I have 6 Springfields. On all of them, the slide stop cannot be inserted without using a punch. I know Kimbers can be easily reassembled without a punch.
2 things may be wrong. Either the plunger tube has a different internal dimension or the plunger itself is out of spec (the shoulder may be too short). You can pull out the plunger assy from your STI and put it in your Kimber just to see where the culprit is.
Hopefully the fix is a new plunger assembly. Otherwise if it really bothers you, then you can have an aftermarket plunger tube installed.