I just saw the second "over-ramped" M4 barrel extension on an SR-15 Carbine to be returned here (KMC)for repair in the last 30 days.
But that is just half the tragety (i.e., original parts can't be repaired). The other half is that the current owner's of these two carbines are not the original ownwers.
They purchased these already "broke" guns from subsequent owners, two of which had dremeled/cut/polished the feed ramps way down into the aluminum receiver, and way too wide in the barrel extension as well. So what happens is the rounds initially feed out of the magazine OK, but then are not ramped up into the chamber void. Instead, they enter the barrel extension so low in position, that the bullet tips strike & stop on the rear edge of the chamber.
Buyer beware!
Check out (hey, print out a copy and put in in your wallet) of a properly ramped barrel extension here on ar14.com, and carry it with you to gun Shows and Pawn Shops.
Don't buy someone elses wall hanger.
And parents, do you know where your dremel tool is right now?