There is indeed a demand for field carriages. Lots of folks want one but very, very few are willing to pay the price. I get requests all the time for a field carriage but when I give a price estimate, I never hear back from anyone.
In addition to the wheel issue, there is the ironwork issue. A properly made #1 field carriage has a whole lot of ironwork that is difficult to make.
Look at these pictures of Max Caliber's gun. I lifted these from one of his very early postings here on the forum. Look at the metal work alone. Look at the metal to wood fit. Look at the number of parts.
Max, this carriage is an incredible example. Very obviously, hundreds of hours went into this work of art.
There are ads in "The Artilleryman" magazine for an ironwork set for a #1 carriage. It cost $3000. That is for full scale, but smaller scale is the same number of pieces with the same degree of difficulty in each piece.
This was an early attempt by me to replicate an 1841 six pounder on a 1/2 scale carriage. Please excuse the wheels. They were my first attempt many years ago. The barrel and most of the hardware here are made of wood. That took a long time too, but nothing like making it out of iron. Even the leveling screw is made of wood.
Zulu