Most mid-length gas systems come with a triangle end cap.
The Magpul Mid-length Moe SL requires a round one. I'm not sure if yours requires a round end cap. Maybe contact Midwest Industries and ask. You can do so here.
http://www.midwestindustriesinc.com/Articles.asp?ID=259
If you do need a round end cap you will have to swap it out. It isn't that difficult if you have some mechanical aptitude. You will need some tools to do it properly and safely. If you watch videos and read about it first you can do it in about 30 minutes.
-Tools Needed-
Upper vice block
Armors wrench or regular 3/4 inch (19mm) wrench
round end cap (0.750 inch Diameter)
crush washer
roll pin punch
flat pin punch
brass hammer
-remove hand guard
-remove flash hider
-pound out front sight post gas tube pin
-Pound out the front sight pins. (this is the most difficult part, but -read up on it. took me five minutes)
-slide off front sight
-slide off triangle end cap
-slide on round end cap and line up gas tube
-slide front sight back on
-install front sight pins, gas tube pin
-install new crush washer and flash hider
-drop in your hand guard
Ammoland has a nice write up on removing the front sight.
Removing an AR 15 A2 Front Sight - Step by Step How To Guide