The closest that we can get to a numerical answer to this question would be:
How fast would the AM bullet need to go, so that if it struck a wall with more mass than itself, then every positron and antiproton is annihilated rather than shot back out from the force of energy released from annihilations happening in front of it?
We can probably assume that the energy released will expand in a relatively spherical burst of gamma and x-ray radiation. (Relativity pun intended, since we're working on high energy EM radiation.) Fortunately, since things are happening at relativistic speeds AND much of the energy is traveling at the speed of light by definition, this means that we don't need to worry about reference frames very much. The energy expands as a sphere, whether your frame of reference is the bullet or the ship... And if you're either, then you won't need to worry about reference frames after the impact, either.
So, in the moment of annihilation for each particle, roughly half of the energy is working to move the ship out of your way, and the other half of the energy is working to slow the bullet down.
In order to figure out how much kinetic energy we need, we take the 86 kilotons ("borrowed" from Salda's excellent answer) of explosive energy, cut it in half, and convert it to joules. In less than than it takes for a human to think, there is going to be 179,912,000,000,000 joules delivered into the bullet that we'll need to overcome.
At 2 grams, if relativity didn't exist our bullet would need to be traveling at ~360,000,000,000,000,000 m/s, or about a billion times the speed of light.
Fortunately with relativity, we don't need to go that fast (but we DO need to pump in that much energy into our 2g bullet). We merely need to accelerate our bullet to 99.999999999% the speed of light, or thereabouts.
Keep in mind that these are all back-of-the-envelope, spherical cow on an infinite, frictionless plane types of calculations. If there is less than 2g of matter in an AM bullet's path, this is how fast it will need to go in order to guarantee that some parts of it makes it through. It won't resemble a bullet, but it will be bits of antimatter that is still going in the right direction.
If the target ship has less shielding, you don't have to slow down quite as much, of course... I.e., if there will only be 1g of matter in front of your antimatter bullet, you'll only need to be going half as fast, about 99.999999995% of c (that is, you'll only have to put half as much kinetic energy behind the bullet, which due to the nature of relativity, only seems like a very minor change in the fraction of the speed of light that our bullet is traveling, despite being quite significant.)