They were made using the melted down blue-grey steel of the sword, Excalibur, and have the rose, the sign of the Eld, engraved in the side. The sandalwood grips of the gun have never lost their fragrance and near the muzzle of each gun can be seen scroll work which translates to "White", which was Arthur's dinh mark.
However there are many other legends to their exact origins and some say the steel and sandalwood are not from Mid-World and brought from an alien world. Other believe the guns came from same Kashamin pyramid that Arthur and his sword Excalibur has been entombed in, with others saying they were a gift from the Dark Tower.
The guns are extraordinarily large by the standards of modern pistols. They are described multiple times as being "comically large". The guns shoot .45 caliber bullets and our world's equivalent of Roland's original bullets are Long Colt .45's.
A few times in the series, Roland has referred to the guns as "the widow makers".
As Roland had to present a sign of Arthur Eld upon entering the Dark Tower, he presented one of the sandalwood guns, which allowed him access (the other having been taken by Susannah when she left to find her own world and her own destiny). He originally wanted to present the Horn of Eld, but he lost it after The Battle of Jericho Hill.
From the way they are usually depicted and described, they are likely twins of the Colt Single Action Army model, otherwise known as The Peacemaker or the SAA in .45 Long Colt. or maybe a .454 casull like this one []But in Midworld, anything is possible.