.90 cal Ammunition


Does anyone have any info on the US .90 cal T1-T4. I’m trying to find out about the weapon systems, ammunition used or any other relevant info. I recently found one at a gun show with an inert projectile and fuze but full up otherwise. Any help would be appreciated. Thanks, Bruce.


There’s this:

From Anthony G (Tony) Williams’ site
The US also produced several automatic guns in the .90 inch series (designated T1-T4) of which the 23x139SR for the T4 is by far the most common, but these remained experimental.

And then on page 9 of this:

scribd.com/doc/3917160/Ospre … llery-1941

Apparently, these guns and this caliber were never put into service. But all I’ve read are these two bits on it, so who knows.


Great topic. Not much to be found on this caliber other than the examples of the T4 that show up at times.

I seem to recall reading somewhere that one or more of the earlier (T1-T3) types was a rebated rim version for an API type aircraft cannon. Does this ring true? Does anyone have any of the T1-T3 items to show a picture of? I’d post a pic of my less than pristine T4 exmple but am not home now. Mine lacks the cool text printed on the case.



As a result of this question I have been doing some more digging in my memory - and my files.

I have found a copy of an undated draft report on the .90 inch gun project, unfortunately it is very short and omits a lot of detail (especially about the ammo). This is a summary of the highlights:

The project commenced in 1937 to develop a .90 inch machine cannon for aircraft use. The initial target was a muzzle velocity of 2,850 fps, a RoF of 150 rpm and a gun weight of 125 lbs. Although the report does not give details, it seems the required RoF was soon increased considerably (one of the Lenselink/De Hek cartridge books states 400 rpm). As a result, the MV was reduced to 2,700 fps to enable a shorter cartridge to be used. A blowback mechanism was selected for T1, the first design, but the gun was never built - it was just “a hypothetical model from which other designs were elaborated”.

Design studies for T2 and T3 were submitted simultaneously by the Artillery Division, Manufacturing Service in January 1938. In T2, the cartridge was fired when the it was still 0.6 inch from its “home” position in the chamber. This is referred to as the “differential recoil” principle but today we would call it Advanced Primer Ignition Blowback (i.e., Becker type). Presumably the cartridge case would have had a rebated rim - either that or a massive base construction to take the pressure when unsupported by the chamber.

T3 was a variation on T2 in which the round was fired 0.125 inches from its “home” position, and used counter-recoiling buffers (exactly how is not clearly explained).

T2 and T3 were developed at the Watervliet Arsenal up to 1942, with a new lightweight gun being mentioned in July 1941 (whether of T2 or T3 is not stated).

The ammo which is commonly available to collectors is the 23x139SR which was only used in the T4, “the only model…to reach the pilot model stage and to actually be tested by firing”. This was a recoil-operated gun similar to the 37mm M1A1 (Browning-pattern long-recoil) designed for a two-way feed by flat clips or a flexible belt in a drum. It was made by Colt (according to a hand-written note on the report). Design seems to have been initiated in 1940 and it was tested in March 1941 at Aberdeen Proving Ground. Certain changes were recommended as a result, but nothing more seems to have been happened. Curiously, it was being considered as an anti-tank gun at that time.

In addition, I recall long ago seeing a photo of the ammo for T2 and T3, but unfortunately I don’t have a copy in my files. As I recall, they were bigger than the T4 round, and may possibly have been developed from the 1.1" AA gun case. No doubt someone on this forum knows more!



Thank you for the additional information. A couple of thoughts…

Regarding the T1 being conceptual with no guns built, it is interesting to note that in issue 465 of the Journal, Fred Butt provided a picture of a crate marked as .90 Cal. T1 Proof. This would lead one to believe that there would have been ammunition made in this version if a marked crate was fabricated.

The fact that Watervliet Arsenal developed the T2 and T3 has given me the idea it’s time to visit the museum there (haven’t been there since before they closed it up after the 9/11 events) and see if I can make some new friends. I don’t know what access they have to the archives, etc. but it should be interesting to find out!

Perhaps others will weigh in with more info on this one.



I took a moment to look over information I had available on this topic.

First off, I need to correct my thinking on Fred Butt’s crate in Journal issue 465 on page 29. Closer inspection (I think I needed to clean my glasses…) reveals that the markings indicate:


(The “4” in “T4” has a stronger vertical component). Based on that realization, I would guess it once housed the Cal. .90 items like that here:

Headstamp reads: “CAL. .90 T4 4430 -F.A.I.W.P.B.” w/ crossed cannon. I’m guessing the “F.A.” is for Frankford Arsenal but must confess my ignorance as to what the “I.W.P.B.” stands for. Anybody?

Also, I accessed George Chinn’s “The Machine Gun, Vol. 1” where on pages 607-609 there is information on the T1 and T2 guns and images of the T2. It indicates the T1 gun was of the long recoil design and the T2 was a straight blowback design. It goes on to state that the T2 resembled the Oerlikon design but was not identical and that firing was initiated with the bolt only .003 inch out of battery. The T3 is mentioned as being so similar to the T2 that no description needed to be added. No mention is made of the T4 or the ammunition for the T1-T3 other than MV and bore diameter.

This information seems to be in conflict with the report that Tony cited. Now I feel I gotta keep diggin’…


Edited to correct spelling.


Please do…on closer examination, the draft report I quoted seems to have been an unofficial study, by someone at Lafayette College. I’ll scan it and send you a copy.


