Hi,
I'm pretty new to shooting guns, but I have put a little time in grip strength. I think every shooter could benefit from an improved grip strength. When people refer to 'grip strength', they don't realize there are many aspects to a strong grip: wrist strength, finger strength, crushing strength, and static strength.
For wrist strength:
Someone already mentioned a wrist roller.
You can try taking a sledgehammer and while keeping your arm straight, lowering the hammer's head towards your head, then raising. It's called levering.
You can do are wrist curls with a barbell, or doing bicep curls while holding a weight plate.
Try tearing a deck of cards in half (or folded newspaper).
For crushing and static strength:
Grippers (don't bother with the cheap Wally World ones). Get a good one from somewhere like the
Captains of Crush,
Heavy Grips, or elsewhere. These can be done for repetitions, or you can get a difficult one and just hold it closed (which will probably help with gripping a gun).
Try wrapping a towel around a barbell or dumbbell and doing exercises with it.
Try putting a towel over a chinup bar and grabbing both ends to do chinups.
Try taking weight plates, and with the smooth sides facing outwards, picking it up by pinching the plates together and lifting. Hold for time.
There are many other things you can do, but this is plenty. I think the most important aspects would be a strong wrist and static grip strength.
Thanks.