Pete et al. Well, I purposely was avoiding this thread, as these rounds confuse the h–l out of me. But, I will give it a try.
Firstly, I believe it is an error to intimate that UMC auto pistol cartridges with no headstamp and a “U” primer are “prototypes.” In some calibers, they simply exist today in far greater quantity then they would if they WERE prototypes. That is not to say that none of them could be, but it would require a lot of provenance to prove it one way or another. In the case of 7.63 Mauser, I have a box with cartridges packed on C96 stripper clips, all rounds having no headstamp and a “U” primer. It is obviously a serial-production box with serially-produced cartridges.
Now, the UMC .25 auto shown on this thread is described as haveing a “bullet appearing to be lead.” I have never seen a “U” primer, unheadstamped .25 with a lead bullet, although that of itself means absolutely nothing. Cartridges I have not seen in my own field, but are known to exist, are legion. It is the doubt registered in the description that becomes a confusion. Regardless, I agree with Pete that the dimensions given are indicative of a standard .25 Auto Pistol cartridge.
Now, regarding “unknown2.” A couple of things make it difficult to decide which cartridge this is.
Firstly, the picture is not clear and it is hard to tell if there is a case-mouth crimp or not with my old eyes. I would bet that it is not crimped. If uncrimped as I THINK it shows, then my specimen has the following measurements:
Head (rim): 0.2995" (7.50 mm)
Base: 0.3015" (7.66 mm)
Mouth (neck): 0.301" (7.65 mm)
Bullet diameter at case mouth: 0.280" (7.11 mm)
Case Length: 0.612" (15.55 mm)
Overall Ctg. Length: 0.9075" (23.06 mm)
I would consider this well within specifications to be the same cartridge as “unknown2.” The question is, what did UMC call it? It appears to me to be the cartridge from Mar 1904 labeled in the UMC Register as the “.30 Cal. Browning,” even though the entry shows a bullet diameter of 0.288". Without pulling the bullet from the case, which I will not do, it is hard to know its true diameter at the largest point. Case mouth measurements are often smaller than diameters of bullet bases.
However, there is a very similar cartridge, also with no headstamp and “U” primer that has the following dimensions:
Head (rim): 0.2975" (7.56 mm)
Base: 0.2985" (7.59 mm)
Mouth (neck): 0.295" (7.49 mm) (measurement taken below heavy roll crimp)
Bullet Diameter: 0.259" (6.58 mm) (measurement taken just about very, very thin line of lead
from half-mantel bullet base. This is a Smith & Wesson-type bullet with half-mantel bullet
having two opposing-lead flanges just above the base of the bullet jacket)
Case Length: 0.6165" (15.66 mm)
Overall cartridge length: 0.895" (22.73 mm)
Unfortunately, the UMC Register, often oddly lacking in technical information, does not tell the bullet type for the 7 mm Browning, produced in July and August of 1908. It does seem to mention (I say “seem” because it is hard to read) that the case length is the same as the .25 caliber Browning, and it IS certainly close to that. My UMC no headstamp, “U” primer .25 Auto round has a case length of 0.6175" (15.68 mm), well within specification tolerances with the above cartridge. Can this cartridge with the heavy mouth crimp and half-mantel, Smith & Wesson-type bullet, be the “7 mm Browning”? I don’t know.
Then there are the rimless .32 APs, one with “UMC” headstamp and “.32 ACP” caliber designation and one with “REM-UMC” headstamp and “.32 AP” caliber designation. But those are for another question, another time.
All dimensions taken as carefully as I can with a Lyman electronic digital caliper.
I know I have not positively answered your question - as I said, this subject is a confusion to me, and many others that I know. From the scant documentation available, it is difficult to be dogmatic with one’s answer.
Edited to correct spelling error only