My experience with J&P Springs on both of my BT-99's. I ordered the springs online and 2 days latter I received them.
I documented my trigger pull weights on each gun prior to taking them apart. I disassembled the receivers completely and cleaned them, finding them both to be filthy. I did one at a time and they were put back together with the new main springs (Hammer Springs) to some people. I checked the trigger pull weights again and found both had went up an average of 7 ounces. I expected that because of the new springs that replaced the old worn weaker ones.
I have a couple of photos of the old and new springs. The 2 shorter springs are out of my 1977 (RR) and 1978 (RP) BT-99's that have been shot many thousands of rounds each. The new spring (Longest of the 3) is shown along side of them. The old springs are 3 1/2" long and the new ones from J&P are 4 1/8" long. You can see the amount of set the old springs took in the photo. Any coils springs over time will take a set and become shorter especially if left compressed. Jim says his springs are made no longer than the original Browning springs. They are just made out of better material and will last longer. I have no reason to doubt him.