The arguments against having dot sights deal primarily with defensive situations. You may have heard these before:
1. Dependence on batteries. Murphy is alive and well -- your battery may die on you at a very inopportune time.
Note that the Trijicon dot sights generally do not suffer from this problem. Tritium elements are used instead of batteries. These have a life of ten years or so -- maybe you could say that the batteries last ten years.
The US Army's new CCO M68 sight (the Aimpoint Comp M-XD) has a battery life of 100-1,000 hours. Not bad.
2. Switching. To use most dot sights, you must power up the unit. If you need to get your weapon into action quickly (i.e., from slung position), turning the unit on will add more time in a critical situation.
Again, the Trijicon tritium sights do not have this problem. They're always on. The M68 sight requires that you rotate a small knob to power up the sight.
3. Durability. Generally speaking, electronic sights are more vulnerable to moisture, particles and impact.
Again, most Trijicon (especially the ACOG) dot sights are heavily armored and sealed against moisture and particles. The M68 sight is also very shock-resistant and is also submersible. Other dot sights may not fare as well under adverse conditions.
Damage to the lens of any dot sight (including those mentioned above) will damage the sight. Glass is still glass when it comes down to it. The M68 sight, by the way, comes with two rubber flip-up covers for both ends of the M68 sight.
4. Camouflage. The glint of light off the objective lens of your dot sight may alert your target to your presence. Aimpoint manufactures a kill-flash for the M68 sight which supposedly alleviates this problem.
5. Cost. There are some economical dot sights out there, but the serious ones can run you a good bit of cash.
The aforementioned ACOG can cost you over $700-800. The M68 sight is a bit more reasonable, running around $300-400. There is a version of the M68 without night-vision capability that will run you $250-350.
In addition, you may have to buy a special mount for your dot sight. Typical mounting options from Aimpoint for the M68 sight can run you $80-100 for the flattop model and $125-175 for the "scout" carry handle mount.
Note that this analysis has ignored all the benefits of dot sights and has focused exclusively on the liabilities.
For non-LE or military, dot sights on long arms can make sense. Most likely, your situation will demand the use of a long arm in stationary defense -- aiming your rifle or shotgun at the door of your safe room doesn't implicate many of the concerns above (except for the batteries, make sure they're fresh!). However, in the instance of a Hurricane Andrew (?) where you may have to "patrol" (i.e., your mission is no longer stationary defense), a dot sight may not be such a good idea, for the reasons stated above.
Backup iron sights are always a good idea if you depend primarily upon the dot. No idea if anyone makes backup iron sights for shotguns -- check out the offerings from GG&G for rifles.
By the way, for competition, there's no problem with dot sights, as long as you don't mind shooting in Open class.
Justin
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Justin T. Huang, Esq.
late of Kennett Square, Pennsylvania