[quote]Headstamp reads: “CAL. .90 T4 4430 -F.A.I.W.P.B.” w/ crossed cannon. I’m guessing the “F.A.” is for Frankford Arsenal but must confess my ignorance as to what the “I.W.P.B.” stands for. Anybody?
Dave , notice the I has no dot but has to be read slash, F.A. /, W.P.B. These initials should be from the inspector. Please keep diggin’


Yep, it’s very strange that Chinn failed to mention the T4 at all, despite the fact that this seemed to get closest to production - and of course, the ammo for it is common whereas that for the earlier designs is pretty well non-existent.

I have to say that while Chinn is an essential resource it also contains many errors and omissions, so is always worth checking against other sources (while being careful to ensure that the other sources didn’t just copy from Chinn, as a lot did!).



Thank you for that clarification. On closer inspection, the marks around the “I” aren’t really dots. Now if we could only find “W.P.B.” to interview on the subject!


Had missed it before but in Chinn’s Vol.3 there is a very brief section on the series under “Caliber .90 Machine Guns with T Numbers” on page 30. There it indicates:

The T1 was built at Rock Island Gun Arsenal and used a necked down 1.1" case for 2,850 fps and 150 RPM.
The T2 series (including the T2E1 and T2E2) were based on the Oerlikon design and developed by Watervliet. No mention of ammunition.
The T3 was similar to the T2 but included a blowback retarding device.
The T4 was built by Colt based on the Browning long-recoil cannon and while intended as an aircraft cannon, was considered for anti-tank use. Again, no mention of ammunition.

Some of this information seems to fit with the report you have. Thank you for offering to provide a copy of that. I’ll see if I can make any contact with the folks at the Watervliet Arsenal Museum regarding archives, etc. and will post any relative findings.

Also, I failed to mention that Fred Butt’s crate is also lot marked “4430” which is the same as on the headstamp I showed above. Any other Caliber .90’s out there in collections with a different number? Maybe Fred’s box is the source for all of them!..



My .90 T4, purchased from Jim Tillinghast in the 1990’s, shows the same lot 4430 on the headstamp.

Stenciled in black on the head: AMM. LOT E-4430-2P-
Stenciled in black on the side of the case: HERC. NH-M2 HERC. LOT 4148_1939 CAL. .90G T4 MV 2700
Stamped on the projectile: LOT 2536-1 F.A. CAL. .90, T1, E.D.B.


Didn’t you get it? I sent it a few days ago.



Thank you very much for the info on your example’s markings. Hope some other folks can chime in with numbers from their Cal. .90 items.

I’m guessing the number on the case side is a loading (perhaps powder) lot and the the popular “4430” is case and/or primer related as it is stamped in the brass too. The crate referenced earlier has the full lot number “E-4430-3”. Again, I guess. The “E” would be for experimental?

I have a feeling there are not too many varieties of T4 lot numbers that survived and it would be interesting to see if there is much for variety at all.



I dug out the catalog from which I purchased my round hoping that it would list more data on the cartridge. Unfortunately it only gives the headstamp. I’m not sure what these sell for today but it was only $17.50 in Jim Tillinghast’s List #23, November 1988. I wish I could go back and buy a few more!


Looking at the similarity in design? of the US T 1-4 Cal.90 cartridges and the Soviet 23mm VYa Anti-Aircraft cartridge ( calibre and case size), and the idea of a “23mm” in any case ( the Soviets at the time were using some pretty Low Power 20mm Aircraft cartridges), would anybody know as to any connection has been assumed, confirmed, or even thought of, between the US and USSR Calibres.??.. WE do know that the Rimless 23x115 NS Cartridge was derived by “Necking up” the 14,5x114 AT cartridge…But the totally different Rimmed 23x137 AA cartridge …is it a “parallel development”? or a " upgrade" of the German 20x138RB ( " 2cmFlak30 & 37)???

Anybody of the “over 20mm” Specialists have any comments?

Doc AV


The first 23mm calibre brass-cased cartridge may have been the British .9 inch Elswick (23x198R) developed around the end of WW1, probably for anti-tank purposes.

The first of the 23mm automatic cannon cartridges was probably the 23x106 Madsen, made by necking up and shortening the 20x120 case. This was offered in an aircraft gun and featured in many aircraft armament proposals (including American ones) in the 1930s but seems to have seen little or no use. Compared with the 20mm, the 23mm sacrificied some muzzle velocity to obtain a much more destructive shell; it contained about double the HE quantity. Possibly this inspired the US .90 inch and Russian 23mm developments, and also the French experimental HS 406/407 guns (23x122).

The Russian 23x152B was developed for the VYa aircraft gun which saw extensive WW2 use in the Il-2 ground attack planes, as well as some others. Neither the gun nor the ammunition appears to bear any relationship to the US .90 series; the two were developed in roughly the same time period and it seems very unlikely that the design teams will have known about each other’s projects. The case dimensions are also completely different from the 20x138B (the cartridge case was originally rimless, but modified during development to solve a technical problem).

The Russian 23x115 emerged late in WW2, a relatively simple matter of necking out the 14.5x114 anti-tank rifle case to take the shells from the VYa.

You can see them illustrated here: quarry.nildram.co.uk/collect … 3-28mm.htm


U.S. 90 Cal. I have a fired T2 case H/S CAL 90T2E 37 ORD.Mark F.A.
size same as T4 but rim dia.29mm same as base ,deeper and wider ext. groove