Got a sales e-mail this morning from IMA and found this for sale. One of the competitors to the original Allin trapdoors.
Springfield M-1863 Rifle Converted to Robert's Patent 1867 Breech loader
https://www.ima-usa.com/products/original-u-s-civil-war-springfield-m-1863-rifle-converted-to-roberts-patent-1867-breechloader?variant=12159861817413&trk_msg=F6U5LM3O8U045ERR0GURQQCL00&trk_contact=6EJ5NRGQIMV1SSVNVA58085SDK&trk_sid=UVQO6JVM49UKKGBBMDO1PT9MOK&utm_source=Listrak&utm_medium=Email&utm_term=Product&utm_campaign=20180518&utm_content=20180518 (https://www.ima-usa.com/products/original-u-s-civil-war-springfield-m-1863-rifle-converted-to-roberts-patent-1867-breechloader?variant=12159861817413&trk_msg=F6U5LM3O8U045ERR0GURQQCL00&trk_contact=6EJ5NRGQIMV1SSVNVA58085SDK&trk_sid=UVQO6JVM49UKKGBBMDO1PT9MOK&utm_source=Listrak&utm_medium=Email&utm_term=Product&utm_campaign=20180518&utm_content=20180518)
Those are quite rare and a fascinting piece of history. While the rifle was not comercially successfuly, the .58 Roberts cartridge was used by Remington in some of their Transformed rolling blocks. These were sold to state militias and overseas. I have a very early Transformed rifle and shoot it a fair bit. Here is the .58 Roberts cartridge next to .32 long colt and another next to a .22 short for comparison.