How much will you be able to upgrade your sentry, if at all?

For example, I was thinking that pneumatic bolt/drill would be a nice upgrade for the sentry, it would make the sentry unable to be knocked down, or higher rate of fire, or armor piercing/explosive bullets. There are loads of upgrades I can think off just off the top of my head.

Will there be any upgrades at all like that for the sentry and bots?