I invested in a 300 lb. spring scale, a "come-along" ratchet pulling device, and a strong board as a testing bench.  Below, you will see this setup used in the calibration of one spring used in another testing device.

Testing bench can be used to check all aspects of your home built big game speargun. All parts of the test bench must be thoroughly bolted into the board. With a few basic pieces of equipment you will be able to verify that your home built gun is strong enough to be safe and work effectively.

We have tested everything from rubber band strength and soak down to ultimate failure of shooting lines and swaging techniques.  Equipment failure is no excuse for losing a fish.  If something does go wrong, analyze, test and change until there is no possibility of the failure happening again.  Shooting a gun in a swimming pool at a target is an excellent way to verify if your gun is accurate.  You need to give it a fair test by being quite far from your target.  Any gun can hit a target at 15 feet.  A big game gun should be shot into a target at 20 feet or more, if that is the shot you are planning.  You need to be critical and honest with yourself when you are testing your gun.

Below you will see a testing setup that evaluates trigger pull pressure.  The Prodanovich mechanism has a "firm" and deliberate pull of between 9 and 12 pounds.  It is not a "hair trigger".

Below you will see the small spring scale in position to verify that the trigger pull is proper.

If you are planning to load up your home built big game speargun with extra rubber bands, you should verify that the handle and mechanism can withstand the pressures without collapsing.  I built this mechanism to check the security of using 1000 pounds of rubber on one of the special guns.

By counting the revolutions on the nut, I was able to adjust pressures that pulled on the small, test spear shaft.  Four double valve springs were compressed to simulate rubber bands.
I designed and built this mechanism with the assistance of Don Paul Gaboury.  This testing bench has a maximum pressure of 1600 lb. The Modified Prodanovich handle and mechanism successfully and repeatedly survived firing at this maximum pressure.  This tester was built to verify that a special 1000 lb. gun would hold together. When the handle and test shot are made the shaft (with 9 square inch steel plate on the tip) travels 1/8" into a large rubber pad that absorbs the energy. The results of a 1600 lb. shot is a very firm clack sound.

It is essential to know that your equipment is safe.  The only way you can do this is by testing at required levels in excess of the design pressure levels.  You need to know there is a "safety factor".  Be sure to be safe with designing and building your own testing equipment, it too can be dangerous, as a highly loaded tester, is just like a real loaded speargun.