I have the BSA 30mm--it seems to be the cheapest at $30, and $30 is practically disposeable as far as I'm concerned. It works just fine on a Mk II so far (500? shots), but others have said it comes apart on bigger-caliber handguns. It has integral weaver mounts and mounts pretty low, and the 11 dot levels have a good, wide range to them. --I found several reviews of the BSA 42mm that said it has poor internal reflections.
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Before that, I bought a Tasco Mini-Mag 25mm for about $45 on an impulse. It uses an intermount weaver block (as below), the 25mm window is rather cramped (I think it's really closer to 20mm, internally), and the 5 dot levels don't have a lot of range between them (the lowest is nearly as bright as the highest). I am keeping it as a "stand-in sight", but it does not live permanently on any gun.
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I would warn you to make sure whatever you buy can mount as low to the gun as possible--get something that will fit directly onto the mounts you have. Some of the cheaper red dots have integral "22/mini"-type mounts, and then include a weaver-rail adaptor block but they end up sitting rather high. It works but feels odd with the sights a couple inches above the boreline--most people agree that "lower=better" in this matter. -And other cheap red-dots don't include a weaver adaptor block at all, so check this before buying.
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