Entirely depends on the rifle. Mancraft SDiKs are pretty widespread and your main option for most rifles, but there are a handful of other engines for other rifles. Personally, I like the Wolverine Bolt (NOT the Bolt M!) because I think the VSR is the tried and tested best platform (but not my favourite) and that's what the Bolt is for. I have a VSR with a Wolverine Bolt and it's probably my quietest and best shooting gun. The mechanical systems aren't bad, but they're a bit louder (still very quiet) and not quite as effortless with the bolt pull (still very easy).
The other option is to go with a gas rifle like a KJ m700 (my favourite platform!) and HPA tap the magazines, but this is a lot less practical and is basically heresy to me. It's heavier and you need to detach the line every time you change the magazine, unless you handload the BBs like you would a real m700, and you still have a line and tank to contend with.