I am coming home on this route and would like to know that since SAA Airlink and United are both Star Alliance, can my luggage and firearm be checked all the way through to EWR or do I have to collect it in JNB and and re-check both for the following flight to EWR?

I think there is a 50/50 chance of being checked thru. Two trips this past year on SAAirlink, one each from Zim and Moz, trip from Zim was checked thru to DFW and the trip from Moz we had to claim and recheck baggage in Joburg (even though gate agent in Beira said it was checked thru).

Wow I suggest that you ask your travel agent.

In the past I have been able to check my rifle all the way through, however I had to re-ticket myself on all trips.

For my Checked bags they were routed all the way through. That being my luggage bag and firearms case. I have had to wait at the recheck area in Joberg to obtain new boarding passes.