1 Use a stress ball to warm up. And warm down. And recovery. Hell, just use it all the time.
2 Stick your hand in a bucket of dry loose sand, and do smooth movements opening out (start in a fist, and spread your fingers open). You can do this explosively too.
3 It may not be a muscular problem, or even a tendon/ligament problem. If there are malformed bones, or shards of broken bone from a previous injury, this is the way to irritate them. Maybe see a doc. I used to get a stabbing pain in the end of one finger from heavy lifts and COC. Turns out I had tiny splinters of glass that the skin had grown over. One quick operation to remove them, and no more problems.
4 If you can do 5x5, it’s time for the No.2 gripper. Heavy and Hard.
5 The best exercise for grip? Wide Plate Pinches. Two 45s together, smooth sides out, lots of chalk, lift, and hold until you drop them on your foot.
You can also do pinch maxes (like a deadlift) by putting a bar through the 45s and adding more plates symetrically. My pb is 180lbs. After a a cycle of these, I closed the No.3 no problems.