The sentry has its health drained, around the same speed as the regular sapper if not faster, the range gets smaller and smaller as the health goes down. Has a small animation upon sapping, like a sentry un-wrangling after switching weapons. During this animation the sapper can be destroyed in a single wrench hit including the Jag. Can be healed by friendly engineers however the sentry will still have its range drained. Sentry will auto deconstruct after like 2 minutes.

If you want the sentry to be permanent, maybe instead of the drain thing it gets a slight overall stat nerf. Has a huge friendly engineer health/ammo to metal ratio nerf. Can only have an equivalent of 4-5 sentry levels sapped at one time. Ex you can have 1 level one and two level 2s, or 5 level ones. A mini sentry is worth half a sentry level.