Honestly I would like the iron sights better than the dandy little red dot thats on the DMR.
However the underbarrel shotgun may not work because the thing below the barrel there could be an important part of the DMR, maybe a gas tube. Of course it could also be a useless bipod mount.
I have always wanted to do something like this, like using a shorter range DMR sight on the sniper, or using an assault rifle mag and the full auto setting of the DMR, or even a short barreled sniper rifle with a supressor!