1) Your assuming you have more bullets then the enemy has critters and that the enemy doesn't have the mentality of army ants.
Bullets don't need time to grow or nurturing. They are ready for action the second they step off the assembly line.
Drones, tanks? Same thing - you only need gas.
We allready have enough bullets to kill off the entire human population several times over.
So no. Despite what you've seen in Starcraft and Alien, an organism doesn't grow from a baby/larva stage to a ready warrior stage in minutes/hours.
That's actually probably the worst argument against biology. If there's one area where biology trumps technology, it's manufacturing. Forget bullets per person, do you know how many
bugs there are per person?
Your problem here is you're comparing a full-sized warrior to a bullet, when you should be comparing a warrior (Zergling, xenomorph, whatever) with human beings. Let's instead compare a bullet and its production mechanism with a bullet-sized attack organism that some hypothetical alien race uses as an area denial weapon against infantry.
A bullet is a great way to kill things. It's congealed energy. Fired from a gun, it's pretty reasonably lethal. And you can fire a lot of them with great accuracy. When it comes to elegance, not much beats the good ol' human bullet.
To make a bullet, you need to set up a factory, which takes days to months. You need to ship in refined materials, which in turn requires refining plants in other areas, as well as skilled labor (or good machines) to run them. Those in turn require raw materials from mines. All of this requires infrastructure and transportation, which probably requires roads. You then need to mate the bullet with its symbiote, the gun, which requires a whole 'nother set of infrastructure.
You then have to give the bullet and gun to a soldier, who's been raised from birth as a generalist and only recently been instructed on how to use his bullet and gun. Said soldier represents thousands of man-hours of investment, as well as thousands (millions?) of regular dollars. Your overall man-gun-bullet system requires a huge amount of effort, just to get that single bullet to the point where it's actually ready to use.
So, your big advantage is that once you have a factory up and running, and a secure flow of raw materials, you can make bullets really fast and in great volume. Your disadvantage is that production is centralized, and that each individual bullet has no capability to create any more bullets.
Now.
Meet the bullet bug. It's an insect-type critter - not very big. It eats biomatter and certain chemical compounds that it hunts for. It has a keen sense of smell, swarm behavior, decent vision, excellent navigation (like a bee!) and a number of substrains. It is asexual, and born pregnant. Our hypothetical Biotic Tech Enemy has designed it, but it works alone just fine.
Introduce a bullet bug to an area, and it immediately begans consuming biomass. When ready, it dies, producing live bullet bug 'larvae' that mature within a day. These hatch new bullet bugs. They consume biomass as well.
By simple exponential growth the bullet bugs rapidly enter the triple and quadruple digits. At this point, pheremone signals trigger gamete differentiation. Some newborn bullet bugs are specialists. They may gather chemical materials for the bullet bug swarm. They may develop acid sacs, hardened tips, gunpowder charges, noisemakers, explosive phosphorous compounds, burrowers, or an electromagnetic sense.
Throughout all this they continue to multiply - remember, they're born pregnant. Burrow specialists create underground hives so that the bullet bugs can feed on topsoil and move without easy detection, but these hives are non-centralized, and the majority of the bugs simply roam. Other bugs spit digestive compounds that render normally inedible materials useful to the bullet bug swarm. Solar bugs gather in enormous mats and convert sunlight into simple organics for feeding. Gas bugs metabolize compounds they're fed into toxic plumes (working, obviously, in large numbers.) Current bugs clamp onto power lines and sap electricity for more insidious broods, including lightning bugs that can actually work in swarms to carry considerable charge.
Now, the development of a good swarm requires a great deal of energy. But because each individual bullet bug is capable of feeding and reproducing, the effort is non-centralized and limited only by local resources.
In the meantime they engage in area denial behaviors. Some bullet bugs dig long tunnels in metal, wood, or concrete buildings. Gunpowder bullet bugs hide in these areas; they will explode when triggered, acting as improvised bullets. Other bugs deliver potent neurotoxins to unprotected enemies. Armored humans can be smeared in phosphorous compounds, and unless their armor is top-grade, they will be vulnerable to depolymerizing compounds or the destruction of their sensors. Flare bugs are the swarm's armor-piercing solution; they achieve physical contact with hostile armor and then ignite, much like anti-material grenades. Either that or digestion bugs can splat themselves on armor, releasing tailored solvents - oh, and the chemicals released from the dissolving armor by a successful solvent attack cue the rest of the swarm to amp up production of that particular solvent type. Adaptive anti-armor chemicals!
The simplest weapon of the swarm is sheer numbers. Due to rapid diversification and adaptation, they can be retarded but not eliminated through the deployment of toxins. The only effective countermeasure to their presence is fire (or possibly very high wind!)
All this, from a single bug, in a matter of days - without intelligence, preparation, or outside control.
Because, remember: DNA is the best damn storage medium there is. You can fit the information required for all these bullet bug variants into one little cell. That little bullet bug that drops in from space (oh, sure, they can survive in space too, they've got a vacuum strain, why not, it just hitches a ride in on a meteorite or whatever) is a frighteningly pluripotent weapon.
Now, I'd like to stress that these bugs are not particularly more effective than bullets. In a straight-up battle situation you can kill a lot more human beings firing bullets than firing bugs. But the bugs require no infrastructure to create. You can get millions of bugs in the same time that it takes you to just dig the
hole that you're going to put a factory in. And while they don't work well against a crafty human foe with armored infantry and good tanks, well, neither do plain infantry bullets.
So, there you go. The bullet bug: an effective, economical, and competitive biological answer to the bullet. It has its own advantages and disadvantages. But, unlike a bullet, it requires no infrastructure, no training, no human element, no additional components, no storage area, no proper usage, and no serious investment of...anything. Drop a single bullet bug on an unready planet and you could take out an entire colony just by crop destruction.
Game over, man! Game over!