Well, I am guilty of an assumption. I had assumed that because the 43 Spanish chamber casting shows the botleneck I had assumed all bottlenecked cartridge chambers did. Spain did reload their cartridges, so often that many surviving rounds have blurred headstamps. So, I assumed. The idea of a bottle-necked case having a straight chamber hadn't occurred to me.
Then today, new information came to me. I was told it was not that the Baby carbines chambered in 44-40 had short chambers, but rather instead that their chambers were shorter than the chambers of 43 Spanish rifles. So where i was thinking a 44-40 chamber would be not more than 2" (based on an OAL cartridge of 1.92"), actually 'shorter' means anything less than 3". So I combine your information about your friend's Comblain with straight chamber walls and I did more digging.
The Baby Carbine used a type 1 1/2 frame. Other military carbines mostly used the type 1 frame. Mr. Layman's book and other sources merely say the Baby Carbines had a thinner frame than the Type 1. But he provided no measurements.
Today I found this (very valuable info IMO).
(You need to Login or Register to view media files and links) There's a comparison photo, but I will quote "oldremguy"'s measurements from there below..
"The top rifle is a Sporting Model Creedmoor Musket chambered in 44-90???? The thickness at the front of
the receiver is 1.22", the thickness at the back of the receiver is 1.24". This rifle has a longer bottom tang.
The 2nd rifle from the top is a No.1 Sporting rifle chambered in 32 Long rimfire. The thickness at
the front of the receiver is 1.23" and at the back of the receiver is 1.24". This rifle has the longer bottom tang.
The 3rd rifle from the top is a NYS Model rifle chambered in 50-70. The thickness at the front of
the receiver is 1.32" and at the back of the receiver is 1.36". This rifle does not have the longer bottom tang.
But some of the early made rolling block rifles that were chambered in .58 rimfire or centerfire using Model 1861
and 1863 Springfield musket parts (wood, barrel, barrel bands, buttplate and rear sight) do have a longer bottom tang,
as found on the sporting models.
The 4th from the top is a baby carbine, built on a No. 1-1/2 frame in 44-40 caliber. The thickness at the front of
the receiver is 1.14" and at the back of the receiver is 1.13". This carbine does not have a longer bottom tang."
At last, numbers I can use. What are my gun's measurements?