If you can't continually remember to reload before the clip runs out, there's a great place to grind this, and you can get it while you're getting your Dragon Sniper kills as well.
On Chapter 8, you'll run into some guys firing RPGs at you. You'll have to snipe 5 rocket shooters here, and there's a Dragon Sniper close by with exactly 5 bullets in it. Grab that, and snipe 4 of them -- DO NOT KILL ALL 5. Afterwards, jump off the building you're on, and repeat.
This spot works excellently because that Dragon Sniper pickup can't auto-reload. It has a full clip in it but nothing else. Even if you miss a shot and empty the clip on the 4 rocket shooters, the gun will be empty so it won't reload.