The HK light is OK, but it is exactly the same light as the Insight M2, which cost about $30 less due to no "HK" branding on the off side. It also does not need an adapter.
If you are anti-adapter then you really limit your choices. Laser Devices makes a couple but the BLAST unit is huge. Excellent quality but very large, but it does not need an adapter. SureFire also makes one that is a direct fit but you will have to research that one. Last time I looked I couldn't find it.
As to adapters. There is nothing wrong with them. I like the GG&G best followed by the equally good but more expensive SureFire. LaserMax has one that looks to be slim-lined also but I have not tried it. The Pro-Mag adapter works good too, is only $20, but is large and hangs the light down pretty far. I don't like it only because of that.
As to lights, I have a few but really cannot recommend any for serious use. I much prefer night sights and a hand held light, especially for civilians. Be truthful, as a civilian you are not going to be sweeping crack houses, office buildings, and warehouses, and if you are sweeping a room, it is more likely a bedroom in your home so better to do it with a hand held light so you do not end up pointing your gun at a family member which is more than likely what you are going to find.
Be that as it may, the lights I like best are the Insight/Streamlight line, the TLR-1 for light only, TLR-2 for light and laser, and the TLR-3 for a diode light. I have and use the TLR_1 and 2 and love them for a kicks and giggles toy. They are metal housed and have stood up to anything I have thrown at them. The laser also holds zero. The TLR-3 I have not seen but if it is anything like the TLR-1 and 2 it is a dandy.
The Insight/M6 was a good light at first, but mine is the old spring loaded plunger type and it has gotten loose so it really does not hold zero any more. At first it was tight but with time it works loose and zero on the laser shifts. I kept moving from gun to gun and from rail to rail, finding a tighter rail each time and now it only fits the HK rail on the USC, so I guess it will stay there until time for the trash heap. IMO get one that clamps on instead of using a spring loaded bar to hold it on. You can get the M6 either way now and except for the unit getting loose it works just fine. I don't think it can take a blow like the TLRs can since it has a plastic housing like the HK and the M2. But I have a feeling that anything that would put them out of commission would be so serious that lack of a light would be the least of your worries.