Updated in October 2016
This is a fairly lively category, but I am not finding a whole lot of interesting stuff here. It seems like too many companies introduce scopes in this price range that are high on features, but weak on fundamentals.
Still, a few classics stand out.
If you want a simple fixed power scope for a rifle that really kicks, Leupold FX-II2.5x20mm is a really nice little scope.
For general purpose hunting, another fixed power Leupold, Leupold FX-II 6x36mm, has a lot to recommend itself.
If you want a tactical scope, or you want to use the same scope for hunting and precision, SWFA SS 6×42 is your best bet. If you lean more toward target shooting SWFA SS 10×42 and SWFA SS 12×42 are both good. Generally, if you need reliable tracking for $300, any of the SWFA SS scopes will do.
Updated in October 2016
Weirdly, many recommendations I had five years ago are still valid.
If you want a low range variable, get a Hi-Lux/Leatherwood CMR 1-4×24.
If you want a red dot, go back to Hi-Lux Leatherwood and check out the Micro-Max B-dot. It is a long and strange name, but a very sturdy reflex sight. I think that the same OEM makes this sight, Vortex Spark AR (also a nice red dot, if you are OK with the AR-specific mounting height) and a few others. I have tested a half dozen and they have not given me any trouble whatsoever. Do I prefer the Trijicon MRO? in principle yes, but if I am spending my own money… maybe not so much.
If what you are looking for is a general purpose hunting scope, there is a major problem with reticles in this segment: they are a bit on the thin side. I suggest you give Vortex Diamondback HP 3-12×42 a good look. You could do much worse than this one. Another option Is Nikon ProStaff5 or 7 in a 2.5-10×40 configuration. I am not a huge fan of Nikon scopes, but the new ProStaff impressed me as being very good for the money.
If you are looking for “tactical” use where you will spend a lot of time twisting the knobs, the obviosu choice is one of SWFA SS Classic fixed power scopes. I am partial to the 10×42 and 6×42 models with the MilQuad reticle.
Previous recommendations: December 2011
Interestingly, the choices in the “3-9×40 or similar” category expand considerably in the <$300 range, also there is a very nice tweener available. However, as far as other scope types go, there isn’t all that much that is really worthwhile. One exception to that is that for $300 you can get the cheapest “tactical” scope I feel comfortable recommending.
Weaver Grand Slam 3-10×40
Nikon Monarch 2-8×32
Nikon Monarch African 1-4×20
Fixed Power (hunting):
Leupold FX-2 2.5×20, 4×33 and 6×36
Leatherwood CMR 1-4×24 (conditional recommendation since the scope is fairly new)
Ultimately, in this price range, two scopes really stand out: SWFA S.S. 10×42 and 6×42. These are exceptionally robust tactical scope that can focus as close as 10 yards. They are suitable for anything from a precision rimfire to 50BMG.