Hey. Great question!
Firearms aren't a basic skill. They are an expert skill. So normally a character without a rifle expert skill simply wouldn't have a rifle or know how to use one.
If however, a character did pick up a rifle and want to use it, we would normally rule that the skill would start out at 20%. (like untrained basic skills). Unless they revealed it as an expert skill.
Since this situation is not included in the booklet, the GM is allowed to make the call. They may choose to rule that an untrained skill starts at 1%. Which is how basic BRP would resolve it. We've chosen not to get too nitty gritty with Embers and leave some stuff up to the GM to rule.