I have a bushmaster M4A3 14.5" upper with a mini-Y on my lower. Anyway, how does the mini-Y affect velocity as opposed to a plain muzzle? And how about a flash hider?

http://www.ar15.com/forums/topic.html?b=3&f=118&t=165287&w=searchPop

this post references a velocity of 3064 fps for a 14.5" barrel. With "win. 55 gr." ammo. Is that the sort of velocity I should expect with Q3131A? How about Fed/LC XM193?

Thanks for the help.

The muzzle brake shouldn't have any effect on velocity.

As for the actual velocity you will receive depends on a lot of other factors.  Your barrel may run a little fast or slow.  Ammo from the lot you're firing made be a little faster or slower than the average.  The weather conditions and altitude you are firing under can effect it.  It is safe to assume that you'll getting somewhere between 3000 and 3100 fps but the only way you'll really know is if you set up a chronograph and measure it.