It's by no means a new idea.
One problem with "light" (low-density) bullets is stabilization. I went through this in trying wood for short range bullets, with which you could not hit a barn from the inside. (A relatively fast powder was needed -- I recall starting with Unique and ending up with Green Dot shotgun powder, and covering the range of burning speeds in between.)
Aluminum bullets require twice as much spin for stabilization as jacketed lead bullets do.
"For bullets made of other materials than lead, the spin (snip) should be multiplied by the square root of the number found by dividing the specific gravity of the (lead) bullet by the specific gravity of the material of (the other) bullet.
"Thus if it is made of aluminum (aluminium), whose specific gravity is 2.7, we would divide 10.9 (the specific gravity of the lead bullet) by 2.7, obtaining 4. Then take the square root of this number, which is 2, and multiply the spin by that, showing that the aluminum bullet would require twice as much spin as the usual jacketed lead one."
(Hatcher, p 556, paraquoted)
So, with the usual military rifling twist of around ten turns per inch, an aluminum bullet would need a twist of one turn in five inches.
However, I'm pretty sure that accuracy is not a prime consideration when using these kinds of light bullets in that application. Many countries have used wooden bullets as "guard cartridges," whose density is even less than aluminum.
BTW, at even only 1000 feeet per second, the rate of spin of the aluminum bullet would be 144,000 RPM with this one turn in five inch rifling twist.
One problem with "light" (low-density) bullets is stabilization. I went through this in trying wood for short range bullets, with which you could not hit a barn from the inside. (A relatively fast powder was needed -- I recall starting with Unique and ending up with Green Dot shotgun powder, and covering the range of burning speeds in between.)
Aluminum bullets require twice as much spin for stabilization as jacketed lead bullets do.
"For bullets made of other materials than lead, the spin (snip) should be multiplied by the square root of the number found by dividing the specific gravity of the (lead) bullet by the specific gravity of the material of (the other) bullet.
"Thus if it is made of aluminum (aluminium), whose specific gravity is 2.7, we would divide 10.9 (the specific gravity of the lead bullet) by 2.7, obtaining 4. Then take the square root of this number, which is 2, and multiply the spin by that, showing that the aluminum bullet would require twice as much spin as the usual jacketed lead one."
(Hatcher, p 556, paraquoted)
So, with the usual military rifling twist of around ten turns per inch, an aluminum bullet would need a twist of one turn in five inches.
However, I'm pretty sure that accuracy is not a prime consideration when using these kinds of light bullets in that application. Many countries have used wooden bullets as "guard cartridges," whose density is even less than aluminum.
BTW, at even only 1000 feeet per second, the rate of spin of the aluminum bullet would be 144,000 RPM with this one turn in five inch rifling twist.