The bottom photo shows what appears to be an AR15 with standard Magpul 30 round magazines; one in the rifle, one out. However, the angle of view is such that there appears to be more girth to the mags and a slightly straighter body shape, which might make it an AR10 or one of the ~30 caliber AR15 variants, like 6.8spcII.
BTW, the slide fire stock does not make the gun a full auto, it is still one round per trigger pull and the trigger must be pulled. It is fully approved by the BATFE (ATF) and has been for many years. It also does not add any capability that these rifles cannot already do without it by what is called bump firing.
“... the angle of view is such that there appears to be more girth to the mags and a slightly straighter body shape, which might make it an AR10 or one of the ~30 caliber AR15 variants, like 6.8spcII....” [xander, post 59]
The top image in maggief’s initial post (with the reflex sight and the bump-fire stock) does not show a rifle of different caliber. The magazine with the wider body below the magazine well is an extra-capacity magazine: 50 or 60 rounds, in 5.56mm size.
The wide-body concept typically incorporates two columns of cartridges with a separate follower for each. The basic design is not new: it was pioneered in Finland and produced by Suomi, for submachine guns (1950s or 1960s). Later, the Swedes adopted a version for their M45 submachine gun, modifying the magazine well to permit removal to accommodate the wider magazine. Affords greater capacity with no major increase in length, and far less complexity (hence lower cost and weight) than drum magazines.
Adaptation to 5.56x45 NATO and the AR-15-style rifles is a recent development (past decade or so).