I think Rob62 is about right, I think it's about 100 FPS per inch or so. Check out
www.balliisticsbytheinch.com, they have a breakdown of .223 velocities there.
The other issue you're going to have if you start cutting your barrel down is gas pressure. The length of barrel in front of the gas port is referred to as "dwell time"; that's the length of time that the bullet has passed the gas block and gas pressure builds up in the gas tube before the pressure drops off when the bullet exits the barrel. The longer the dwell time the higher the gas pressure. Conversely, the shorter the dwell time the lower the gas pressure.
If I remember correctly, the DDM4 V7 has a mid-length gas system. The general rule of thumb is that the ideal gas system for a 14.5" barrel is carbine-length and for 16" it's mid-length. If there is too much gas because you have a 16" barrel and carbine-length gas system it will probably run just fine; most ARs with this configuration are a little over-gassed and still run fine. And it's easy to fix a rifle that is malfunctioning due to being over-gassed.
But a 16" barrel with a mid-length gas system generally isn't over-gassed, so if you start cutting the barrel down it won't take long until your dwell time is too short and your rifle is under-gassed. And an under-gassed rifle is usually much worse than an over-gassed rifle; at best an under-gassed rifle will be ammo-sensitive, will have lackluster ejection, and might not lock open on the last shot. At worst it will short-stroke and not work at all.
If you have a mid-length gas system and you start cutting your barrel down to 14" or even 12", you're going to have under-gassing issues. You might be able to fix this with a lighter buffer and/or spring weight, but at that point your rifle will be more ammo sensitive than it was before.