To add my 2c as a local - it is a very easy drive as the route is mostly dual carriageway, so the RH drive staying on the left hand side of the road is less of an issue. I would try to avoid departing the airport at rush hour as the first 50km is infamous for extremely high peak traffic density (rush hour is typically 6:30-8am in the morning). Once you hit the N1 highway north, it is an easy drive. I would advise using a GPS/Waze for peace of mind on the route. It should be a very straight forward 3 hr drive.
The firearms concern is extremely minor IMO - you will have the requisite paperwork already from SAPS at the airport. Yes, you need to be cognisant of the potential safety/robbery risk here, as has been mentioned already. EZRider has given good, pragmatic advice.
If you do get stopped by the authorities on the road, wear your mask, be courteous and have your drivers licence to hand, stick to answering any questions they ask (don't volunteer info about carrying guns or going hunting unless you want to risk prompting them into a whole lot more questions - however do not lie or pay bribes (asking for a Coke or some food is a police favourite here!)), be patient and you'll be on your merry way in a few minutes.