It's the castle nut. How can i know if it's stakeddfergus8 said:So you removed the castlenut and the tube won't back out of the reciever?
If that's the case, I agree with the poster above that suggested that it has locktite.
Soultion- apply heat to the threads with a heat gun then remove.
If the castle nut is still on there and not backing out, check to make sure that it has not been staked.
Can't say there, don't know what DPMS does but wouldn't be surprised if they use Loctite. The solution in the video I posted would work for you too. He does mention he was installing on a milspec tube like the Colt. If they have both sizes you'd want commercial. The castle nut itself is the same size but you do have to be able to slide over the tube.I have a dpms