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This may have already been answered, but why are so many modern assault rifles gas-operated, when blowback-operated designs are (generally speaking) simpler/cheaper to manufacture and require less maintenance? I've been doing some research and can't seem to figure out why for the life of me. Any assistance is greatly appreciated.
Let's say you're developing a tank with a unique (AKA non-historical) gun for one of our competitions here on SH. It would be nice to have an idea of the size of the gun, its shells, and what their performance both in terms of shell weight and velocity but also penetration, wouldn't it? Well, fortunately there is a way to do this with reasonably accurate results using your solid modeling software and some free to use browser tools. First, you want to have a general idea of the size and performance of your gun. For this example, I decided I wanted an optimized, high velocity 85mm caliber gun with a case about as big as the 7.5cm KwK 42 (as it happened, I ended up with a case that had significantly greater volume, but that fact is unimportant for this example). The cartridge I decided on has a 130mm wide rim and a 640mm long case, of course in 85mm caliber. My first step was to model this case in SolidWorks: You will also need to model your projectile, in this case a tungsten-carbide cored APCR round: Next, we need a bit of freeware: A Powley computer. Originally developed by DuPont engineers for small arms ammunition, the Powley computer is an accurate enough tool to use for much larger tank rounds as well! When you click the link, you'll be greeted with this screen: You'll note the dimensions are in inches and this thing called "grains" (abbreviated "gn"). The grain is an archaic Imperial mass unit equal to 1/7000th of a pound which is still used in the small arms field, today. Another quirk of small arms has the case capacity - a volume measurement - listed in grains as well. This is in fact grains of water (gn H2O), or the weight of water that will fill the case to the top. To find this, simply multiply the volume in cubic centimeters by 15.43 - which is also the exchange rate between the metric gram and grains mass. Finding the volume of the case is easy with a solid modeling program; simply model the interior as a solid and find the volume of that solid: Filling in my Powley inputs gives me this: Note that I typically use the diameter of the projectile across the driving bands for "Bullet Diameter", but it really makes very little difference. So far, though, we haven't actually produced any results. That's because our gun is well outside the bounds of DuPont production IMR powders, hence the output "Much slower than (IMR) 4831" in the lower left. So, we need to override the computer by checking the box next to the blue "Pressure" function, and typing in a pressure value in CUP that is reflective of tank guns of whatever era we are trying to represent. My tank gun is trying to represent something from about the late 1940s/early 1950s, so I'm going to use 45500 CUP EDIT: USE 41000 CUP for APCBC and 42800 CUP FOR APCR (or better yet, do your own calibration!): This gives me an estimated muzzle velocity of 3,964 ft/s for my L/50 barrel. Not bad! Note the outputs on the left, which tell you a bunch of fun facts about your round but aren't terribly relevant to what we're doing here today. Next, we need to put this gun's performance in terms of penetration. The way I like to do this is through comparative analysis. The first thing we need is to know to find penetration the ballistic performance of our round. We can estimate this using JBM's ballistic calculator and a few rules of thumb. When opening the calculator, the first thing you'll see is this: We care about basically none of these settings except BC, velocity, and maximum range. Caliber, projectile weight, chronograph distance, etc are all pretty irrelevant to us. Keep the environmental settings (temperature, pressure, etc.) set to their defaults. First, change the ballistic coefficient type from G1 to G7 using the dropdown menu. Then, change the muzzle velocity from 3000 to whatever the muzzle velocity was that was calculated by the Powley computer. Finally, set the maximum range to your desired distance - in my case 2,000 yards. For my round, I now have inputs that look like this: We also need to get some idea of how fast our projectile loses velocity, something we can't know for certain without actually building a real gun and test firing it - or at least without some really sophisticated simulations. However, projectiles with the same shape tend to fly the same way, and that's something we can exploit here. To figure this out, we need a graph showing us the performance of a real-life gun. Fortunately, there is a handy one for an IRL gun similar to what I'm designing, the 90mm M3 from World War II, and its M304 HVAP-T, which is broadly similar in construction and shape to my 85mm APCR projectile: Based on this chart, we see that the M304 should drop from its 3,350 ft/s muzzle velocity to about 2,500 ft/s at 2,000 yards. Doing a little trial and error with JBM tells me that this means the M304 has a G7 ballistic coefficient of about 1.13. Now, our projectile will not have the same ballistic coefficient, due to it being a different size and mass. But, we can figure out what its ballistic coefficient would be by finding its sectional density and comparing that to the sectional density of M304. To find sectional density, take the projectile's weight in grains and divide it by the square of the projectile's diameter in inches, times 7000. So for M304, we get: And for my 85mm, we get: This means that the ballistic coefficient for an identical-shape projectile with our size and weight will be about 1.019/1.330 - or 76.6% as much - as that of the 90mm M304. That means a BC of 0.866 G7 should be approximately correct for my 85mm APCR round. Let's plug that in: And then scroll down to the bottom to click "calculate", which gives us a big ol' chart that goes out to 2,000 yards: O-Kay! Now we have some data. It looks like at 2,000 yards, my projectile holds about 2,800 ft/s striking velocity. It's important to note here that what we really care about isn't the striking velocity of the projectile per se, but the velocity and energy of the projectile's core. The core is what's actually doing a lot of work to the armor, so for now let's stop thinking in terms of the whole projectile, and take a look at these two cores, that of the M304 90mm HVAP, and that of my 85mm APCR round. The core of the 90mm M304 is an approximately 8 pound lump of tungsten-carbide that is about 45mm in width. My penetrator is also 8 pounds, but it's longer and thinner in proportion - just 40mm wide, rather than 45mm. This means my penetrator will penetrate more armor at a given striking velocity, and we can estimate how much more by taking the specific energy of the rounds and comparing them. That is, the energy in Joules of the penetrator alone, divided by the penetrator's diameter squared: So the specific energy at 2,000 yards is about 826J/mm^2. Now, we need to find out at what impact velocity the M304 penetrator produces this same specific energy. Do do that, we go backwards, using the figures for M304: Therefore, the equivalent impact velocity for my 85mm APCR round at 2,000 yards is 3,150 ft/s for the M304. That means, in theory, that the M304 would have to impact a target at 3,150 ft/s to produce equivalent penetration of RHA to my 85mm APCR striking at just 2,800 ft/s. Now, we head back to that chart: On the left side of the graph, we put our cursor on the line that corresponds to approximately 3,150 ft/s velocity, and follow it over until it hits the curved line that corresponds with the angle of plate we care about - arbitrarily, let's pick 20 degrees. Then, we follow that point straight down until it hits the x-axis: Therefore, we estimate that at 2,000 yards, my 85mm has just over 10 inches of RHA penetration - not bad at all for a lowly APCR round!
