I know that you will hear different views on this--I honestly feel that yes the springs should be un-cocked BUT you do not have to use a snap cap, just make sure that the chamber is empty and pull or set and release the trigger. Will not harm the firing pin.
I asked my buddy who has a Master's Degree in Mechanical Engineering, specializing in machine design and strength of materials.
It depends on the material and the level of stress seen in the sprung (cocked position) and the unsprung (hammer down) position.
Metals will tend to take a set, that is, if kept in a stressed condition for long periods of time, the metal will actually "creep" (and yes, that really is an engineering term.) Which is why your hammer springs get shorter over time, because regardless of whether the hammer is cocked or uncocked, the hammer spring is under stress. Its just under higher stress when its cocked.
However, cycles (each time you take a shot) will fatigue a spring as well. And in the case of someone who shoots 10,000 targets a year, spring fatigue due to cycles may indeed be the governing cause of spring weakening, and not creep.
So, for guns used occasionally, snap caps may indeed extend the life of the hammer spring.
On the other hand, for guns that are used regularly, like most of your trap guns, chances are that using a snap cap may not make any difference at all.
Will a snap cap harm anything? Oh, probably not.
As with all things...your mileage may vary.
Probably the right answer is, replace your hammer springs regularly.
The notion that gun springs will take a set that will somehow disable them is a myth. Do you leave your garage door open so the springs don't take a set? Do you unwind your watch (assuming it's not electronic) so its springs don't take a set? The whole idea is absurd. Cycling is the enemy of springs, not compression.
As noted in The Modern Shotgun, by Burrard, any loss of strength by keeping the mainspring compressed is negligible. He points out that all the shooters who worry about decompressing the mainspring never seem to worry about the ejector springs, which are generally kept compressed almost all the time. His advice is not to worry about it.
Burrard was the leading authority on shotguns in his day (the 1930's and 1940's). If anything, metal technology has improved in the last 70 years!
Only if the spring or design is defective in the first place is there any issue with taking a set, and if there's a defect, you've got a problem that snap caps won't cure.
With all due respect to Burrard and his theories.....I KNOW if I leave one of my .45s with a full magazine for a year or two, the magazine won't feed the shells as crisply to the chamber as a magazine that has not been compressed over time......it takes significant time, but yes, it does make a difference, a NEGATIVE difference in magazine springs....
In terms of trigger sprins, it takes even longer, but yes, it does make a difference.....I use Snap Caps.....
Another guy with a degree in mechanical engineering here... specializing in mechanics of materials.
Creep is a very real thing, and depending on material selection/condition, can happen very quickly. We have some springs that we use here at work that will creep after only a few hundred cycles and then left with a light load (~5% of UTS).
I can explain more... but I'll just end it by saying that, both as an engineer and a shooter, I use snap caps in all my guns.
Springs do tent to lose their spring if stored under tension. I always release the tension on my hammer springs before I put my gun in a case, but I do not use a snap cap. I just use the trigger and an empty chamber. I am not stating this is the best way to release the tension, only that it is the way I do it.
Ricky mentioned garage door springs as en example of a spring that is kept under tension for a long period of time. He suggested this an an example of tension not damaging a spring. I would use garage door springs as an example of how springs are damaged when kept under tension. Garage door springs are very heavy springs that frequently break after several hundred cycles. Replacement springs are fast selling items at Lowes.
When I was 14 my Dad bought me a shotgun, my first one. It was a 16 guage Stevens. The night before opening day, I was all excited and we were getting ready for bird season.
Dad put the gun up to his shoulder and dry fired it. The end of the firing pin came rolling out of the barrel. He and I both were stressed.
He made a pin out of a bolt, whch took him some time with just a file and an electric drill. Then he cut open a shell and took all the components out, and fired the primer. the primer pierced, and he gave up. Needless to say, I did not carry a gun the next day. Later that week he went and got a firing pin locally and put it in.
Now, 56 years later, I do not dry fire my 90T. I do my 1100's but there are enough organ donor 1100's around so it's not a problem.
I dry fire my rifles, as I was told it would not hurt anything. But there is always that nagging doubt.
When you use snap caps when storing, (better yet casing a gun), how do you use them? Are you just sticking them in the barrel(s), or do you dry fire the gun first to relieve the spring pressure, and then remove the barrels?
1. Insert snap cap like a cartridge and pull trigger. Leave snap cap in barrel. Otherwise, when you open the action to remove the snap cap, you are likely cocking the mainspring again.
2. With my Mossberg 500 and Rem 1100 (and I assume with most pumps or autos), remove barrel and hold snap cap in place against the standing breech with your fingers. Pull trigger. Remove snap cap.
3. With my Beretta O/U, insert snap cap like a cartridge and pull trigger. Then remove the forend and open the barrel, holding onto the barrel so it doesn't fall off. Remove snap cap from barrel. The forend must be removed first because opening the action with the forend attached will re-cock the hammer. Other O/U or double guns may work differently.
Alternate method for any gun with removable barrel (no snap cap needed): Remove barrel. Hold piece of wood flat against the standing breech with your fingers and pull trigger. Remove wood.