I've got an issue with my Pietta 1860 army not wanting to let the cylinder rotate at half cock. It rotates about a quarter turn then locks up. When this first started occurring I thought the action was fouled up (I normally only take the barrel and cylinder off and clean after the range) and took it completely apart, cleaned everything and reassembled, still the same.

Any idea what this could be? I've realistically only put about 50 rounds through this firearm since I bought it new, so it shouldn't be worn.

Edit: I should also note if I hold the hammer at about 1/4 cock It lets it rotate freely. It's almost as if something is not catching that should be. The bolt will actually stay up at half cock, which I don't think should be happening.

My first pietta wore out in about 50 rounds also. Cabelas sells a kit with I think all of the internal parts. Other than that I can't help ya.

Take the cylinder off and see of the hand sticks out or if the bolt extends too far when at 1/2 cock. for the cylinder to turn.  Check to see if the cylinder gets stuck at the same place every time which would indicate an issue with the cylinder.

It does not appear to be stuck at the same place, I can replicate it completely around the cylinder. I noticied the bolt is all the way up at half cock, it appears this is what is stopping it from moving. Shouldn't the bolt be recessed at half cock?

Your bolt is probably slipping off the nub on the hammer ..... it's either the tail of the bolt, or the protrusion on the hammer that is either broken or worn.

remove everything but the bolt, trigger, bolt/trigger spring, and hammer. Look thru the underside of the frame and see where the bolt disconnects from the hammer. It should disconnect almost simultaneously as the hammer reaches full cock.

Everybody loves to talk about "4 clicks"...... but I tune mine so the "3rd  click" (bolt disconnecting / rising) is almost imperceptible..... I don't like drag lines on my cylinders.

It appears the hammer protrusion is almost sheared off, the bolt slips past this protrusion at half cock and locks up. It would appear that this is my issue.

Any recommendations on where to source a new hammer?

VTI will have all the parts you need

https://www.vtigunparts.com/store/shopdisplayproducts.asp?id=70&cat=Pietta+1860+Army+1861+Navy

Some tips on timing/tuning when you have to replace the bolt or hammer:

(cont)

I have (my son has, actually) a Pietta '51 that the exact same thing happens when the hammer is at 1/2 cock: the bolt has slipped off the cam allowing the bolt to rise into the bolt slots on the cylinder.

Ours also started happening at around 50rds.

Looking into the bottom of the cylinder frame at the moving parts with the grip frame removed the bolt is falling off the cam on the hammer at the very tip of the bolt leg; as if the leg isn't long enough or the cam (not the cam surfaces, but the actual cam itself) is in the wrong location on the hammer.

I keep forgetting to get the parts ordered to fix it.
We've just been cocking the hammer to "1/4 cock" to drop the bolt and advance the cylinder for loading.

This is great information, thanks for all the help!

@Villafuego, which reference book is that?

It's David Chicoine's "Gunsmithing Guns of The Old West"

Out of print, but pops up on Ebay every now and then......