Let $W(r,n)$ be the set of all words of length $r$ on $n$ letters.

For two words $a,b \in W(r,n)$ define the Hamming distance $d(a,b)$ as the number of places that they have different letters.

Let $M(r,n,k)$ be the maximum $|S|$ such that $S \subseteq W(r,n)$ and $\forall a,b \in S$ $d(a,b) \ge k$ (I believe this is usually denoted $A_n(r,k)$ in coding theory)

Most of the results I have seen about this value have been in the case $n=2$ since most applications of this idea are in binary codes. I am not particularly interested in $n=2$ and am wondering if any general bounds are known (lower or upper) or even a list of known exact values for small numbers would be nice. I do know that the Singelton bound says $M(r,n,k) \le n^{r-k+1}$. Any other information you could give me would be greatly appreciated. General bounds or exact values are good, or even if $M(r,n,k)$ has a "name" attached to it?

Thanks!