I have a Walther P990 that I got over 10 years ago. A friend gave me a P99 upper they had. Question is, will the P99 upper fit the P990? It obviously fits, but I have not tried to fire it. The single action doesn't seem to lock. Don't know if will after cycling, or is there any modification that can be done to the lower to convert it to work with the P99 upper??