I rather suspect you will want to think this through again. It seems to me that you are somewhat backwards in your logic. But not entirely. I get what your concerns are. Here are the options and what values are bought forward.
Forwarding the .com to .rs.
You will likely gain nothing or very little doing this. The .com site will have no value to pass and will not likely perform well in the SERPs as a result of apparent duplicate content which you would likely use a canonical link pointing to your .rs site to resolve.
Moving your content to the .com and forwarding .rs to .com.
This will allow your content to penetrate the search engines globally which is what you want. Any value of the existing .rs site will be passed to the .com site. However, you will lose some of your more regional appeal.
Creating two sites .com and .rs.
If you are able, you can create a more global version of your site for .com and a more regional version of your site for .rs. This will depend entirely on whether you feel you can do this. You want to avoid duplicate content, however, if your topic is such that it can be covered uniquely in two different ways; globally and regionally, this may be an option. More on this later.
The reality.
Generally speaking, adding a second domain name, in this case via the gTLD and ccTLD, you can realistically only rank one site unless each site is unique. If your Serbian site is another language, then you can easily have two unique sites by using English on your .com site and making your .rs for your language.
If you are going for a more global reach, you will only get this through the .com site. If you are doing business, this may be your best option.
Redirects.
You will only want to do a 301 redirect. Forget 302. Why people mention the two options is beyond me. There is a place for a 302 redirect, but not for what you are doing.
If both sites are unique.
You will likely not have to do anything but create your sites.
If both sites will not be unique.
Then in order to reach a wider audience, you will likely want to redirect your .rs site to .com using a blanket redirect. This will preserve any value your existing site has and pass it to the .com site. You can add a location and section of your site devoted to your Serbian audience. This may help with regional searches. I recommend using schema.org mark-up to help with your geo-targeting so that you retain as much as possible your existing audience. There may be other options too.
Here is an example of a blanket redirect in this answer: When aquiring a company and website, how should their website be redirected?
Wrap-up.
While there will likely be other input from users here, I suggest listening to all the experienced voices. I do not specialize in geo-targeting as much as others do, for example. I wanted to make sure you got an answer. But sometimes the best answers come from a chorus of experience.
