Beginning in 2020, we have introduced a replacement for the bumper and disk spacers in the buttstock.
The one piece bumper is designed to replace both the rubber bumper and the nylon disk that were inserted into the buttstock tube previously. These items were in there to cushion the back of the tube from the buffer slamming into it. The disk was there to protect the rubber from the spring. The downside to having these pieces in there was people would remove them or reverse the order of assembly and cause the bolt to overtravel and allow the reciprocating charge handle to slam into the back of the receiver causing damage. The new bumper is a press fit to prevent accidental removal.
Whenever you remove a buffer tube from the JRC, upon reassembly you should check for proper function and spacing. Ensuring you have removed all live ammo from the weapon, cycle the bolt all the way to the rear. You should have the charge handle fall past the lockdown notch, and yet not all the way back to the keyhole in the back of the receiver. It should fall approximately half way in between those to landmarks. If not, then something is wrong and you should investigate before firing the weapon.