Howdy
I agree with everything rcmodel and Craig said. Normal factory springs for Uberti single action revolvers are stronger than they really need to be, to overcome the friction left behind in the mechanism. The mechanisms are not tuned, and there are enough tiny burrs and rough surfaces left over to cause some friction that affects how easily the hammer falls. So the standard factory springs are purposely made heavier to overcome the extra friction inherent in standard factory guns. A lightened hammer spring is only recommended if the gun has been tuned to remove the extra friction.
Sometimes it takes a little bit of experience to get the feel for what is a 'light' hammer pull and what is not. I have a 2nd Gen Colt that I bought used a few years ago. It still had the factory spring in it, and it was truly amazing how much strength it took to pull that hammer back. But I had the luxury of another Colt and a couple of Ubertis to compare it to. The factory spring in that Colt could have been used for the suspension of a Mack Truck. I replaced it with another spring, and to tell you the truth I do not remember exactly what spring it was, but that is the one that I only use Federal primers for.
Yes, it is true that using only a particular brand of primers is only addressing the symptoms, not the cause. But since you have some Federals anyway, it will not hurt to try some. Hopefully you have Federal Large Pistol primers, not Large Rifle. Since you reload anyway, try seating a few Federal primers in some empty cases and dropping the hammer on them. No need to load complete ammo, all you need is primed cases. That means you can find out what is happening in the basement, rather than going to the range. Only load one at a time because the primers will back out and make it difficult to rotate the cylinder. That is what happens when you fire primers without a bullet or powder charge. But if you just load one at a time you should be able to drop the hammer, and if the primer backs out you can rotate the cylinder by hand to get the fired case to the loading gate. You may be right on the hairy edge right now where the spring will fire Federal primers reliably, but it will not reliably fire Winchesters. Yes, Federals are easier to set off than Winchesters. Try a few in empty cases and see what happens.
Your idea of adding a strain screw like a S&W has will probably not work. The spring in a S&W revolver is retained at both ends, and the hammer end is captured by the stirrup on the hammer. The strain screw then adds tension to the spring between the two fixed points. A Colt clone hammer uses the same little roller that a Colt does. The roller rides on the end of the spring. If you have to bend the spring a tiny amount, it might work, but if you have to bend it too much the spring might pop out from under the little roller when the hammer falls.
Looking back over the earlier comments, it looks like when you measured firing pin protrusion it came up a little bit short. This may have something to do with what is going on. In this case, another pin might be a tad longer and might fix everything. I will tell you that once the firing pin in one of my Colts broke. The replacement pin I installed was a tad too short. Rather than finding another pin, I ground some metal off the frame so the hammer could fall a little bit father. This approach did work, but I do not recommend it. It is usually better to alter the cheaper part, the pin, rather than the expensive part, the frame.
Cylinder endshake might also be playing a role. Ideally there should be zero endshake, but a couple of thousandths is not unusual. Obviously, if the firing pin expends some of its energy pushing the cylinder forward, that is energy that is not available to dent the firing pin. If you have excessive endshake there are shims available from Brownells. They go down inside the cylinder where the removable bushing is inserted. The right thickness of shims will keep your cylinder all the way back without endshake, but they will keep your barrel/cylinder gap open a little bit larger than is normal.
Be sure your primers are completely seated, they should be a few thousandths subflush to the case head. If the firing pin wastes part of its energy seating primers, that can lead to unreliable ignition too.
P.S. One more thing. Look for a raised burr on the hole in the frame where the firing pin protrudes through. This is very common with Ubertis, it is often caused by excessive dry firing. The guy you bought the gun from might have been doing a lot of dry firing. Colt and clone firing pins are free to rotate slightly up and down in the hammer. As they go through the hole in the frame, the firing pin 'finds its way' through the hole. This is completely normal and is part of the design. As the firing pin flies through the hole, it is contacting the inner surface of the hole. Over time, material gets displaced and ends up as a raised burr surrounding the hole. Colts have a hardened insert to prevent this happening, Ubertis do not. Just an odd chance that a raised burr may be adding to the quotient of friction the firing pin is trying to overcome.
