Ultimate tensile strength of forehead skin is 5.7 to 12.6 MPa, yield strain is 27-59% UTS.
So, CD is ~1mm thick weighs 1g.
Let's say 1" penetration to get it to stick. Let's call skin about 9MPa UTS for purposes of this approximation. Let's use 45% yield strain.
So pressure needed is 4.05 MPa (call it 4 its an estimate).
Arc length at 25mm (1") deep is 62mm, let's average at half for 31mm. (That's all geometry, using chords and triangles, not going to reproduce here.)
So that's an area of 31mm2.
To convert to square meters (Pascals being 1N force over 1 square meter) that's 31e-6 m2 so we need 14.5 Newtons force approximately 3.14 lbf.
So we need a deceleration F=ma Mass is 1g (0.001kg) a=F/m for CD would be 14,500m/s2
Deceleration over 25mm v2 = u2 +2aS Final velocity v is the sum of the squared initial velocity plus twice the acceleration multiplied by distance
Ok V is zero we're solving for initial velocity U is unknown a is 14,500 S is 25mm
Plug those in we get 27m/s Or 60mph.
However that's just skin, bone would need greater than that, but I don't have figures for that, so, let's take a WAG and say 5 times that. For 300mph.
That could be overkill might just shoot right through, or not, there's a huge amount of approximation in the tensile strengths etc. And it also requires the the impact is entirely in plane of the disk, and that the disk can withstand the impact.