So I came here for help (and i visit this site all the time so why not join). I recently decided to switch my ar to a mid-length system and im kind of regretting it. Just as the subject says its undergassed.

List of parts:
-Davidson Defense 5r rifled 16" mid-length stainless h-bar fluted barrel
-Carbine buffer
-Stainless gas tube
-mil spec gas block
-fail zero bcg
-YHM billet upper
-Competition Howitzer Brake

Problem:
- Gun cycles sometimes (75% of the time) but when it does the brass is almost straight back and only kicks to the side after it hits the shell deflector.
- Other 25% of the time I will get a double feed (mags used are US gi twenty rounders and Troy Battlemags)
- Almost every time the bolt carrier itself gets locked open on, not the bolt, or nothing catches at all.

Analysis:
- The gas block is aligned properly on the barrel (carbon ring around gas hole when removed)
- Gas key is properly tightened (its a Fail Zero bcg, id be pissed if it wasnt)
- Went from a Heavy buffer i was running in my carbine to a carbine buffer with minor improvement but significantly more recoil.

Please Help! love this build and its super accurate but I need it to run reliably for it to be enjoyable! Any help would be appreciated!

What ammo?

Is there any resistance or drag when hand cycling the BCG?

I've ran PMC bronze 55gr, Armscore 62gr, Tula copper jacket 55gr, Federal 55gr, PMC x-tac 62gr, ss109, and even some hot handloads which were maxed out. It cycled the handloads the best but still had problems and if it can't run factory ammo it's not worth it in my opinion. Bolt does seem to be a little snug in the upper but when I do the 45 degree test it stops until I lightly touch it then goes forward. You'd think under spring pressure it wouldn't matter?

Do you have a different BCG to swap in?  Not faulting FailZero here, just helping troubleshoot.  If nothing else, you can eliminate it as the culprit.  

Try firing single shots with the magazine removed from the mag well and check the ejection pattern.  If the mag well is out of spec, the BCG might be dragging a little bit.  
If it ejects fine with the magazine removed, you'll have your answer.  Also, check the alignment of the gas tube to the gas key.  

Have you checked the diameter of the gas port on the barrel and/or checked to see if there's a burr that might be partially blocking the port?  
A burr would likely have been eroded off by gas cutting by now, but an undersized port could lead to weak cycling.

I did try swapping the bcg and upper onto my buddies lower, same thing. The gas hole measures about .062. I could try and ream all the holes for burrs in the gas block but the barrel is fine, checked it with a light and eye piece. I'm thinking it's either the buffer spring, gas block and tube are clogged, the brake, or it just needs break in. Thoughts?

Look for unrecognized gas leaks.  Things can be tight but still leak.:

Start by removing the bolt from the BCG and see if the BCG slides smoothly in the upper receiver.  

Next, Use some CLP around the gas key, put some rubber tubing on the key and blow or use air to see if there are any bubbles indicating a leak.

Then put the tubing on the gas tube in the receiver, and put plenty of CLP around the tube to block and block to barrel assembly and use 100 psi to pressurize and look for leaks.

Also, make sure the BCG goes back far enough; you should see about 1/8"-1/4" of bolt when the BCG is fully back.

Dano has a copy and paste version he uses that is more eloquent but this should get you started.

So there is no leakage and burrs but I did find some carbon on the step just where the tube enter the gas block. Loose perhaps? Small amount of play, no more than 5 thou I'd say

Just read on tactical machining that a 16 inch midy should have a gas port size of at least .078 and mines about .065. Should I open it up to that you think?

First double check your gas block location to make sure it's perfectly centered over the gas port. Some barrel shoulders are cut with the expectation that the end user will be using a standard hand guard with the front end cap and tower style front sight base. You may need to shim the gas block forward a little.

I have never needed to open a 5.56 mm gas port from the factory. I have been forced to on occasion when using 6.8SPC, 7.62x39mm and 6.5 Grendel. Your barrel warranty will be void once you perform any home gunsmithing technique.

Contact the barrel maker if moving the gas block doesn't correct the short stroking.

