Not bad
That "weird thing" you refer to is called the ejection port ... where spent cases come out. They're experimenting more and more with caseless ammunition these days, where each round is surrounded by propellent instead of the round being shoved into a case which holds the propellent. The gun fires, the propellent is spent firing the round off, and you don't end up with any ejecting brass. Caseless ammo is not only lighter, and uses less resources to make (no case), but depending on the propellent used, you may or may not be able to fire a gun under water. The guns most folks are familiar with are actually quite old. EG: AK47 originated in USSR quite a while back (60's or 70's I think.) The SPAS shotgun, which seems pretty modern, is an 80's gun. M16 & M60 are from the late 60's to 70's. Since war has unfortunately been going on for so long in so many places, guns were refined pretty quickly early on. There hasn't been a need to re-invent the wheel, so to speak, with them as often as, say, computers. Older guns, when maintained properly, are reliable and keep going. An AK-47 built in the USSR could have been circulated to many other conflicts across the world and still seeing service today. And even fav guns, like the AK-47, have variants and knock-offs (EG: Chinese variants), since it's such a popular and reliable design.
That's TMI, though. Your flash was good, although I've never seen a sniper rifle that loads the way you animated yours. Usually they're bolt-action (EG: Springfield), or clip-based (dragunov SVD, or AR .50 cal).