At "the outskirts of the solar system", there is, as everyone has written, no real big deal.
What about in orbit around an inhabited planet, though?
Even there, the problem will not be the bullets, missiles, railgun slugs, or whatever, since there's only one of them per bullet.
The problem instead will be the debris that other people have mentioned in passing.
The bullets will be fired from something moving at orbital speed, to something else moving at orbital speed. The bullets won't be significantly faster or slower than orbital speed, because once you're moving at a good few miles (or kilometres) a second there's really no point using a lot of energy to go much faster.
If you're in orbit around an inhabited planet, this could be devastating.
By destroying a single satellite, the Chinese in 2007 added over 3,000 pieces of trackable debris into orbit around Earth, as well as a cloud of countless smaller particles. At orbital velocities, paintflecks strike with the same kinetic energy as a rifle bullet. A 4in (10cm) object would impart kinetic energy equivalent to 25 sticks of dynamite.
If the battle is in low orbit, and explosively destroys a significant proportion of two fleets and all the shrapnel from their missiles, then within 15 minutes (or whatever the low orbit period for that planet is) the planet will be surrounded by a cloud of debris: satellites, space stations etc which are hit by those will be shredded, each one adding a few thousand more particles to the cloud as it is hit. The particles in the cloud would also intercollide, creating more and more fragments in a runaway reaction.
But how likely are they to be hit?
For an estimation of the risk: currently, collision avoidance maneuvers are performed a "couple of times a year" on the ISS (I'll call this "once every 200 days": anyone got more specific stats?) and are performed if the chance of collision is higher than 1 in 100,000. There are about half a million pieces of debris being tracked.
So naively, to get a probable hit once a day you'd need 200 * 100,000 * 500,000 = 10M particles. Statistics isn't linear, so multiplying by 100,000 doesn't change that 1:100,000 into a 1:1, but it's good enough for our purposes.
To get a probably hit once an hour, which is what you'd need to have a good chance of hitting anything that launched, you'd need about a quarter billion trackable particles to blockade the planet.
This would make launches completely infeasible and wipe out all satellites and space stations at that level. Whatever level the battle happened at, it would likely destroy everything below it and an almost equal distance above it, through elliptical orbits and orbital degradation. Anything happening at a significantly higher levels much, as if the orbits were that elliptical then the debris would collide with the planet. So if the battle happened in low orbit, geostationary sats would be mostly safe, for example.
You'd need rather more particles as your orbital radius increased, though, as every doubling of the altitude would cube the volume.
This leads to an obvious military approach of "dusting" planets with a few billion particles each to prevent them launching against you, when you attack their solar system. At that point, you then have air superiority, and all the planets have to do what you want or you drop bigger rocks on them.