Hi Dean, according to owners manual, you can use any ammunition that is within SAAMI specs. No where in the entire manual does it use the terms "cowboy ammo only". It is unlikely than any standard factory ammo from a major USA maker like Federal, Remington, Speer, Winchester-Western is going to damage your gun, unless the conversion cylinder’s maker specifies only so called mild “Cowboy Action Shooting” ammunition is to be used. So, the answer to your question is, yes, Signature Target 45 Long Colt, 250 grain, Total Metal Coating will work fine in your Uberti 858. Stay safe & have a great day!

Hi Dean and welcome. Any standard load that is intended for SAAMI spec will be safe in your Uberti 1858. There will be no issues and the overall pressures will be slightly lower than similarly loaded lead projectiles with significantly less lead fouling. I have a Similar Pietta and have experienced zero issues with TMC loads. Cheers and stay safe!

Dean, I wouldn't recommend using them. Uberti recommends lead projectiles with a velocity under 850fps. The signature ammo only has a velocity of 842fps but it has a total metal jacket. I'd recommend a lead cowboy load. Here's a link to a cowboy load by fiocchi that would work if interested. Sorry but hope this helps, link here.................... budsgunshop.com/product_info.php/products_id/76207/fiocchi+pistol+shooting+dynamics+45+long+colt+cowboy+250gr+50rd+box