Sorry if i am not following site rules…newbie just trying to find some help.
Can you provide dimensions? Do you have a micrometer or calibers to measure the diameter of the projectile, case body, and the rebated head?
This should be a 30x136RR. Will try to find it in my files if noone will be faster with a propper ID.
Length is 8 inches. Taking calipers from work home with me today.
No visible headstamp
What is the exact case length?
I will provide more measurements this evening. It is at home.
Any idea if it is old?
It is a 30 mm round developed by United Shoe Machinery Corp. for the T168 gun series. Looks like the short variant with 119/120 mm case.
Wow!!! Thank you so much.
Any idea how old and/or value?
"Here are two assembled rounds and two empty cases from the U.S. Shirgun development program. This program began in the early 1940’s centered around development of a 20mm aircraft cannon. The Navy let a contract to the Schirgun Corporation and somehow, in spite of the fact that was the spelling on the company letterheads, the Navy changed it to Shirgun and that name carried forth. A prototype EX 1 weapon was developed, but the program died in 1949, only to be revived in 1953, as both a 20mm EX 5 weapon and a 30mm Mk 3 Mod 0 weapon. The 20mm system used the standard 20mm Hispano-Suiza cartridges, but as you can see, the 30mm system went down a totally different path. The program once again was later dropped. The left hand 30mm cartridge is the electric primed T268 round with an FAT32E1 case that is about 133mm long, a T306E10 projectile that was fired from the T168 gun under an Army parallel program. The next cartridge has a 119mm long electric primed case from the FAT32 series and a Practice projectile. I do not have the model numbers for most of the rounds components. The first empty case is 136mm long. The far right case is the original FAT32 case design and it is 123mm long. None of the cases have a headstamp. Unique rounds at best. The right hand cartridge is a 30mm DEFA round for comparison. "
So, let me hazard a pure guess here: Very rare?
May I guess “very uncommon”. I go to a LOT of shows and I’ve never seen such a thing.
What was the engineering reason for the tapered/rounded base with the extremely rebated head?
That is really cool! Never saw one of these. Neat find!
the basic idea is the Becker principle. The cartridge is fired while it still is moving forward in the chamber. So recoil first has to stop the forward moving bolt. This is intended to reduce felt recoil.
The case has to be cylindrical to make longitudinal movement in the chamber possible. But you also need room for an extractor to be able to pull out a misfire from the chamber. The extremely rebated head leaves enough space for a strong extractor between chamber wall and extractor groove.
Interesting. I’ve not heard of the Becker principle before… looks like I have some homework for this evening! I can imagine the bolt thrust and PSI on the bolt face must be enormous! All that pressure against such a small surface area (compared to a “normal” cartridge-case sized head). Thanks for the explanation.
I understand that a severely reduced head of the cartridge leaves more space for a bigger and therefore stronger extractor. But why then other rimmed and rimless systems exist? Also small head saves metal for each cartridge.
Past Gunbroker auction, Item=458056619
Inert US experimental 30x120mm rebated rim cartridge that was developed for a prototype aircraft cannon that was never adopted. Very scarce cartridge. Point-detonating nose fuze unscrews. Brass cartridge case.
(Note: The fuze description is incorrect, the fuze is a solid aluminum dummy fuze)
Starting bid was $400, no bids.
I would certainly classify these as “rare” at least.
Granted I have not been in the collecting game as long as many of the others here, but I have been searching for one of these for nearly 10 years and only recently found a complete TP and an empty case myself. On top of that I have only seen another 2 in collections. Just my 2 cents.
Lovely cartridge as well, love the experimental stuff myself!