Had a student fight his way through this over two days recently. He was a smaller gentleman, and was having a hell of a lot of trouble with the M4's collapsing stock run out nearly all the way. He (with great frustration evident) pointed out that collapsing the stock raises the cheek weld fairly significantly, so that if he shortened it enough to be proper LOP his face couldn't get down low enough to use the optic, much less the irons. Suboptimal indeed.
On the upside, it's a lot harder to accidentally trip the bolt release![]()