Look closely at the gas tube/gas key interface. I had a factory gun that the gas tube was off kilter ever so slightly and sometimes the gas key would strike the gas tube. Sometimes not.

Measured the block and port and they line up. Might ream it out to .075 on my Bridgeport tomorrow. Not to worried as the barrel was only $120 and didn't have a warranty anyways. Hopefully it works. My thinking is I'd rather have it over gassed  then under.

I cannot tell from your sentence structure......

Have you:

Placed ONE round in a single magazine that is known good, and fired it?  
Does it lock back on the empty magazine?

How many rounds total, and what are you using for lubricant EVERYWHERE?

If you manually pull back the charging handle on an empty mag, does it lock back?

When you cycle the action with the charging handle, does it feel smooth going back and forth slowly?

With one round it won't lock back and I'm using troy battlemags and improved gi 20 rounders

It will lock back if I pull back manually

Have tried 300-400 rounds of assorted ammo to the same degree of effectiveness

I tried clp everywhere on all major lube points

Action feels smooth going back and forth with the only resistance coming from the bolt engaging

Thinking the gas port is undersized at .065 when most say for a 16" midy it should be at least .078-.08. Thoughts?

Have you tried swapping just the BCG?  (I know earlier you mentioned swapping the upper and BCG to your buddy's lower, but did you swap out and use your buddy's BCG?)  

If you've got an extra gas block and tube, that might not be a bad thing to try, too.  
I know that's a lot of extra work, OP, just trying to hit the easier stuff before you decide to drill out your gas port.  

If both of those swaps fail, I'd say break out the drill.  Like you said, the barrel was a decent deal and no warranty, so what could it hurt?  

Sounds like undergassed to me, given all those other points.

How are you measuring this?

I'd increase it to .075 and re-test, doing only the single cartridge in the mag, bolt lock test.

My BCM's are .082 and they are significantly overgassed.

Ya I tried my buddies palmetto bcg as well and no go. Checked out my tube and block and they seem 100% compared to my other ar. Even ran a thick wire down the tube and it went smoothly. Going to ream out the whole and try it then. Wish me luck!

I'm using a set of gauge pins I have that increase by 5 thou

Good luck, OP.  Let us know how it turns out!

If everything checks out okay I would open that gas port up.

I do it at the range with  a drill and bits using low powered/cheap ammo. (Tula)


Open it up one step at a time. .065-.070-etc.

Keep in mind that small changes in port diameter make big changes in gas flow.

That said, .063 is supposedly the minimum size for a 16" carbine gas barrel so would seem to be undersize for a 16" midlength. .078 seems to be a common size for 16" midlengths, so .075 might be enough. If you have something like .070 or .073 it might be worthwhile to give them a try first. Can always drill a bigger hole but never a smaller one.

When I was playing with gas port sizes, PMC Bronze was the weakest factory ammo I tested, so might use it as a reference. Avoid using Tula as a reference, as it has an odd pressure curve and can be somewhat variable.

First and foremost, gas ports are the size they are because of the tolerances created by the manufacturer into the chamber and the bore - it has nothing to do with "standard" sizes! Just because one mid (or any other sized barrel) has a .072 gas port doesn't mean another similar barrel from a different manufacturer should have the same size gas port. This is why all these charts floating around vary wildly. Be careful before you start drilling open gas ports, 90% of the time it's not the gas port and when it is, it's because the port is too big.

Having said that, at first I read "Daniel Defense" - my heart skipped a beat when I saw "opening up the gas port" combined with Daniel Defense. Then I realized it was Davidson Defense. LOL.

gas port size?

Well I drilled it out and... Success! Drilled it out to .078 and perfect functioning and ejection! Thanks for the help guys!

Heres some pics of the build:
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And the accuracy at 100 yards and get this, when reassembled the rifle I forgot to lock the quick detatch on my optic and it was loose the whole time! Ran out of ammo so i couldnt retry the accuracy testing.
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Let me know what you guys think!

Congrats, OP!  That's a slick looking setup.