If you can, whack them and quickly cut the throat in case you didn't whack hard enough. They have a small head and sturdy little body with quick jerky movements so shooting and trying to dislocate the neck are difficult. Trust me shooting them is a very bad idea. That was one of my most traumatic experiments ever. Did not go as planned and I went in to with a very good knowledge of guinea pig abilities since we'd been raising them for the pet market from the time I was born. I've also seen several pups dropped from 5' on to concrete and walk it off even though they tend to land head first because they lack long enough legs to catch themselves.
I had some 3lb boars as adults. The show culls were a little smaller versus pet bred but dirt cheap. In fact I got a buy a sow get a boar free from one breeder who was doing a heavy cull that year. The mismarks just looked interesting on the pelts. I probably did most of them around 4months. Some 6 months. Then a couple I culled after breeding who were a year or 2 old. The ones a year or more were rather fatty and kind of disgusting looking meat. Dogs and cats say it was great though. If you did 1 per person you could probably butcher at 3months depending on the line. They grow rather slow after 3-4months so it's only cost effective to let them grow out to 6-8months when you can provide a bunch of fresh food and hay instead of pellets. Corn plants, cattails, and vegetable leftovers fed them pretty well from summer through fall and unlike rabbits they handle the addition of fresh food even in sudden large amounts quite well. In south america they graze them and feed vegetable scraps especially potato peels. We used to let them run wild in our backyard and even though they could fit through the chainlink fence they only did so long enough to dodge a person or animal and then popped back in to the yard. They lived under a low deck and in the woodpile eating the tall grass and weeds around those areas, the garage which sat 3/4ths in the fenced area, and around the pool. We didn't need weed whackers then.
