Charli Carpenter points to an ominous infographic from the UK nonprofit Action On Armed Violence:

IGp-DD-Overview

It shows some of the worst conventional weapons used in Syria:

What NGOs are calling attention to is what Nina Tannenwald refers to as a perverse effect of international prohibitionary norms. While norms can constrain bad behavior, such norms also exert “permissive effects” whereby the social sanction against one type of action implicitly legitimizes others. The international reaction against chemical weapons in Syria, to the exclusion of attention to other types of civilian harm, constitutes such a “permissive effect” on civilian targeting and the indiscriminate use of explosives which should rightly be questioned by human security advocates.

The organization says such weapons have been responsible for about two out of every five deaths in the civil war. Ninety percent of those victims have been civilians.