I'm not sure if it was the artist's intent or not (I can't find any mention on Pixiv, but then, I can't find much of anything on Pixiv), but the top of the magazine would indicate that this is an airsoft gun?

That Aimpoint is also a bit high'ish

Where is this "assault rifle" mentioned in the tags?

hungkok2007 said:

Where is this "assault rifle" mentioned in the tags?

It's still in the list 5.4 hours later, so I guess you found it.