This is a question someone asked over on Quora.com. Since the answer is rather obvious, I wanted to add a stipulation: Suppose you’re wearing Super-Duper™ Kryptonian body armor that for all human purposes is inpenetrable and indestructible. The 30mm depleted uranium round hits you in the center of your chest plate. Exactly how fast are you hurled back as a result? Alas, the math is beyond me.
ETA: the inelastic vs. elastic impact results would be nice
You are hit with 268 horsepower, exactly the same as a Subaru WRX If that much HP can accelerate a 3400 lb car from 0-60 in about 5 seconds, it will accelerate a 200 lb person (soon to be ex-person) 15 times faster. 0-60 in 1/3 of a second.
A tad over 1 second after you are hit, you are tumbling along the ground 100 yards away.
Dennis
According to this page, the armor-piercing discarding sabot round for the Goalkeeper system weighs 0.225 kg and has a muzzle velocity of 1225 m/s. Assuming a 70 kg person and inelastic collision, that gives a final velocity of 3.9 m/s, or about 9 miles/hour. An elastic collision would be double that, but seems pretty unlikely (at those speeds, even depleted uranium would “splash”).
Seems survivable if you really do have unobtanium armor that can spread the impact over a wide part of your body.
What’s the max Gs though?
IOW, if the body is accelerated (utter guess, not realistic) at 1000G for a small fraction of a second all the way up to 9mph, the brain will still be mush from the 1000G hit.
Is this for Shadowrun or D20 Modern? 
As far as your head is concerned, it’s not that different from being in a 9 mph auto accident. The body stops moving basically instantaneously (due to the seat belts) while the head keeps moving forward at 9 mph. Your neck gives the brain a bit more distance to decelerate through.
The organs in your chest have a tougher time but being squishy and suspended in fluid, it still shouldn’t be too bad.
If we imagine the armor to weigh 10 kg, then it’ll hit you at 27 m/s. Sorta like being hit by 70 baseballs at 60 mph. It’s gonna leave a mark and break some ribs, but again, probably not life threatening. If the armor weighs as much as you do, then it’s just like a guy running into you at 9 mph. Might knock you over but not much more.