The maim main things you cannot have are a pistol grip, flash hider, vertical forward grip, adjustable stock. You can have an adjustable stock but it has to be "permanently fixed" which you can do with a stock stop or other methods. Or you can just have a fixed stock. Which you should anyway because nyet rifle is fine. For grips just get a well known featureless grip. There are several designs to choose from, not only fins. But the definition of "pistol grip" is somewhat vague so best to go with a well known and widely used option imo. For front grips you can have like angled grips or handstops. Again the definition is vague but it is generally held that anything advertised as a verticle grip is illegal, even if it's clearly more of an angled grip. Same for flash hiders. You don't want anything that says it hides flash even if it looks more like a brake. But also you don't really want something that loonks like a flash hider even if it is advertised only as a brake. No one has ever actually been prosecuted for having a flash hider so we do not have much of a definition to go off here.

You also can't have flare or grenade launchers, or attachment points for them, but unless you are into milsurp you probanly aren't worried abiut this. Means you can't buy Yugo SKS's which sucks, and some other less popular milsurp rifles.

You can have rails, lasers, lights, scopes, etc.