First of all, I think we need to clear up the science a little bit.
Anti-matter doesn't 'explode' when it comes in contact with matter - it mutually annihilates both itself and the matter that it comes into contact with in equal quantities of mass, becoming pure energy.
This means that in essence, the anti-matter bullet is far more efficient at creating energy than a fission reactor, and probably just as lethal given that most of that energy would probably be released as gamma radiation (this is based on current theory - we have little practical experience with creating anti-matter explosions).
The formula E=mc2 tells us that for every gram of antimatter in the bullet, we multiply that by twice the speed of light squared (because the antimatter is only half the mass annihilated) to get an energy release value.
This is also in line with how thermonuclear explosions work in that a nuclear bomb doesn't really 'explode' as it does release a massive amount of energy in the form of heat. Fission is effectively just breaking large complex molecules atoms up into smaller ones, resulting in a minor decrease in overall mass, the remainder of the mass becoming heat energy to be released in the process. What makes the blast waves and 'explosion' is that heat increases the atmospheric pressure and this form of sudden and uncontrolled release of heat results in a massive and sudden increase in the atmospheric pressure, not to mention the creation of plasma out of existing mass around the blast - all in all it's a bad outcome.
In the case of your antimatter bullet however, the bullet won't just create fission - molecules of it literally cease to exist when reacting with a normal matter counterpart. The ENTIRE AM molecule, and the molecule it reacts with, become pure energy. With fission, no actual protons, electrons or neutrons go missing as such, but they are reconfigured in a lower energy state as the complex molecules become multiple simpler ones. in the case of antimatter the effect would be far more devastating because mass is literally being converted to energy.
As such, the AM bullet is not a kinetic weapon per se; in other words, you can't just sharpen it and fire it with a really high velocity (even relativistic speeds) to make it bypass some of the armour and annihilate mass behind it. Antimatter just doesn't work that way.
Edit it is important to note that some of the energy being released is going to actually push the bullet back, or push other molecules out of the way to some degree. That said, the more velocity you put into the bullet, the more of it will be annihilated because the gamma radiation released has to counter a greater initial momentum. What fragments may make it through would do so only because it would be riding a bow wave of gamma radiation and plasma, but it won't be a bullet anymore in either case.
The good (?) news is that with a mass of (say) 10 grams forming the 7mm 'shell' when it hits the armour, the energy release is going to be so massive it's unlikely that the armour will be sufficiently strong or robust so as to withstand the sudden onslaught of gamma radiation, meaning that all the people behind it are likely dead from the radiation generated even if they don't flash burn because of heat release, which they most likely would.
The short answer is that you can't have your AM bullet look like bullet after passing through mass really quickly. Contact is all that is required to set off the reaction and as such, they'll go off with the first contact with ANY regular mass. They are not kinetic weapons, and you can't think of them as such. They're energy release weapons with a contact trigger.