I have built well over a dozen AR15s over the last 20 years or so. Most of those from 80% lowers.  I completed my first ar10 and am having a fail to feed issue.  

-Lower 80% Ares Armory
-Upper Ares Armory
-Barrel Ares 18" nitride coated
-Lower build kit from Brownell's
-Buffer and buffer spring from Brownells are AR10
-Buttstock LuthAR adjustable
-Magazine Magpul 20 round PMAG 20 GEN M3

I purchased all this stuff last year when they were still Ares Armory, but just got the time to mill it out and put it together.

The gun chambers and shoots and ejects when I load up by hand.  But when I load from the magazine the bolt seems to slow down too much and fails to go into battery.  I can hit the forward assist and it will chamber and shoot and eject but again when it picks up the next round from the magazine it slows down and will not go into battery.

I was thinking of trying out some other mags, but I don't want to go and start spending money if there is something obvious I am missing for this platform.

Any thoughts and advice greatly appreciated.

I'm no expert on the .308 guns, but some things that I would consider if it were mine.

Does the carrier move freely within the upper without a mag inserted while cycling with the charging handle? Is the gas tube aligned properly and not contacting the gas key? Are you positive you have the right action/buffer spring installed? Rifle or M4 feed ramps? Have you tried any other magazines?

What happens when you lock the bolt back, insert a loaded mag, then drop the bolt using the bolt release? Does it chamber a round or does it act the same way it does when firing? Does this failure to feed happen every time, or is in intermittent? Also, if you load one round in the mag and fire, does the bolt lock back?

The carrier moves freely without a mag
The carrier moves freely with an empty mag
The bolt will lock up with an inserted cartridge in the chamber as long as the mag is empty or not in the gun
The bolt goes into battery even when a cartridge is chambered, so I think the gas tube is is aligned
I don't know how to verify the spring is for an AR10.  It came with a kit for AR10 buffer and build kit
M4 feed ramps.  But the bolt carrier seems to slow down before the bullet even contracts the feed ramps
The same ftfeed occurs whether I manually run the bolt or use the bolt release
It acts the same whether feeding from scratch or firing
The ftf is every time, not intermittent
The bolt does lock back on an empty mag
The gun functions properly as long as I load the cartridge into the chamber by hand.  The bolt will pick up the new cartridge but it slows way down and does not seem to have enough umph to go all the
 way into battery.  I considered the spring was wrong or too weak....but again, I don't want to go off buying parts that may or may not solve the problem.

Relevant Update:

I discovered another clue while trying uncover the truth on this one.  When I load a round into the chamber either by hand or from the magazine (with forward assistance) but DO NOT fire the gun, cartridge gets stuck in the chamber and requires the butt stock slammed on the ground method of removing the cartridge.  Yes, I thoroughly cleaned the chamber and barrel before loading any ammo.

Click bait.  Not an Armalite, not an AR-10.  Hell, not even a factory rifle or lower.

What are we supposed to help you with?

Fail.

So you have two issues. You said the bolt stops before it even strips a round far enough out of the mag to even contact the feed ramps AND there's an issue with your chamber or bad ammo that even once a round is chambered, you have to mortar the gun to extract the case? Sounds like a combination of a couple different issues that aren't even related.

I say they're unrelated because if the carrier comes to a complete stop before the tip of the bullet even touches the feed ramps, then it's a separate issue from the one you're having once the round is actually chambered.

If you remove the magazine, drop a round in the chamber and close the bolt on the round, then pull the charging handle to the rear in an effort to extract the live round from the chamber, what happens? Do you still have to mortar the gun even without the mag inserted?

ETA: Obviously, be very careful when chambering live rounds in and out of the gun. This should be a given, but you never know with some people.

The bolt doesn't stop.....it slows down.

The ammo isn't bad.  It is new PPU 150g and also tried out Remingtion 150g

I have to mortar the gun to extract no matter what

Thanks for help.  Going to gunsmith tomorrow.  At the very least will need the chamber cleaned up.

Yup.  This is why I haven't visited arfcom in several years.......I am, after all,  posting in the "Build It Yourself" forum.  It's haters like you who spoil the true spirit of give and take on the ole interwebs.  I hope you die a lonely, slow and painful death.  Creeper.

You roped it, you ride it.

Lots of AR-308 data in the build-it-yourself, troubleshooting, and variants forums.

But of course you knew that.

I built 2 Ares Armor lowers into AR10s. First one I assembled and it worked flawlessly right out of the gate. The second one I had a bolt over round problem.

With any build I always load one round in the magazine and make sure the bolt locks back. I usually do this 5-10 times. Mine did lock back everytime.

When I loaded the magazines full. After the 4th or 5th round I would get the bolt over round malfunction. Occurred with 3 different brands of magazine. I swapped lowers and BCGs to narrow it down. Slow motion video revealed the bolt was not always going back far enough. I opened up the gas port in the barrel and all is well.

You say the bolt is slowing down. How do you know?

Do you have a carbine receiver extension or rifle?

I would remove the spring and buffer and then reassemble. Manually cycle the gun, you do not need a round just make sure that the BCG moves freely with no binding. Then insert the buffer but no spring, keep the gun tilted muzzle down and do the same. Does the bolt go all the way back or are you feeling the buffer bottoming out on the receiver extension?

If these tests prove there is no binding or bottoming out then you need to make sure the gas system is working correctly. Is the hole in the gas block lined up with the port in the barrel? Do you have any leaks? Is the gas key leaking?

If you have to mortar the gun to remove a live round that could be the cause. I would do the simple tests I mentioned but you also need to address the tight chamber. First thing you need to do is check headspace with gauges. If the headspace is tight this could be jamming the bolt at the lugs. If you don't have gauges have a gunsmith check it or rent them. There are a couple places that rent gauges cheap.

Are the rounds scratched or marked up that were "stuck" in the chamber? If so, there is a burr. If not, and it head spaces ok then you probably can polish the chamber with some JB Bore Paste and a shotgun mop on a drill.

You have to love the idiots who post things like "buy a factory rifle". Well some of us enjoy the build and trouble shooting. I also have no choice but to assemble mine from 80% lowers. My state has banned the sale of new ARs. I am sure that idiot will post something like "just move" or "you voted for them". Ignore the idiots. I know it is not easy they are multiplying at a fast rate.

Helpful.

Have him headspace the gun and clean/polish the chamber while you're there.