I have come into possession of an 1874 Sharps by Pedretti & Sons chambered for 45 Colt.

The manual with this rifle says to use only cartridges loaded with BP, BP equivalent, or nitro for BP.  What gives?

A falling block action is as strong as they come.  This rifle looks like you could use level III 45 Colt loads or even ream it out to 454 Casull and it would be happy.

What’s the consensus on how hot a load it will take?

A Sharps action can crack apart in the thin part of the receiver under the barrel, along the internal angles
of those sponson-like sidewall buttresses that look so Victorian, or between the (supported by the hammer) half of the firing pin the hammer hits and holds down, and the (unsupported part of the firing pin) that's right in front of your eye while it hits the primer.

Use black powder level loads only in the Pedersoli, it is not built to handle high pressure loads. The proof marks on the barrel tell it all and none that I have seen have high pressure markings. A Italian made Sharps is not in the same league with a Shiloh, C Sharps or Jap Browning/Winchester when it comes to handling high pressure loads but in the same light the Italian guns will handle black pressure loads all day long. Do yourself a favor and buy a gun made for the pressures you wish to use, cheap insurance in the long run.

Quote:
What gives?  Lawyers!
* 45 LC has a maximum pressure of 14,000 psi
* All Italian rifles are proof tested.  A 45-70 has a maximum pressure of 18,000 psi.
* The rifle being a '74 Sharps would be proof tested at 24,000 psi
* Sounds like you want to shoot smokeless reloads...
* A 250gr Pb bullet with a velocity under 1050 fps would be safe haven
Trail Boss 4.5 gr.     606fps
Trail Boss 5.8 gr.     727fps
Blackhorn 206 22.2 gr. 869fps Max (full case)
Unique     8.5 gr.     963fps
42gr FFFg 1050fps