Just finished my 300 Blackout build......

[Grassman]

....But it won't cycle.

I'm wondering if its the ammo? The upper, 16" 1/7 twist barrel.
Only ammo I could find locally is some Hornady 110 gr 300 Whisper V-Max.

It would not cycle it, and half the time not even eject the cartridge. Is there much difference in 300 Blackout and 300 Whisper? Doe's it have anything to do with bullet weight? I am going to start reloading for Blackout, just havn't found any dies locally yet. Also, doe's buffer and buffer spring have any bearing on this?

Any thoughts?

 

[Rusty Luck]

Is the gas tube properly aligned? Some whisper ammo won't cycle.

 

[wally]

Also, doe's buffer and buffer spring have any bearing on this?

Not usually unless you've started out with a heavier than standard buffer and/or "extra power" spring.

 

[Grassman]

Is the gas tube properly aligned? Some whisper ammo won't cycle.

It looks to be fine.

 

[wally]

It looks to be fine.

The issues is usually a misalignment of the port, gas block, and tube that either "leaks" gas or partially blocks the flow. The leaks usually leave evidence after some shooting, the cause of restricted flow can be really hard to see after assembly -- say the port in the gas block doesn't line up right with the port in the barrel.

With the current high demand its not as uncommon as it should be to get some parts that probably should not have been let out the door.

I have a 7.62x39 11" barrel that was under gassed. Turns out they drilled an 0.065" port (correct for a 5.56 carbine) I needed to open it up to about 0.093" to get perfect function with a wide variety of ammo.

 

[Rusty Luck]

I had a similar problem when installing a low profile gas block on a new barrel, the holes didn't match up correctly. It resulted in the same problem it sounds like you're having.

 

[hentown]

What's the brand of the OP's barrel? Also, if the OP is using an adjustable gas block, it should be wide open, of course.

 

[Walkalong]

What BCG? What weight buffer?

My supersonic loads, including the 110 Gr V Max will cycle an AR Semi auto BCG (Lighter than FA) and a standard rifle buffer, but will not cycle with an M16 FA BCG and the same rifle buffer.

 

[hentown]

I'm using a standard collapsible stock spring and buffer and an M16 BCG with my .300 Blackout. I'm also using an adjustable gas block. For supersonic rounds, the agb doesn't have to be but about 1/2 open for reliable cycling.

The gas port on my .300 Blackout barrel is .120"

 

[Walkalong]

The combined weight, BCG plus buffer, the OP is using is what I am curious to know.