I have a 1917 New service that has been turned into a single action target pistol.
I replaced the lug on the hammer to return it to double action.
The main spring has been lightened to where it fires fine with CCI primers but sometimes with double action shooting the primer fails to fire leaving just a small dent in the primer.
What I 'm looking for is what make primers is easiest to set off?

I had the same problem with occasional FTF and my old revolvers reloading with CCI primers. Switched to Federal for these guns, no problem since. I still like and use CCI, just not for the oldies.

I, too, have heard that CCI primers are harder to set off and Federal's are easier.

In order from easiest to hardest to ignite, my experience indicates
Federal
Remington
Winchester
CCI

Federal ..

You mentioned that the mainspring has been lightened.
It has been lightened too much! FTF is what happens when you lighten the mainspring too much.
The easiest thing to do is to cut a shim out of shimstock (such as automotive feeler gauges, and place it between the mainspring and the tip of the strain screw. Try a 0.005" thick shim. It needs to be large enough only to extend the tip of the strain screw and be large enough to handle while you are inserting it.

I would check with Numrich Gun Parts to see if you can buy a factory original mainspring. Currently $10.25 and in stock.

Thanks to all that replied.
Looks like I'll be getting some federals
I got a new main spring off gun broker ,but it wasn't factory.
It was so week that I think I could have made a better one out of a hair pin .

Results definately vary on Gunbroker. Probably someone found their spring was weak, so bought a new one and stuck you with the old spring. I would not shange primers, just buy the right spring from Numrich.