New AR Build Problems

[DFinan]

Ok looking for help from all the black gun experts out there. Finally got to go to the range and tune the gas system test the rifle and work on zeroing it in. We ran into a problem with the gun failing to go into battery

We looked at and tried the following

1. 3 different magazines (all 2nd gen magpull 30 round mags with 3-5 rounds loaded)

2. Two different bullets, a 55 grain hornady soft point (exposed lead) and a Ultramax 55 grain fmj (fmj ones appeared top go into battery consistently to start with but started having problems after about 10 rounds.

3. If the bolt was release with the paddle while empty it was pretty consistent and went into battery (though it failed a few times).

4. The gas system was tuned to one click more then needed to lock the bolt back on an empty magazine.

One additional note on the soft point bullets two of them showed gouges or grooves in the the jacket towards the base of the bullet prior to where the case starts, and the case had no marks or scoring. In all cases all rounds fired correctly when manually dropped in and the bolt was allowed to close while the gun barrel was pointed down.

I have pictures below and a vidoe of the issue if anyone has any suggestions. Aside form this issue, the gun was fun to shoot had zero rise and felt like I was shooting a .22 but sounded like something much larger .

Only picture I have is to large to upload I can email it if needed.

Edited September 20, 2015 by DFinan

[BitchinCamaro]

What upper/BCG/Buffer/spring?

[Stlhead]

3. If the bolt was release with the paddle while empty it was pretty consistent and went into battery (though it failed a few times

Can you explain exactly what you mean by this? It sounds like there where times when you drop the bolt with the bolt catch with no magazine inserted and the bolt failed to go fully into battery. Was this the case?

[DFinan]

BitchinCamaro

PSA Upper (no forward assist), Boomfab Titanium Low Mass BCG and a Jp Low Mass Buffer cant find the invoice on the spring (rifle Length gas system, 18" barrel)

Stihead

Correct no mag inserted, bolt locked back, hit the paddle to release bolt and it would on occasion fail to fully go into battery almost as if I was riding the charging handle.

[Larry White]

Check if the gas key is hitting the gas tube.---------------Larry

[BitchinCamaro]

I second Larry. See if the gas tube is entering the upper without any torsion.

Then pull the rear takedown pin and see if there is any obvious sign of the tube getting smashed by the key.

Did you check headspacing?

If it's not physical interference, it may be the carrier/spring/gas combo.

This is just pure conjecture, but if the ultralight carrier has just enough gas to engage the bolt release, it may not gain enough momentum in the forward direction to send the bolt back into battery reliably. I think the only way to check is to add mass, or increase the rearward stroke in order to give it more time in the forward direction to gain adequate velocity.

It may seem counterintuitive, but try increasing your gas far beyond the bleeding edge and see if that makes a difference.

Edited September 21, 2015 by BitchinCamaro

[warpspeed]

Check if the gas key is hitting the gas tube.---------------Larry

This -

[Stlhead]

If it won't go into battery under it's own spring pressure, it is not a gas issue. Here is how I would troubleshoot it. Take the upper off the lower and unlock the charging handle latch, gently pull back on the charging handle feeling for resistance. Remove the bolt carrier and the charging handle, then reinsert the bolt carrier without the charging handle and slowly close it, paying careful attention for any hang ups. If nothing comes up so far, take the BCG back out and swap it for a known good one from a different gun, try the new carrier in the other gun as well. Thus should rule out the BCG as the culprit and let you move on, unless you found a problem already.

[BitchinCamaro]

If it won't go into battery under it's own spring pressure, it is not a gas issue.

You're right, I got ahead of myself thinking about the Boomfab.

Difinan, does your bolt move relatively freely in the carrier?

[Absocold]

Recoil spring weak/binding, buffer binding in buffer tube, carrier bind, cam pin bind, out of spec hammer (seen it, sat too high and rubbed the carrier something fierce), barrel not indexed properly, front face of upper not square causing barrel to be out of true (extremely common on forged uppers but only very rarely is it bad enough to cause feeding issues), barrel not fully seated in upper, bolt lugs or barrel lugs out of spec... the list goes on and on. But the number one cause of constant failure to go into battery on a clean new gun is firing pin retaining pin not fully inserted or above flush.

After a full assembly verification check, I'd just sit and work the action back and forth over and over, checking for rub marks every dozen cycles or so until the problem showed itself.

[DFinan]

If it won't go into battery under it's own spring pressure, it is not a gas issue.

You're right, I got ahead of myself thinking about the Boomfab.

Difinan, does your bolt move relatively freely in the carrier?

It does not, no hang ups or roughness in operating the bolt

Edited September 21, 2015 by DFinan

[dauntedfuture]

Provided that there is not an issue with the gas key hitting the BCG i would try another bolt with the same BCG that you are using and see what happens. On my last build i had to swap out the new coated bolt with an older one to get things running. I suspect but i dont know, that the new high speed coated bolt is a little on the big or stiff side of things.