Brand new Kolar pins and springs. Spring cut by a few coils. Pin passageways cleaned.

50% of the time bottom barrel fails to fire with a light strike to the primer. Closing the gun again and keeping same wheel in bottom barrel successfully fires.

Any thoughts or similar experiences?

Why would you cut coils off of your mainsprings?

Right there you pretty much answered your own question is to what's causing the issue.....

Send your trigger group back to Kolar and have them install new hammer springs.

My gun was annualed in December 2019 and has been shot about 10,000 rounds since. Recently, I have been getting an occassional fail-to-fire in the bottom barrel. I called Kolar and talked to one of the technicians. He told me that in October of last year, they made a change to the tip profile of the firing pins as well as increased the installed load of the hammer springs by about 20%. The real answer to your problem, most likely, is the increased driving force of the hammer springs.

Agree 100%.

Firing pin springs, not main springs. Firing pin spring clipping is something Kolar frequently does to reduce inertia against the pin moving forward toward the shell.

Did you cut the springs or did Kolar?

When's the last time the mainsprings were replaced and/or you had a complete annual done on the gun?

I cut the Kolar supplied springs copying Kolar cut springs exactly. Full annual 2.5 years ago with reasonable rounds through the gun since.

Time to go back.