It's a good idea to use your finger (with some grease on it) to lube up in the inside of the plunger tube.

Continue applying lube and testing the plunger tube on the breech until they slide nicely.

It's also a good idea to apply grease to areas where the metal is in constant contact with the plastic, like the plunger tube guide tabs.

The bottom rails on the bolt sled also slide past the plastic a lot, so might as well lube them up, too.