[DonVoigt]

Miles,
Since I built my "7shot" baby myself, I have been down this road. A few observations
that may help :
-there is not enough room for a double spring
-a shortened spring is just as "strong", but lacks the tension at the end of the stroke for closing
-all springs are not the same, I was fortunate enough to have a handfull of old, take off, or out springs to experiment with.
-You will just have to "play" with the spring/ammo combination; but it sounds like what you have is working well
-Yes, some impact at the rear is normal- I have seen it in many/most lugers and have seen many that are quite a bit worse than yours.

Short answer, if it feeds and cycles now, leave it alone; and don't stress over a little impact!

You could show a picture of the back of the frame where the toggle bears against the frame. This will help understand if the spring is really weak, or you have a parts length/space mismatch inside.