So on my recent trip to Japan (protip: don't fly for 19 hours with your kids), I took some time out from family to visit Yushukan museum. To bring you all up to speed, this place is the museum attached to the controversial shrine that Japan and China are in a perpetual snit over. The shrine itsef is actually pretty anodyne, if fairly imposing and charmless. The museum, however, is pretty fucking sinister. Anyway, I'm sure you didn't click this just to see me repost content, so here are my impressions. 1. Revisionism deluxe If you've gotten the impression by now that this place has an agenda, you are absolutely correct. Japanese soldiers are always described in glowing terms ('honourable actions', 'noble warriors', 'honoured dead' etc.), war crimes are ignored whenever they aren't completely rewritten as laudable or necessary (Manchuria is described as an operation to bring regional stability, for instance) and the Emperor was a saint. It gets to the point of being almost admirably ballsy, such as the train from the Burma railway parked at the entrance without any comment whatsoever. Or when the brochure specifically highlights a Japanese flag signed by 25 of the most well-known ‘alleged’ war criminals as a key exhibit. In terms of the content of the museum, it is at pains to remind the viewer about Japan’s glorious martial past (glossing over the whole civil war aspect), how it was pushed into a hopeless war by the perdifery of the US/colonial powers, and how the noble sacrifice of its people/Emperor lead to... something, I guess? Sadly, a lot of the place is off-limits to cameras so I can’t show you some of the truly egregious stuff. Finally, the amount of memorialisation gets to sort of strange levels. There are statues, displays and plaques commemorating the brave souls who died in the war – including, and I can’t make this stuff up, a special statue depicting the sailors who died testing a suicide diving suit that the empire was working on in its final hours. There is an entire wing of the museum dedicated to photos and mementos of dead soldiers, sailors and airmen. There is also a section devoted to providing bibliographical accounts (including displays of uniforms and equipment) of the men – again eliding any reference to crimes or atrocities. 2. Suicidepalooza Part of this focus on heroic struggle seems to be to include every possible reference to suicidal actions that it can. Every field gun displayed, for instance, helpfully included a note on how the crew had fought to the last man. This also extended to suicide weapons. The museum has an Ohka sitting up in the hall (which I wasn’t supposed to photograph, but did anyway), a Kaiten at the centre of the same room, a Shinyo sitting to the side and a model of Kairyu sitting next to it. Each helpfully notes the exact number of airmen/sailors who perished during testing or use. Finally, the Zero sitting in the entranceway and the Judy sitting in the hall both make mention of their later careers as planes intended for ‘special mission’ purposes. There was also an interview with one of the surviving kamikaze pilots playing on repeat in the main hall. My suspicion here is that the obsessive focus on suicide craft has some special meaning to the Japanese nationalists who effectively fund and run the place that I am unable to grasp. This is interesting, as I’ve generally found that the best possible way to get people to contemplate the insanity of total industrial warfare is to talk about Japanese suicide craft and the reasoning that went into their creation. Generally, once you’ve explained this stuff in detail to a person they’re, like, 50% of the way to either total pacifism or a wholehearted embrace of America’s post-war role as the most munificent empire in history. 3. Odds and sods Every museum has some interesting little bits and pieces hidden away, and this one was no different. For me, it was seeing the astonishingly crude nature of pre-Edo bows (which were, sadly, verboten for purposes of photography). One of them was literally a bronze-capped branch (complete with copious knots) about 25mm in diameter at the handle and steamed into the familiar yumi shape. The others were various iterations of brutalist single-piece bowering, culminating in a square cross-section bow that looks like a direct ancestor of the modern Japanese bow. For the small arms nerds, there are a few machineguns and cannons to look at. There was also a single, lonely Chi-Ha to give the armour nerds some succour. Finally, outside of the museum there was an example of a weirdo-gun: a bronze cannon which was taken in an re-rifled at the end of its life. 4. Conclusion All in all, I found the visit interesting but a bit ominous. Worse, I fear that this sort of thing is more portentous in terms of where Japan is headed than anyone wants to admit. I guess I can only hope that the country, which seems to be going through some sort of transition, doesn’t begin indulging in its worst tendencies again as pax Americana wanes.