t see the post at all.
That said, a fixed FSB can sometimes reflect light back into the objective. When I’ve run 10x scopes behind an FSB, there are some circumstances where reflected light briefly distorts my sight picture (though not enough to actually cause a problem). For most users, under most circumstances, this simply isn’t something to worry about.
That brings me to necessity. Do we actually need backups? When you see optics mounted behind fixed sights on military rifles, it is because that is how the rifle is issued to the user. Military users don’t have much leeway on how their rifles are configured, so they “make it work.” On civilian rifles, it is mostly done either because the user purchased a complete rifle with FSB and just ran with it, or they are copying the look of military weapons. For most people choosing a configuration with a lot of leeway, going without fixed sights (or even backup) will probably work just fine. For new users, the uncluttered sight picture offers a slight boost in speed.
I will readily admit that I prefer using optics without FSB shadow (both RDS or magnified), but I’ve come to accept the trade off. The fixed front sight tower is simply the strongest front sight solution available. If I choose to move back to irons, then that sturdy front sight will be there for me. But that is me.
Just because you see everyone slapping backup sights on optically-sighted rifles doesn’t mean you need to as well. Analyze your situation, your needs, and make a decision. If you have an optic that has short eye relief, like a TA31 ACOG, it is okay to delete the backup sights in order to position the scope for comfortable shooting. As far as I’m concerned, backup sights are a nicety, not a necessity. I’m sure some will argue with me on that, but it really comes down to personal preference.
As always, look at how you actually use your rifle and what your needs are.