Magazine related double feed - velocity loss?
 

Would I get a loss in velocity from a strictly magazine related double feed issue? Right now I'm getting a consistent double shot feed from some of my mags. Is the second shot or even the first shot affected resulting in a loss of velocity? Or is it not a big deal and now I have burst mags :D

Only certain mags? That seems strange. A double-feed issue would usually be cause by the nozzle/tappet plate or hopup.

Is it possible that it could be double feeding from a weak motor and a 11.1v lipo? I'm set up on an ics sig 552 with a 11.1v lipo (all I had when I bought it), I don't like how fast it shoots so I'm stepping down to a 7.4v lipo. Will this solve the double feed problem? It feeds fine on full-auto.

over cycling due to the 11.1v?

Yes if you have too high of a voltage your gun could over-cycle, or in your case even cycle twice on semi.

As long as your gun propels two BBs at once, you will always get a velocity drop because you're moving twice the mass per shot. It obviously means loss of range and accuracy :(

The double feeding issues with the ICS SIGs, are related to the tappet plate being shaped slightly different than the normal ones. It releases the nozzle forward softer, and thus also slower. When you get a little overspin, which you do in most cases, this particular tappet design keeps the nozzle fully retracted longer. This leaves the hop-up open and the pressure from the magazine feeding spring causes two BBs to be fed into the hop-up. If you're adventurous you can grab a file and tune it slightly. That or just get a new one ;)

I've also noticed that the stock piston has an abnormally high pickup tooth (as well as some strange reinforcement of the first 2-3 teeth). This causes binding with the sector gear on all three gear sets I tried out. Your best bet is to reduce the tooth heigh or get a new piston. Also remember to remove the 2nd tooth and reduce the height of the 3rd tooth!* The higher the RoF, the more important it gets. The gun can totally handle the 11v lipo, but port your piston head and get a bearing spring guide :)

* Preferably shim the necessary amount between piston and piston head too. This is called AoE-adjustment. Check out the many guides on the web if you're new to it.