Sounds like yours is closed bolt (post a pic of the ejection port with the bolt carrier locked back and we can confirm).
If it is, then as Hectic pointed out you can upgrade it to Gen4 (open bolt)
Is it worth it? Depends on you. I really prefer open bolt, better feel and better realism IMO. But is it worth $150 to you? Is this your first GBBR? If so, maybe just try running the closed bolt system and see if you like GBBRs before you sink more money into it. If you're hooked then you can upgrade.