I have seen the phrase “lightweight carbine” mean different things to different people, so as a first matter of business, I will define what I mean by it as it applies to AR-type rifles.
A lightweight carbine, to me, is built primarily for compactness and light weight with other considerations (aside from reliability) secondary.
There is no hard and fast rule on what “light weight” really means and there is no hard cutoff here. Besides, it also varies between AR-15 and AR-10 variants. One mistake that people make is in thinking that a lightweight AR carbine should be the same weight as an ultralight bolt-action rifle. The weight correlation is not direct, since an AR-15 of equivalent weight is usually shorter than a bolt action gun and has a more neutral balance. As an exercise, I spent some time experimenting with a variety of rifles and realized that the perception of “light weight” has nearly as much to do with balance and fit as it does with actual weight.
It sports an Adams Arms ultra-lightweight upper with piston gas system, and I could not be happier with it.
I use it to test sights and scopes of all sorts and, believe it or not, I actually do not have a dedicated scope for it right now. It almost always has something I am writing an article about mounted. However, that also gives me a good perspective on how different sights and scopes work on this gun.
In the picture above, there is a small non-magnifying red dot sight mounted on the rifle and if your rifle’s primary purpose in life is home defense (i.e. you do not expect shoot past 100 yards with any regularity and most shooting will be notably closer than that). That was the original purpose I had for the gun, but after I put it together it turned out that the little thing is quite accurate and handles like a dream, so I had to think a little more about the best sight for it.
Here are some options I came up with for a lightweight carbine, depending on the specific application and budget.
If you want a carbine primarily for home defense and fast shooting, get a red dot. I am not a big fan of full size red dots, but I really like the smaller ones. They are unobtrusive and perform exceptionally well. The king of miniature red dot sights is, in my mind, Aimpoint Micro. It is not cheap, but the battery lasts forever, brightness adjustment is very natural and the dot is remarkably fast and easy to pick up.
For a flattop AR, you will want to get a riser, while for a carry handle mounting, all you need is a base (I prefer flat top ARs as a general consideration).
Most miniature red dot sights are really designed with secondary sight application in mind (with a regular scope being a primary), but the Aimpoint Micro does a fine job as a primary sight. A couple of other miniature red dot sights I like in primary applications are Leupold Deltapoint and Vortex Razor (mounted on the rifle in the picture above). They are even smaller than the Aimpoint and a bit less expensive, though still not cheap.
Deltapoint is a bit smaller, while the Razor has an ingenious battery tray for easy battery replacement.
There are some less expensive miniature red dots out there, but I have not had good luck with the ones I have seen. There are some I still need to test though.
Ultimately, if you want a red dot on a budget that does not allow for one of the sights I already mentioned, you need to be looking at full size models, since there are more options there.
I have heard good things about the Lucid HD, which is a very full featured sight considering the price.
Vortex Strikefire and Sparc are not quite as full featured, but are less expensive and in the sub-$200 category I would take these two over anything else out there. I am not crazy about the control buttons on either of these. However, they work well enough, and it is really a matter of personal preference. Sparc is the smaller one of the two and I like it a little more.
If all you want to do with your carbine is plink at the range, just about any scope you have lying around will do as long as it holds zero. I have seen all manner of hunting scopes on AR carbines. However, if you are looking to make your carbine into more of a general purpose gun, you need something that can perform with almost as much speed as a red dot sight, while giving you some magnification for precision shooting when needed.
On the surface, it would seem to be quite straightforward to go pick a low range variable scope of some sort (1-4x24, 1-6x24 or thereabouts), slap it onto your gun and be all set. In practice, it gets a little more involved since these carbines are pretty light and mounting a heavy scope on it upsets the handling and balance of this lightweight rifle in short order (something, I learned the hard way). As I go through the options below, you will notice that I largely stay away from the newly fashionable 1-6x and 1-8x scopes. They tend to be both heavy and expensive, so I try to stick to scopes that top out at 4x.
Here are some recommendations I can make that keep both weight and performance in mind.
Trijicon Accupoint 1-4x24 that retails for around $900 is comparatively light for a 30mm tube scope at a bit over 14 ounces and with its bright reticle illumination, it is very fast to use, while dialing up to 4x give you some precision.
Leupold Patrol VX-R 1.25-4x20 is the lightest good quality variable scope I know of at 11 ounces or so, and if you can swing the $570 price tag it is probably your best choice in magnified optics for lightweight carbines. It does not give you true 1x operation, but it is close enough and brightly illuminated dot in the center of the reticle really helps with speed.
The $500 Vortex Viper PST 1-4x24 is one of my favorite scopes in terms of bang for the buck. It has every feature under the sun along with a well-designed reticle. It is a touch heavier than Trijicon at 16 ounces, but still very serviceable and less expensive.
Hi-Lux/Leatherwood CMR is similarly sized to the Vortex and is also very fully-featured. It is a little less expensive at $350 or so, but unusually good for the money. I tested a few versions of this scope and liked the value and the reliability.
One of the scope I mentioned above, would, in my opinion offer the most for your money. However, there are other options with scopes specifically designed for ARs and featuring compact dimensions and fixed magnification. I am talking about the famous (due to extensive military use) Trijicon ACOG and its many competitors: Elcan Spectre, Leupold HAMR, Browe etc.
Still in terms of light weight performance, Trijicon is king. Using it for close distance targets requires a little more training than a red dot, but it is doable and effective. If you are willing to put in the effort, a 4x32 ACOG is a superb, albeit pricy, light weight option.
If you are not especially interested in holdover reticles (i.e. you do not expect to want to shoot beyond 300 yards a whole lot), an even smaller compact ACOG might be an even better option. I have a lot of mileage with the 3x24 model that has been discontinued, but I have also dabbled quite a bit with a diminutive 1.5x24 an 2x20 (pictured below) compact ACOGs and walked away impressed. With the compact ACOGs retailing right around $1k and the 4x32 ACOG even more expensive, these are not for everyone. However, there is a reason the military uses them: they are light sturdy and optically sound.
If you are wondering what I decided to put on my person lightweight carbine after all... well, to be honest, I am still a little conflicted. I started out full intending to mount a low range varible scope on it, but I am leaning toward either the ACOG or Leupold HAMR or the new Hensoldt 4x30 AR scope. Once I test them side by side, I will pick one of them and buy it.