I went to the range yesterday with my AR i just finished building a friend of mine took some video i wanted to know if its normal to have inconsistent muzzle flash i noticed in the video about 1 out of every 4 shots there would be a big muzzle flash other shots none at all iam shooting a pretty basic 16" barrel ar15 with a 3 port muzzle brake ammo mid grade 5.56  62gr

Video records in frames and muzzle flash is so fast that you might not have it captured properly on any particular frame. Don't worry about it.

Normal to see some flash, get a high speed camera if your worried about it.

Don't overthink things.  

Does it operate properly?

What is "ammo mid grade 5.56 62gr"?   Is this true full power 5.56 M855 green tip from Federal LC, or something else?  Who made it and what does it say on the box?  Is this factory ammo or handloads?   My guess is that variations in muzzle flash may also indicate that the ammo has been indifferently loaded with variations in powder level.   That would not be good.   How does it shoot?   Even true M855 is usually no better than 2 MOA on a good day.  If it shoots reasonably tight groups, its probably not the ammo.

It also may simply be a matter of perception.  Post above about video capture rate (frame by frame) is correct.  Also,  depending on the design of your muzzle device, viewing the rifle from the side can produce noticeably different amounts of flash depending on whether you change position, even a few degrees.  And, obviously, the amount of ambient light makes a big difference.  Far less visible flash at noon on a bright day than in late afternoon or dusk.   Muzzle brakes tend to not be so good at flash mitigation, either.  It's often an either/or.  If you want a brake, it is likely to be loud and have a good bit of flash.  If you want a flash hider, it may not be very good at reducing muzzle rise or recoil.

As mentioned camera frame rate can only so much. 

Another thing is the gasses expanding from the muzzle blast, and speed shooting (sometimes separately, sometimes together) can create mach rings. 

Thanks i appreciate it like i said it was a new build everything felt good just that video concerned me thanks again

The first round of every mag always creates a lot more of a fireball / blast out of my DD rifle, than all the others hardly have any flash at all

Depending on what kind of camera (and camera settings) you have, you may be shooting at 60-frames per second (either interlaced, which is basically 60-half frames, or progressive, which is 60 full frames), or possibly as slow as 30, or even 24, both progressive. Unless you've got something real nice, the likelihood that you're shooting faster than 60 is slim. Depending on the type of sensor (CMOS or CCD), that can also have an effect; CCD basically captures still images, while CMOS scans downward over the coarse of the interval (which can result in weird lighting and/or skewing issues as a result).

My point is that, unless you've got the right equipment (60 fps or greater with CCD camcorder), you're probably not going to capture muzzle flashes spectacularly on a consistent basis.

did the flash look the same in person?

Normal