I once had an Italian-made 1860 “Colt” with the very same problem.

I fixed it by removing some metal from the frame where the hammer falls.

This allowed the hammer to travel a few thousandths of an inch further forward, therefore striking the caps more firmly.

Problem solved!

 

You sure it’s got the right length nipples?

 

Maybe show us a struck cap in situ?
And a nipple with no cap.

 

The hammer must extend into the cylinder area enough to strike the nipples. The frame or the hammer is not correct. It is possible the hammer was filed back to prevent firing. Pictures would be a big help.

 

It is possible the hammer was filed back to prevent firing.

That's an interesting point.
I run into a fair amount of firearms here in Arizona that have been rendered 'safe' in just such a manner. Pins, nipples, hammers and strikers.

Some I know to have been done to keep Grandpas outa trouble. Some done by Grandpas to keep kids outa trouble.

 

Thanks, everyone for your help and suggestions. I agree that it's the hammer/frame causing most of the problem, and partly the replacement wedge. There's a fair amount of cylinder gap also. Thanks again.

 

It 'looks' like the striking face of the hammer has been working in the past but it is currently not running as far forward as it should.

Can you show us a photo with the hammer drawn back and showing the inside of the frame where the hammer rests when forward?

I wonder of anyone has replaced or otherwise messed with the cylinder pin. After that, I'd wonder about internal debris catching the hammer short.
Here's a properly resting hammer on a 2Gen Colt. Note how much farther forward it is resting than yours. You can click on my pick and scroll directly through yours for comparison.

 

Make sure the cylinder pin has not been screwed too far into the frame to the point where it hits the hammer and causes it not to go far enough forwards.

 

A cap that doesn't set down flush on the nipple often will not go off. Make sure the nipple cone is correct for your cap.

 

I think he answered his own question in first post and #7

"Non matching wedge resulting in gap at barrel cylinder junction."

If you can see the gap then cylinder is being pushed too far foward out of reach for strike.

 

I purchased a Colt 2nd generation 1851 Navy "C" model that doesn't strike the percussion caps hard enough to ignite them. The wedge is not original to the gun which may be part of the problem. I have a new one ordered from Taylor's, but I don't believe that is all the problem. When checking the distance between the dropped hammer and the back of the nipple, I noticed that the striking face of the hammer does not extend past the inner surface of the recoil shield - like it might not be reaching the cap and nipple to create ignition. The hammer won't even engage the little locking pins between the chambers.
Is this the problem? Is the hammer mis-made/defective so that the striking surface doesn't extend far enough to hit the percussion cap, or is there another cause?
I removed some of the nipples and found that at some point small washers had been placed behind the nipples in an attempt to extend them closer to the hammer.
Gun came in the early tan box.

Is this 51 Navy a true 2nd gen made by Colt or a Signature Series which were not made by Colt , but licensed by Colt and were greatly inferior to the true 2nd gens (which were for the most part made from Uberti parts by the way)
Are you using the correct size caps and made by a reliable source? Have you checked the mainspring?

 

I measured the hammer face protrusion beyond the recoil shield on two new very lightly used (I might be the 1st one to fire them) Colt 2nd gen C series among the 8 I have and my dial indicator reads: 0.108" on the 7xxx one and 0.113" on the 4xxx one. My 3rd early C series is much more heavily used (CAS frequently and for many years) is off at the gunsmith having a loose arbor repaired. Both of the ones I measured are operational and were test fired when I got them with the factory nipples, as they're destined to be in my stable of CAS guns and modified accordingly. I have another C series gun, much later in that series, (21,xxx), but it's also been used in SASS with a filled hammer slot, Manhattan conversion & Ruger hand coil spring & plunger. Its hammer face protrusion beyond the recoil shield is only 0.076", but some of that smaller protrusion may be due to the Manhattan conversion.

Make sure the one you have coming from Taylor's is for a Uberti pistol. I have an Italian copy (made of left over parts Uberti supplied Colt for these pistols that I needed a new wedge for. I've checked and dimensionally they're the same as the Colt parts, (if as some claim Colt sourced some of their parts locally).