If the hammer is hitting square on the cap I would not guess it the hammer spring. I would be more inclined to think the caps are no good, or the nipple. The first thing I do when I get a new sidelock it take the factory nipple off the rifle and put a hot shot nipple on them. Many times the factory nipples are too short. Although being too tall can be just as bad.

You did not mention what kind of caps you were shooting or how old they were. I used to use a German Brand Cap but that seems like ages ago. I am no longer able to find them so I went to CCI Magnum #11's. they are a good hot cap. Also be careful about getting any oil on or in the caps with your fingers. I see a lot of people with bore butter or lube on their fingers then grab the cap and set it. This can cause some troubles.

You did not mention how well the rifle shot. What kind of load were you shooting and how well did it shoot? Mine is a real nail driver with round ball. Actually for $39.95 which is what I paid for the rifle, this thing will shoot roundballs so well, it would compete with my Renegade or Hawkins any day of the week.

Cleaning them is a lot easier then cleaning my in lines. I would clean a side lock any day of the week before I had to clean a sidelock. Although in all fairness, I have not shot or cleaned a Optima or Omega type rifle. I think those might be real nice reference cleaning....

I shot my Renegade yesterday shooting 415 grain paper patched Minnie Balls that I hand casted, and talk about recoil and big holes. These rifles are really enjoyable to shoot and hunt with. After I started shooting them, the center fires kind of lost their charm to me.....