Finsihed putting together my SPR and took it to the range with upsetting results.  Accuracy was sub moa, but the rifle has the most sporatic cycling problems I have ever seen.

With three other AR's, 10 mags and 4 types of ammo I was able to switch carriers, springs, lowers, mags and ammo combinations until my head was spinnig.  

No matter what I did, behavior consisted of shortstroking, the bolt jamming up on next round in mag, the bolt advancing next round only to be hung up in the lugs, bolt closing without round in the chamber.

There was no incidences of FTF and only one instance of FTE in about 200 rounds.  I checked HS and everything is ok.  I firmly believe this is a gas pressure problem, but I checked the gas tube and there are no leak.  The bolt's gas rings appear to be in good shape (only 200 rounds old).

I am at my whit's end.

Examine the chamber and spent cases for signs of a rough chamber.  You may need to polish your chamber.

Does the bolt lock open on the last round out of the magazine?

First off, you have narrowed it down to just the upper at fault (?).

1. Check headspace and chamber size of the barrel.  Since you stated that your getting sub-moa, you might be trying to feed 5.56 Nato ammo in a 223 chamber. This causes the Nato ammo to work at higher working pressures, and the case to stick on ejection.

2.Check the barrel extension for tooling burs and possible the lugs being out of alignment with the upper.  The easiest way is to just feel the lugs for burs, then drop test a B/C into the upper to see if it free locks and releases.

3.Check the alignment of the block to barrel port, and make sure that the gas tube is free from any port burs that may have been blown into the tube during break in.  I use CLP down the gas tube to check for flow, and to dislodge any burs that may have been fouled inside the tube.

4. Check the chamber for tool marks made during reaming.  A simple way is to just look, then with the upper off the rifle, drop a live round into the chamber, bump the muzzle on a piece of carpet or floor, then turn the muzzle up.  The round should fall out of the chamber, without sticking.  A great test is to use a Wolf round, since the lacquer case will grab any burs better than a shinny brass case.

5. Use the upper on another lower. It may just be that the new built is just tight as hell, and need to keep lubed with CLP until everything mates.

6. On the new receiver, check the disconnector for signs that the hammer tail is too long.  A white line just behind the hook is a dead give away that the hammer is running out of room to drop down and allow the carrier to glide over the top of the hammer. If you find a mark or dent, remove .040 off the tail to solve the problem.

Granted, like Old man said, the chamber may just need to be polish, but the ammo will self-polish the chamber threw live fire in a few hundred rounds.  It's when you start to add up items like a slightly rough chamber, a tight key channel in the upper, and a few burs in the barrel extension lug area, and a hammer that is binding up on the disconnector, that the normal working gas pressure fails to fully cycle the rifle, even when lubed with CLP to reduce the friction of the new/rough bearing surfaces.

Sometimes, its about 50/50.  Acutally sometime, the bolt will lock open, but the bolt catch is locking on the carrier, not the bolt.

With the bolt getting hung up on the next round in the mag, this lead me to believe it is a gas pressure problem.  I also manipulated the gas block (PRI Flip-up, actually) around the barrel to see if I could get the gas port and gas tube opening to align better.  No luck, still very sporatic behavior.  

The rifle seemed to function better (but not near 100%) when using Winchester ammo (white box, not Q3131A) when compared to the 69 gr. Blackhills (absolutely did not work) and my handloads (68 gr Hornady, 23.8 gr. Varget).  The 55 gr. PMC was not that great either, but better than the heavier bullets.

Options?
1.  Try a different gasblock and clean the gas tube
2.  Enlarge gasport
3.  Rebuild upper on another receiver
4.  Throw the whole thing in the dumpster

Don't throw it in the dumpster unless it's near me!!!

It's frustrating to go through this but you learn a lot and very few problems end up costing much.  Invest a little thought, work through Dano's list, and report the results.  

You'll get her workin 100% and you will be building another one before you know it.

1.  All .223 ammo, no mil surplus.
2.  No burrs.  BCG lock freely and releases with minimal force (actually falls out it if is on 70 degree angle.)
3.  Tube is clear and I tried moving GB around.
4.  Wolf round fall out easily.
5.  Tried four diff. lowers.  2 std. trigger and (2) 2-stage.
6.  Plenty of room for the bolt to travel over hammer.

I tried this lower on three diff. uppers and it functioned 100%, so it is safe to assume it is the upper.

Thanks for all the help guys, I have not given up yet.  I have built all my uppers (more than 1 but less than 10, not including any I have sold or uppers for friends )as of now, including my AR10's, but nothing like this has ever happens.  I am pretty thorough in my troubleshooting checklist, but this one is has me stumped.  I am going back to the range tomorrow with a diff. gasblock and gastube.  If I ge the same result, what's next, opening up the gasport a bit more?

Gas port should be the last thing.

Normally the bolt rebounds off of the receiver extension. A gas port that is too large can act just like a short stroke. The bolt comes back too fast for the bolt catch or for the next round in the magazine.

If the problem is a "sticky" chamber, making the gas port larger will result in the extractor ripping part of the rim off or dropping the round too early.

Good starting points
20" barrel .093
16" barrel .063
14.5" barrel .067

a change of .003 is significant. People who make barrels know what they are doing.

If you have the tools, remove the barrel from the receiver and examine the chamber.

I once laid up a flat top upper, squared the gas block against the upper, and proceeded to have short cycles.  
It turned out the barrel port was not dead centered with the upper, and the to get the gas block and barrel ports to align, I had to hold the gas block off the barrel block last cut about .050". Since your block is a flip up, then you may have to re-install the barrel to keep the port top center.  

