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Old 11-02-2016, 08:36 AM
Gray Ghost Gray Ghost is offline
Join Date: Apr 2008
Posts: 3,062
Originally Posted by Archinist View Post
I am wondering how effective a 20mm AP cannon would be against the sidal armour of an M1 Abrams original...

...What about with AP sabot rounds?...

...Or stabbing it with a knife? Can you damage the barrel itself by shoving a knife down inside?...
Originally Posted by DinoR View Post
...I'm not sure that any knife you'll likely have is harder than the surface hardness inside the barrel. The grooves of the rifling are quite a bit deeper than any small scratch you'd get in quickly anyway...
Originally Posted by muldoonthief View Post
...A large rock is a very different attack. According to wikipedia, the largest trebuchets could launch a 90 kg rock up to 300 meters. That's an impact velocity of 54 m/s or 120 mph. I wouldn't be surprised if having 200 lb of rock hitting the top of a tank at 120 mph did a lot of damage.

The Abrams has a smoothbore main gun. Not that it matters - you're not going to be able to do anything to the bore with a knife that will affect it.
Surface hardness of the M1A2's M256 120mm L/44 gun's chrome plated lining is currently (?) around 900-1100 on the Knoop hardness scale. As chrome plating, and specifically hexavalent chrome waste, leads to a bit of a toxic waste issue, the US Army is investigating other methods of surface treatment for large gun barrels like the M256. One of those methods involves sputtering either Tantalum or Chromium onto the surface of those barrels. Measured hardness from either of those sputtered materials is around 200 Knoop (or HK), which is evidently hard enough for the purpose. (See Slide 18 of the linked document.) Without this coating, ultra-high speed kinetic energy penetrator shells like the M829A3 would wear out the barrel in, ballpark, about 150 shots. (Slide 3) FWIW, the A3 looks to be much rougher on the barrel than other ammunition. 5700 FPS comes with a price.

Anyway, kitchen knives' hardnesses are often measured on the Rockwell C scale. 55-60 is typical, though modern powdered metal blades can get higher. Converting between the two scales shows that 55-60 HRc is about 600-700 HK. So, I don't think the knife would scratch the older barrels, but it might scratch the newer sputtered ones.

As to penetrating the armor on a modern MBT with multiple smaller impacts, I'll link to this oldie-but-goodie, the A-10 Pilot's Coloring Book, circa 1977. Not because it's all that helpful in answering the question, but it does show that answering "will it penetrate or not?" really depends on where the tank is struck. I have read accounts from Bradley crew that they were able to penetrate the armor on T-72 on down with 25mm sabot ammunition and judiciously selecting where to shoot the tank. I haven't read of sabot ammunition available for the 20mm x 102 rounds the U.S. M61 rotary cannon uses. Which strikes me as strange, given sabot AP ammunition exists for smaller cartridges like 12.7 and 7.62 mm. I wonder if it's because 20mm is primarily an aircraft or anti/aircraft weapon for the U.S., and sabots are either unneeded to damage aircraft, or pose a FOD hazard when shot from aircraft?

Moreover, sabot ammunition isn't the most friendly to use in a combined arms environment; flying sabot petals evidently pose quite the risk to infantry operating in front of the gun muzzle. (Range safety document, with more info than probably you want on surface danger zones for a large variety of ammunition. From it, at page 98, even plastic sabots from 25mm shells are dangerous up to 100m downrange, and up to 50 m to the side of the muzzle.) Still, if you need to punch through armor, there's no substitute for speed, and sabot rounds are faster than everything else.

As far as the trebuchet and rock go, don't tanks often knock down trees while maneuvering in forests? Are tanks often damaged by tree tops falling onto their top armor?