A2 receiver to flat top

[twing21s]

Good day all,
My question may seem a bit elementary to some but I need to ask any way. I have several AR rifles, and only one is the A2 type upper receiver with carry handle. I want to change the upper to the A3 style I noticed that most receivers these days have M4 cuts would this affect the function with out installing an M4 barrel extention? :brickwall:

[Constructor]

The extension will over-hang the receiver and could catch the tip of the bullet but, you can use a chainsaw file or a dremmel to match the ramps of the extension with the M4 ramps of the receiver.