The best advice I can give you is to pull the flip up sight/block off the barrel, then pencil mark the barrel port on the barrel.  Also, check to see what set back is needed in regards to the block and barrel port for alignment. With these two marks, you should be able to bind install the front sight tower and keep the ports in aligned.

Also, since I don't know which type of gas block you are using for the flip up sight, make sure that it does not have a gas setting screw that may be screwed all the way in and restricting the gas flow.

As for opening up the barrel port, you are much better off sending the barrel back for replacement.  Once you drill the port, it will void any warranty on the barrel.

Your gun is doing exactly what mine was doing before I sent it back to DPMS to fix. It was a gas problem, the port was not big enough and the chamber was too tight. They opened both up and it eats ammo like a pig now!

I tried every thing you tried and I know it is very frustrating! Hang in there, I bet it is your barrel. GOOD LUCK!

OK, it's definately short stroking, the bolt catch hooking the carrier instead of the bolt face is a sure sign.

If you are certain the the gas port in the barrel and the gas port under the GB are aligned then check that the hole in the GB runs completely into the hole in the gas tube. Drop a drill bit of the proper size into the hole and twist it by hand.

Sometimes the gas tube roll pin hole in either the tube or the block is mis drilled. When the pin is driven in it rotates the gas tube and occludes the gas port.

Next check the ID of the gas tube. Compare it to know good ones.

Check the slip fit between the end of the gas tube and the carrier key using a mic.

Pull the carrier key off and check that the gas holes go all the way through and are the right sizes. Make sure that the gas hole in the top of the carrier is the correct size and correctly located.

Swap in a different carrier.

After each check above do the one shot check while pressing in the bottom of the bolt catch.

Sound like you have a live one 223Rem.

What length and make of barrel? Size of existing GP?

Sunday's Range Report:

I have no clue.  Can't begin to understand what is happening.

Spent some time yesterday and rebarreled the upper with a different receiver and a NM 20" Stainless medium weight 1X8 Wilson (good shooting barrel) not the 18" Stainless Hbar, originally used.  Used the same gas tube and PRI flip up front sight as before  Thought for sure this was it, definite winner.

Same ammo, 69 blackhills, 68 handloads, 55 winchester.  I brought a box of 75 hornady handloads and wolf 62 for shits and jiggles.  

Started with the blackhills.  Same occurances as before.
68 handloads same as before.
Wolf acted the same as the 68's and 69's
The winchester cycled correctly, but did not hold the bolt open on one occasion and the bolt catch locked on the carrier the next.  Better but not what it should be.

Now to my 75gr. Hornady handloads with 23.6 grains of Varget (.2 grains less than the 68 gr. hornady handloads) OAL of 2.260".  100% functioning.  WTF? 50 rounds, 5 rounds in 10 different mags, No FTF, FTE and the bolt stayed open on the last round each time (correctly, on the bolt, not the carrier).  Plus the ejected brass was actually hit the ground in a nice little pile about 2 feet across.  Ejected brass shows no sign of abnormal wear.  Average group size was 3/4".  Hip hip hooray!

Figuring the rifle cured itself of this nasty case of constipation, tried the wolf ammo again.  Nope, rifle wanted nothing to do with it, same crap as before.  68's, no luck there either.  Winchester, cycled fine and held the bolt open, but caught on the carrier.  Did not have anymore 75's, so I do not know, but I have a feeling they will be 100% tomorrow when I go.

So riddle me this Batman.  How can a gun function like crap on factory ammo, but function flawlessly with specially catered ammo.

Guys, again thanks for all the help.

223Rem,

Did you take any of the actions I, and others, have outlined?

If you start swapping parts in without a plan you'll never figure out what is wrong.

I see you are still using the same gas tube and gas block. Have you checked either of them?

Tweak,

Yes I have followed the checklist you and other have posted, but a lot of the suggestions I have all ready performed even before I started this thread (ie. swap lowers, carriers, alignment of gasport) stated in my first post.

One thing stuck me though was a "sticky chamber" and misaligned gas port.  The only way I could cure a possible sticky chamber  and bad gasport was to replace it, so I did with a barrel known to shoot well.  The only other 20" upper I have right now is my CMP serive rifle and I do not want to take the FS off and go through zeroing it again.  Maybe I will just have to suck it up and do it anyway.

I am picking up my other 20" AR tomorrow which I can readily swap out the gas tubes and see what happens.

Seriously how could a load with a lower charge cycle a rifle properly as opposed to a higher pressure load.

Will be it touch.

 

I had a problem very simmilar to this type.
It about drove me batty.
Mine would function with Remington but absolutly nothing else including Win, Lapua, Federal, IMI and SA.
I switched alot of parts in and out of it, I had the gas port enlarged a bit, I tried over 30 different mags, new mag springs, on and on and on.
I finally read a posting out here that caused me to look at the stock of the rifle. To make a long story short, someone had forgotten to put a hole in the screw that holds the tube in the stock and it was causing the rifle to short stroke like what you are describing.
I replaced mine and shot it some more and the problem has gone away.
I would have never caught that on my own and would have either scrapped the upper or sold it to someone else and would have still been chasing the problem.

Wow, never would have thought about that one, Shadowsabre.  I will definitely look for that

I switched out the gas tube and the rifle functioned 100% for 100 rounds of assorted ammo.  I am still not convinced that the rifle is 100% though.  I put the original tube in another rifle and that rifle functioned 100% for 50 rounds.  Not going to try and guess what the deal is, just going back to range and see how the rifle performs.

Shadowsabre,

Are you talking about the drain hole in the upper buttstock screw?