Ok, maybe someone can help me here. I purchased a PALMETTO ARMORY Model B4.15A1 this past week. Now the lower receiver is machined from a solid billet and is in great shape (minus the finish). The rifle is configured as a CAR 15 with the short barrel and long FH.
Now my problem is that at least once out of every magazine fired, the bolt/carrier will over ride the round in the magazine or the round will start towards the chamber, but will have the bolt/carrier ontop of it.
I'm using new USGI magazines with the green follers (fresh out of the plastic bags).
I've tried 4 different types of ammuntion (all 5.56mm FMJ).
I have changed the complete upper out and put it on a like new preban Colt and it does the same on it.
The weapon is clean and lubed with CLP.
Now the only difference that I can observe is the piston from the PALMETTO, while the same AOL of the Colt is configured differently. The PALMETTO's is black and the head of the piston isnt near a thick as the Colt's.
Please, I need help with this one!
Thanks,
Russell

Not much action here today, so another New Guy will try to help.  I would push the mag release button in all the way and screw the mag catch in another turn.  Be careful not to scratch the lower.  It might hold the mag a little higher and tighter.  

Good Luck,
Mike

When you get an "over the top" jamb, it is due to the carrier not coming all the way back and the bolt grabs the case on top(not rim) and forces it towards the chamber and the bolt comes to rest on top of the round.  The case will have a dent where the bolt slammed into it.

The correct problem, Your rifle is  short cycling on the jammed rounds.  

Since you have not stated what/where the barrel came from, and since it's having problems on other lowers as well,we will need a little more information on the barrel.  
The gas port may be too small, the chamber may be rough, or the barrel may be indexed incorrectly and causing the bolt to bind on un-lock.    The fact the you are keeping the upper lubed with CLP may be what is allowing to run.

When you start talking about pistons, You have lost me .  The only thing that comes to mind would be the buffer, but it is behind the carrier.  The AR-15 uses a gas tube and a pressurized system to unlock the bolt.  The bolt is not driven open by a piston.

P.S. Since I'm up at the cabin, and don't have my reference web sites loaded on this machine, maybe someone can post the repair manual, so you identify the parts and call them by their correct names.

Dano

Thanks SIGnificant and Dano523. I was wrong for calling the buffer a piston.
The barrel was made by OLYMPIC (as was the rifle itself and produced for PALMETTO ARMORY, excluding the Colt Fire Control Parts).
Thanks
Russell

Let's try to re-organize this a little:

Upper has bolt/carrier riding OVER the round and jamming.
This upper does this on another lower as well.
Different Ammo
Different mags

When you say a short barrel and long Flash Hider...how short of a barrel?  That will make a big difference as to what we look for.

Also, I am bothered by the low recurrance...only one round out of thirty each mag.  Most physical problems occur more often or more predictably.

Is it typically the 1st round, 2nd round...last round?
Do the other 29 rounds eject and chamber OK?

Royce I will answer you questions in order the best I can!
Q. Upper has bolt/carrier riding over the round and jamming.
A. The bolt/carrier will sometimes ride completely over the round in the magazine. It doesn't matter what the round count is. BUT, what it normally does is stop on top of the live round that appears to be feeding into the chamber.
Q. This upper does this on another lower as well.
A. YES (on a factory Colt).
Q. Different Ammo
A. YES (4) different brands of FMJ, 55grn, Winchester white box, S&B, Federal and the dreaded WOLF.
Q. Different Mags.
A. Yes, Bushmaster, Colt, USGI GI with green followers. All mags are new.
Q. When you say a short barrel and long Flash Hider...how short of a barrel?
A. The barrel is 12 inches and the Flash Hider is 5.5 inches.
Q. Also I am bothered by the low recurrence...only one round out of thirty each mag.
A. My only answer to this is that it is not predictable. It can be the 2nd or the 15th. Sometime it happens more than once a mag, but on average its once a mag. It seems that the faster I fire, the more problems I have with this and I'm not "bumping" either.
What I have also observed:
The gas tube is about a quarter inch longer inside the upper than on my Colt (looking down into the upper with the bolt/carrier and charging handle removed).
The buffer is black in color and is the same length as my factory Colt buffer. Except, the buffer head(?) is much thinner than the Colt buffer, and yes it is a carbine buffer and not a rifle buffer.
I was having failures with ejection until I put a WOLFE extractor spring in and it fixed the problem. It was attempting to chamber a live round with the spent casing resting on top of the round attempting to chamber.
My fingers are sore now and I hope that this helped!!!!
Thanks so much,
Russell

RTP - I'd guess that either you have fired way too many rounds filled with dirty Ball powder in the past week, and partially plugged your gas tube/barrel block; that, or the bolt carrier is full of carbon.

