In 2002 I used a Small Martini in 218 Mashburn Bee for Guinea Fowl, Steenbok, Vervet and Springbok. I wanted spring hare and Rock rabbit. Spring hare were left for the kids to chase and catch. Outfitter never made rock rabbits available even though they were expressly requested. They were interested in shooting the money animals.
The Bee worked well on everything except Springbok. Although in fairness the shot was deflected by a thorn bush stock. The animal instead of being chest hit, was butt hit. We chased it for several hours before we could finish it. The wound was minor with little penetration. I did not like that.
The vervet was unplanned and just an opportunity. Chest shoot, dropped the animal right where it sat. My tracker loved monkey meat and was always pointing them out. I shot several more with the Bee and my 338/08 IMP. All dead at the shot. The Bee was devasting inside the body.
Guinea Fowl were all head shot---and excellent eating.
The steenbok is a story of it's own. We stalked that animal several times over a couple days and never got close. The day we got him we were out looking for meerkat. We saw him several hundred yards out in the field. To far to shoot. We saw a heard of cattle moving across the field towards the animal. I suggested we get with herd and move with them. The PH was skeptical, but I assured him I had done this successfully before with Pronghorn antelope. He consented and we got the herd between us and the Steenbok. We walked with the cattle and Steenbok paid no attention. We got in close, set up the crossed sticks and waited for the cattle to move on by. At the shot the steenbok jumped straight up in the air and fell to the ground. THe shot was well less than 50 yards. The bullet penetrated completely through animal, destroying the heart on the way through. The PH told he had tried take this animal before with previous clients and never got close enough for a shot. Frankly I was a bit surprised he hadn't thought of using the cattle for cover before. Different countries different hunting techniques.
If you make a plan for what you want to hunt and fill your bag before your time is up, then a small game rifle can be a handy thing to have with you. Just remember some of these animal are not money animals and the Outfitters don't want to spend time or money on them. Although after getting the guinea fowl the first day, my PH always wanted me to bring the little gun along.
I would use one of the little calibers for small animals like a steenbok and smaller, but never again for Springbok or maybe duiker size and bigger.,