Hi guys, thanks for tuning in to another video on forgottenweapons.com, I’m Ian Mccollum. and I’m here today at the rock island auction house because they have this monstrous 20mm full auto anti-aircraft cannon they’re going to be selling it in their may of 2017 auction and I thought it would be cool to take a look at a few fun facts about the mark 4 mod 1 Oerlikon cannon. Interesting fact #1 would be that these guns were used by both sides in world war 2 Probably more extensive by the United States navy by anyone else but the germans, the americans, the british and the japanese and the italians all had Oerlikon Cannons. In fact, between December 1941 and September 1944 these guns were responsible for 32% of all japanese aircraft shot down by the United States. That is quite a substantial percentage. Interesting fact #2 is that these are actually open bolt guns. Meaning there is no locked breach, despite this being a massive 20mm cartridge. The way this works is actually what is called advanced primer ignition. Now, this is different than what most people think of on submachine guns On those, you’ll have a fixed firing pin that will fire a cartridge immediately upon chambering it. Well, the chamber on this gun is a bit different. It’s an extended, long chamber that’s actually longer than the cartridge is. and the cartridge has a rebated rim so that the entire bolt head fits inside this long chamber. So the cartridge is fully supported, sliding forward, into the last bit of the chamber and still moving when the firing pin protrudes forward, fires the cartridge. The recoil energy then has to counteract the forward movement of the bolt, and then start pushing it backwards. That reduces the felt recoil and makes this a much simpler gun to mount and use. Now, the downside of that arrangement is that its got a really heavy recoil spring in it. Because that is the only thing holding this gun closed. And I’m not kidding when I say really heavy. With the round recoil spring wire, like we have in this gun now, When it is fully locked back, the recoil spring is exerting 396 pounds of force forward on the bolt. and that begs the question, how are you supposed to cock this thing? The bolt right now is all the way forward, so how do you get it back? Its the equivalent of having to dead lift 396 pounds against the gun in order to cock it. So the bolt is connected to these action arms, one on each side of the assembly which then come up to this sleeve which retains the front of this huge recoil spring and in order to cock it, this whole thing has to go backwards. Let’s see if I can get it started. Aaargh, all right, I can get it to move a little bit, but that’s not going to work. Helpfully, they put this little tiny cocking handle on the back of the recoil mechanism. So, what do you do? You take your cocking rope and you hook it onto that hook, You then elevate the gun all the way up to vertical, And then the world war 2 pedestal mounts had a little pully, right here. So you take your rope from this hook, around the pully, and then hook it to the base of the mounting pedestal. Once it’s in place, then you have the leverage from the shoulder rests, pull the gun back to horizontal, That is going to pull your charging handle all the way back and cock the gun. In case that didn’t work, you did have two other options. One was a big old block and tackle system that you could pull out of a crate somewhere and bolt to the side of the gun. And use that to pull the charging handle back by hand without the extra leverage of the mount. The other was actually a ratchet system that you could bolt onto the back of the gun and use a geared ratchet to slowly pull this back with a big lever. Fun fact #3, these were primarily naval guns, which meant they had muzzle covers. But If you’re out at sea and its below freezing and there’s spray coming over the gun You’re going to get ice on the gun. Well what happens when you get ice on the muzzle cover and it freezes to the barrel and you can’t take it off? You’re firing high explosive projectiles so you probably don’t want one of those hitting a big block of ice at the end of the gun. I think that would be bad. More to the point, what if some sailor just forgets to take the muzzle cover off and fires a high explosive round right through it. That stands a pretty decent chance of detonating on the muzzle cover, and that’s a really bad day. Well, they did think about that, and the answer lies in loading the drum. This, by the way, is a 31 pound, empty, drum when full it holds sixty rounds And, the way you’re officially supposed to load this gun is 58 rounds of high explosive in the drum and the last two rounds you load, to top it up to sixty, are both inert bullets. So no high explosive, just a solid projectile. And, according to the manual that will allow you to shoot through, safely, up to 1 inch of ice holding the muzzle cover on. Or, if you just happen to be an idiot, firing through the muzzle cover that you forgot to take off. Two rounds will make sure to clear the muzzle cover enough that you don’t have to worry about blowing yourself up with the next HE round in the drum. Fun fact #4. What is the official navy position on mag dumps from the 20mm Oerlikon gun? Well, the answer is that you could drop five magazines through this as fast as you can and not really do any damage to the barrel. At nine magazines, however, dumped as quickly as you can, You would seriously damage the barrel. So, it’s recommended that you lay off a little bit in between drums. Or, alternatively, change out the barrels. One of the advantages of the Oerlikon gun is that you can actually change the barrel out in about 30 seconds. The other alternatives out there like the hispano suiza required a significant teardown to replace the barrel. Oerlikon, not so much. Grab your spare barrel, throw it in, nice cold barrel and you’re good to go. Alternatively, according to the manual, if you don’t have the spare barrel handy Or you can’t change it, dump water on the gun and keep the barrel cool that way. Well if you happen to have a patrol torpedo boat that is in need of some new armament or perhaps, way too much free space in your living room… and you need something to fill it, take a look at the description text below You’ll find the link there to rock island’s catalog page on this particular gun It is, of course, an NFA registered machine gun But that won’t stop anyone that can pass the background check from puchasing it. So, take a look there. Ah, you can place a bid online, although for something like this you probably want to come down and and participate in the auction live. Thanks for watching!