I have transited many times through Heathrow and never understood all of the negative comments. Now, I haven’t done it this year, but I’ve been told that nothing has changed Since my “COVID” trip and before.
There is an easy way and a hard way to transit LHR with a firearm.
If you want it to be easy, book both the arrival leg and the departure leg on the same airline or at least the same alliance. I have always booked Star Alliance and have never had an issue with the luggage going straight through, regardless of change of terminals.
If you want to make it harder, use different airlines in different alliances. If you want to make it hardest, arrive at Gatwick or Stanstead and leave out of LHR. Still doable, but a significant pain (you have to hire a company to convey the firearms between airports, etc. and it has to be arranged in advance).
Lastly, unless the UN has very recently changed its embargo on firearms to Zimbabwe, you can make your life even more difficult (which is to say, impossible) if the firearm is checked to that country.
Otherwise, pretty simple.
