Basically designed to feed full auto with a bit more reliability....Should help in semi-auto mode too you would think.
If you are looking to build on an upper with m4 feedramps, you MUST use a barrel with m-4 feedramps.
If you are building on an upper without m4 feedramps, you can use a barrel with or without the ramps.
Seems to me a barrel with and a receiver without would be less than ideal.
I like to keep them matched up myself.