I may have the opportunity to dabble with some of the 20mm ordnance out there but know next to nothing about this size round. I need some really good TECHNICAL information about this, such as projectile and case dimensions, twist/groove/etc data on the barrels, all the goods

Who can help?

Many thanks!

Marty-Some info is in the back of Cartridges of the World-

Like AP incendiary , 1540 gr bullet at 3380 fps...Uses

WC-870 powder at up to 60,500 psi.I am now playing with them to make rimmed 4 bore brass and my belted 1000HE.

20mm cases that I have are 4.030 long. rim is 1.160. base is 1.150,

shoulder is 1.050, neck is .830. bullet is about .775.

You could call it the 10 GA FH....Ed.

Marty,

I know the guys that load for the 20mm Lahti will often use surplus 20mm Vulcan projos, and turn down the driving belts by a few thousandths. Also, I've read that guys who shoot 20mm Vulcan ammo will have a bushing turned and pressed into the primer pocket so they can use .50 BMG primers. If you like I have an old issue of Very High Power from around 1992, with an article on reloading for the 20mm. I'll see if I can find it and mail it to you.

-The Urosaurus

Ed, Urosaurus -

Thanks for the info, getting some good stuff from other sources as well. Due to the presence of the driving band, I have some questions on headspace.

Bore is 0.818/0.820 best I can determine
Projo is 0.775

Does it headspace on the shoulder of the case or on the driving band of the bullet?

Uro - copy of that article would be great, need that info on the primer conversion definitely ....

Thanks!

Marty-- From looks of fired cases, they headspace on shoulder.For primer adapter we are using 20mm primer cup
with electrical contact removed and reversed in case, crazy glued in and a ream job to allow BMG primers to fit.
Others have little adapter sleeves built for them.Lar soldered in a bushing that takes shotgun primers.His cases are low pressure.If the thin sleeves don't stay in, we will machine old BMG cases to make a heavy bushing, from the BMG base, and machine and thread it into 20 mm cases.ED.

Marty,
there are at elasst THREE 20mm in service today
the phalanx round, i have one of those cases.. it's the PPC of the 20's

the airforce 20mm ... i think it's the 308

the army 20mm... the 30-06 (though these last two might be switched)

i think the a10 cannon is like the 30x50 bmg , to ruin the analogy

jeffe

Isn't the A-10 32 or 37MM?

Tiggertate, the A10's cannon is a GE 30mm gatling gun. Fires DU projectiles at somewhere around 3700 fps.



jeffeosso is wrong with regard to the Air Force and the Phalanx systems. Both utilize the M61 Vulcan gatling gun and fire the same ammunition aka M103 case and M53 (API) M55 (target) or M56 (HEI) projectiles.



Tony which 20mm are you interested in. A modern round, or one of the WWII vintage, Oerlikon, Lahti, Hispano, etc?



Here is some data on the M55A2 cartridge. A friend of mine has a 20mm Vulcan, I can get more data from him, but that will have to wait till I can get in touch with him. If the Vulcan (M103) is what you are interested in, are there any specific questions you would like me to ask him?



Cartridge - M55A2

Projectile Weight - 1540 grains

Projectile Diameter - 0.79"

Projectile length - 2.98"

Case - M103

Case length - 4.02"

COL - 6.62"

Muzzle Velocity - 3,400 fps (milspec I believe he only gets around 2700 fps due to a shorter barrel).



Scott

Scott--The cases I built 1000 HE from and am building 4 bores from are the M-103 Vulcan cases.Does your friends Vulcan make the base of the case crooked on firing.The
Vulcan fired rounds I have seen are crooked.ED.

Marty,

If you want the article chop chop, let me know, and I can see about faxing it to you. BTW, did you get prices for the GPA projos yet? Did you contact John at Bridger about some .458 projos to load and chrono?

-Uro

I am toying with my own 20mm cartridge (because I can, in case anyone was going to ask, and some of my new clients are interested in this sort of stuff), but the information I had was scarce. Two well-placed posts have yielded copious amounts of information, the crux being the dimensions of the projectile. It would appear that the Vulcan projo is 0.78" diameter, but the OD of the driving band is what I need. Once I have that I can commence final design.

Uro - no need to fax, besides the fax is fubar anyway. On the GPA, I got the price list at SHOT, I am awaiting samples for testing. I have not yet contacted Bridger considering when I got to the shop after SHOT, Robert had fired up the screw lathe to make a stack of solid brass 300 gr boat tail spitzers in 458 caliber, about the same overall length as the Barnes 400 gr solid, figured I would test those first ...

Urodoji--These don't have bent rims, the whole base is
cock-eyed to the centerline of the case and bore.It's
as if the breech on the gun was slanted, and upon firing the case stretched on one side..All of the cases
LAR has are that way also.I straighten them with my case spinner/lathe, which shortens debth of the primer pocket,
to match BMG primers, with sleeve we use.Ed.

I used to fly an airplane that used the M61 cannon. They fire at 6000 rnds per minute, sounds like a sewing machine when you're shooting. The gun was cycled off of the aircraft's hydraulics and electrically fired as mentioned above. I suspect the slanted brass was due to the barrels being cycled off of the breechface (is that a word?) when there is still pressure in the barrel, just before the bullet exits. 6000 rpm is awful fast. Very cool sounding gun, a blast to shoot.

Boltman--Sounds like that may be the cause.Because
years ago I had some fired 20mm from older slower
different designed guns, and they were straight.That sewing machine as you describe it
was the ultimate big bore stitcher!!Ed.

Marty, The electric primed M-103 case has a "cup" around the primer wery like the old Winchester "Protected primer" Deprime the case, press primer out of cup, invert cup & press back into primer pocket (with small drop or red Locktite if you feel it's needed), chamfer edge & ream slightly with 50 BMG primer pocket uniformer. From there on the 50 primers work just like they do in the BMG case. Above procedure has been used for about 10 years now & I haven't heard of any problems.

Do you have info on:
River Valley Ordinance Works (RVOW) in MO?
Big Sky in MT?
Brashco?

They all have 20mm stuff in their own right and could help you greatly.

BMG

I have some information from a friend, but it is at work. Will post it tomorrow. The primer pocket seems to be slightly larger in diameter and deeper than the 50 BMG.

Ed,

His rims are straight and square with the brass.

Scott

Marty--My friend Bob Snapp did a necking job down to 50 cal
on 20mm brass, to make high speed(over 5000 fps) cartridge
for testing armour.Pic below.In pic on left is 50 BMG for comparison.And regular 20mm on the right...This was 35 years ago.I myself am straightening brass for a belted straight 1000 HE and
to make rimmed 4 bore cases.These will meet the term sporting I think.I may call mine Belted Four Bore---BFB---
We will see how it works out.Do have guys needing strong 4 bore cases.Ed.

A belted 20. Maybe the Polte Military ctg site has dimensions of it, too. Notice how much the 20 m/m kid grew in the Wermacht when it left Holland's Home.. That.50 FA headstamp- souvenir from corregidor. I stuffed the 1941 bullet in it for a show and tell in 1944.

That is a great looking belted case.Need to find a barrel
of them, and straighten out for the ultimate belted big bore.Here is pic of belted 1000HE made from 20mm brass.
On the right. For comparison we have 1st 30-06, 2nd 458 Win,
3rd my 700HE, 4th 4bore built from 20mm, and then 1000 HE
case..Ed.