i used to use glaser safety ammo before the unjust handgun ban , they are however not the thing for close range snake encounters , they are designed to not over penetrate when shooting jolly naughty people
what you need is the speer shot capsules and some 12 shot or 9 shot , i had some i used in a winchester 38/357 lever action carbine , the rifling spinning the capsule wasn't great for patterns and realistically i'd say 8-12 feet would be the max effective range
although it's all moot really since adders are protected now and have been for a long while plus as far as i know they tend not to attack you so you are unlikely to need to defend yourself from them unlike mamba's , i did not like them one bit when i was in africa!
if you are determined i may still have a few shot capsules i can send you if i can find them in my incredibly tidy reloading cave,,,,,,,