I’ve been inundated with email from physicists. I really don’t want to be the clearinghouse for this. But I will post this:

Hi Jonah,
The email mentioned in your last post is fundamentally wrong. You
CANNOT convert angular momentum into linear momentum or vice versa–
they are independently conserved quantities. He is correct that you
could solve the “twist” problem by using two counter-rotating
centrifuges, but this doesn’t help with the recoil problem. The gun
could be designed to simultaneously fire rounds in opposite
directions, but this would be extremely dangerous to the user and
other friendlies.
When firing a single round, the recoil wouldn’t be a big problem, but
it becomes serious when firing at a high rate. An infantryman firing
the handheld .308 version with steel ammo at 5,000 rpm with a muzzle
velocity of 2,500 feet / second would experience a constant recoil
force of 28 pounds. I doubt it would be possible to hold it steady.
For the full-size version, firing .50 cal at 120,000 rpm at 8,000
feet / second, the recoil force would be strong enough to send a
humvee skidding backwards–even if it was parked on pavement with its
brakes on. At this rate, power consumption for the device would be
enormous–fifty megawatts–or about 5% of the output of a typical
nuclear reactor.
The inventor’s claims of no heat signature are also highly suspect.
According to the laws of thermodynamics, no device is perfectly
efficient. Even if the DREAD was 99% efficient, the handheld version
would put out about 500 watts of heat and sound during sustained
firing, while the full-size would generate about half a million
watts, which would probably leave you in worse shape than the target.
As a short-burst tank-mounted weapon, it may hold promise, but it’s
hard to believe the military.com article given the credibility issues.