New AR-15 build cycling issue

[smgrendel]

Just finished my first full AR build and im having some problems with it cycling. my first trip out back it would not cycle at all, brought it back in and adjusted the gas block, checked and doudle checked for proprer alignment. It will now eject but only load a round about 50% of the time. All brass is landing in a nice neat pile about 18 inches away. Good news is she can shoot... 100 yard four shot group

Build list:
Rock creek SPR barrel
AAC shorty muzzle brake
Vltor Low profile gas block
Rifle length gas tube
PRI rifle length tube
PRI rifle length forearm rail
Young M16 BCG
BCM Charging handle
Rainier arms ultra match upper
POF Gen 1 lower, JP lower parts kit, Geissele SD-E, Colt A2 stock buffer and buffer tube.

Thanks,
Shawn.

 

[Solid_Squirrel]

Re: New AR-15 build cycling issue

Very nice. What mags are you using?

 

[BigJohn141]

Re: New AR-15 build cycling issue

What kind of ammo are you running and how did you align the gas block to the port? Sound like you might be running under powered ammo such as wolf or something or the gas block is not aligned with the gas port in the barrel and not getting enough flow back the the BCG causing a short stroke.

Solid_Squirrel, that is a 20 round MagPul P-Mag and looks to be flat dark earth in color.

 

[smgrendel]

Re: New AR-15 build cycling issue

Pmag 20 round mags

 

[Smith44699]

Re: New AR-15 build cycling issue

It might be the buffer spring. Just a thought. I ran into that on a build once. I had to stiff of a buffer spring. I was using a hydraulic buffer though. Merry Christmas

 

[smgrendel]

Re: New AR-15 build cycling issue

Ammo is HSM. The Vltor gas block has a hole in the bottom that matches where the gas port is, i aligned that hole with the dimple on the bottom of the barrel.

 

[Desert_Racer]

Re: New AR-15 build cycling issue

I had a similar problem when I built my 6.5 Grendel. Turned out to be a bad bolt.

 

[smgrendel]

Re: New AR-15 build cycling issue

Smith,
Thanks ill grab another spring and see if that helps.

 

[Solid_Squirrel]

Re: New AR-15 build cycling issue

I thought it was a 20 round Pmag, but wanted to make sure. I have read about instances where it seemed like, when a gun is already on the edge of cycling, whether one is using a magazine with or without constant curvature internals, can seem to make the difference. That is to say, it isn't the root cause, but does seem to make at least an incremental difference.

20 round Pmags don't have constant curvature internals. 30 rounders do.

 

[BigJohn141]

Re: New AR-15 build cycling issue

Do you have a air compressor? If so, use a blow gun and pressurize the barrel and move the gas block around till you hear the most air flow coming out.

 

[b7tac]

Re: New AR-15 build cycling issue

I had a similar problem with my AR when I first got it. I was loading the mags to full capacity before the mag springs had a chance to break in. I ended up only loading to 25 instead of 30 rounds for a while until the mag springs loosened up a little and ever since not one cycling problem.

Nice group by the way.

 

[smgrendel]

Re: New AR-15 build cycling issue

Bigjohn thanks,
Great idea never thought of that.

 

[smgrendel]

Re: New AR-15 build cycling issue

b7tac,
all the mags ive used are just out of the wrapper. ill try some of my older 30 round mags tomorrow.
Thanks

 

[bigwheeler]

Re: New AR-15 build cycling issue

You do have the buffer tube vented? And not too much lube in the buffer tube. I've
seen grease plug the vent in cold weather on a new ar. Just some thoughts, an A2
should be vented, unlike a carbine stock.

 

[Dbltapipsc]

Re: New AR-15 build cycling issue

What diameter is your gas hole in the barrel? is there any debris blocking it. Double check your screws on the gas key, and make sure your buffer and spring are for a rifle length butt stock. I wouldnt trust a witness mark on barrel with your gas block, I would make sure they are aligned.

 

[mtrmn]

Re: New AR-15 build cycling issue

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BigJohn141</div><div class="ubbcode-body">Do you have a air compressor? If so, use a blow gun and pressurize the barrel and move the gas block around till you hear the most air flow coming out. </div></div>

Now THAT'S news you can use. EXcellent idea. THANK YOU!!

 

[smgrendel]

Re: New AR-15 build cycling issue

Thanks for all the info, one of the reason i love this site. im sure with all the info here she will be running just fine tomorrow. Thanks and Merry Christmas!

 

[375rgr]

Re: New AR-15 build cycling issue

Have you shot it with just a single round in the magazine? It should lock to the rear. If it doesn't it is under gassed and may have a gas leak. Check the gas tube, gas block etc. If it does lock to the rear. When it is cycling with multiple rounds does the bolt face pass the base of the cartridge when feeding it has to much gas. The magazine cannot keep up with the bolt carrier velocity. The gas block needs a smaller hole or the buffer / stiffer spring to slow it down. If it is not either of these then mechanical is next

 

[HPLLC]

Re: New AR-15 build cycling issue

Some of the newer premium barrels will just about undergas stock. It may also be a small gas port. New carriers need to be cycled 20 times to smooth up sometimes - the gas rings can be tight.

A lighter buffer may solve the problem.

If the gas block is not aligned it will be undergassed.

 

[Guest]

Re: New AR-15 build cycling issue

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: smgrendel</div><div class="ubbcode-body">Just finished my first full AR build and im having some problems with it cycling. my first trip out back it would not cycle at all, brought it back in and adjusted the gas block, checked and doudle checked for proprer alignment. </div></div>
My money says you have a gas issue. I've found it more prevalent with chrome lined but it can happen with a std CM tube. Many times the mfg will insert a small piece of plastic into the gas hole to prevent any down the line process material from getting into an attaching to the hole wall, an effectively reducing the port size. At times the plastic is not totally removed, properly. Start at the beginning an move rearward, don't use a shotgun approach.
The following are factory specs that many barrel makers use.
* 10.5"barrel - 0.093
* 11.5" ------- 0.081
* 14.5" --------0.063
* 16" --------- 0.063
* 20"---------- 0.063
If you over gas an AR and it will show up in the cyclic rate first, Carrier second, bolt third. All based on the buffer vent open with the correct buffer an spring.

 

[smgrendel]

Re: New AR-15 build cycling issue

Think im going to call this a bad batch of ammo and maybe a shitty pmag. Ran perfect today with Southwest ammo 77gr run and gun and federal. Thanks for all the advice.