So its the year 4000 and we are under attack by the dreaded Dramulans and we need new attack missles ASAP. Having developed two holy grail technologies, we need to know just how fast is it possile for a missile to accelerate to its target. We of course haven't figured out how to go faster than C but we now have the FAD and the matter-antimatter flipper reactor. The FAD is the Field accelerator drive which accelerates you as fast as you can pump energy into the craft and the flipper reactor is a very lightweight device that is truly magical, it can take ordinary matter and flip it into its anti-matter analog, then just turn around and mix regular matter with the newly flipped anti-matter and you have instant energy maximus. So we know we can get about 26 Terawatt hours out of a KG of matter/antimatter reacting so we can use that figure to find out how much power we can get into our missle. Figure about 10% for the power supply and another 10% for the FAD drive and the rest, 80% is pure hellcat, another flipper but this time ALL its energy will be used in one gigantic explosion you don't even want to be in the same SOLAR SYSTEM with. They will be attacked about a light year away from Earth.
So How many G's can a device like this pull? See if you can figure out how many watts per KG it takes to accel one Kg at one G first. Also, it takes roughly 30 million G's to get to C in one second. You may not even have one second however, these Dramulans are NASTY.
BTW the FAD drive accelerates every atom in its field at exactly the same rate so there is no feeling of accel because there is exactly zero differential acceleration felt anywhere in the ship so it can be manned if need be but of course that would lower the potential accel because of having to add life support to the percentage list so an unmanned missile is used to maximise the destructive potential.