This problem is sometimes solved by installing a pigtail, or opening the barrel's gas port to allow a better impulse. First, though, try scraping the inside of the BC with a plastic spoon handle or something to check for carbon buildup
or roughness from machining...could need lapped a bit...crudcutter like Hoppe's usually does not remove the caked deposits (graphite used as flash retardant on Ball powder) from this area, but that would usually take a few thousand rounds to accumulate.

Be sure not to scratch anything in the area where the bolt's gas rings seat.

BTW, is the bolt locking back on an empty mag every time? If so, it isn't likely to be the mag catch's fault.

Good luck...hope this is helpful.

FastVFR

RTP, What is the distance of the brass on ejection.

The reason that I ask  is that your cases should be thrown at least 10' from the ejection port.  If the brass is less than this, then the new extractor was not the only thing that should have been fixed.  

As you know the short barrel will have a short/violent ejection.  The stronger extractor spring will keep the extractor from being ripped over the rim of the case on ejection, but the original problem was due to a tight/tool marked chamber.  The fact that you are using CLP to keep the upper/chamber lubed is saving you most of the time, but not always.  Every once in a while, the chamber becomes dry and this is when the rifle short strokes.

Try polishing the chamber to loosen it up, and to relieve the tool marks.  As far as increasing the port size, lets check the chamber first.


And, No cutting the buffer spring.  The problem is in the barrel, and not the back of the rifle!!!!!

Your problem started when you installed the 12" barrel.  There is a long history of 11" barrels having all sorts of gas-related problems, prmarily ending up with the barrel gas port diameter. You have a 12" barrel, whichh is already suspect since that is not a standard length.
Now, I do not own, nor have I owned, any barrel under 16", so I am speaking from observation, not experience.  Dano has a greater technical knowledge so he can tell you the physics of the short-barrel  problems better than most.  If you ask him nicely he may tell you.
As Sano said, try looening up your chamber first, but I believe you are headed for the gas port with a lightly wider drill bit.

I want ot thank each of you for your time in helping me try to figure this thing out.
I may have it running now (I hope!). I cleaned the gas system with carb cleaner (B12) and did it run black out the muzzle. I then replaced the recoil (action) spring and buffer with a factory new Colt products. It now runs with no problems....I hope. I'm going to give it a work out this weekend and let you know how it went.
I had stated that the barrel is 12 inches, its actually 12.5.
Thanks again!
Russell

RTP,

Just incase the rifle does start acting up, get a 30cal mop and polish out the chamber.  Since you were having problems with the extractor, I still believe that your chamber is still a bit rough.

Or, if you can keep the rifle running for a few hundred rounds, the chamber will polish out it's self.

Also, when you clean the rifle, give a shot of CLP down the gas tube and let it run into the barrel.  The CLP will break down the burn on fouling and will be blown out on your first shot.  But, keep in mind that it will be blown into your gas chamber, so make sure that the gas chamber is also lubed before shooting.  This way the crud is blown out the exhaust ports to exit the rifle.

Hope this helps.

RTP, I had one of those Palmetto/OLY lowers with a 100% Colt M4 upper on it. Mine was double and triple firing! I ended up buying all new fire control parts for it, then I had the same problem you are having.
When I compared the hammers I found that the original hammer had been "shaved" about 3/16's on an inch from the topside when the hammer is locked down.
The new hammer was being forced down and it was causing the bolt/carrier assembly to jam-up and it was also jamming with the round going over the top.
The problem is that on come of those machined lowers the fire control pin holes are not in the "right" place. I think that Palmetto fixed this problem by "modifying" the uppeers, lowers and fire control parts to make it work.
Not that it's a bad thing, it just makes swapping things over very difficult.
I ended up getting mine remanufactured by Oly.
That's the nice thing about that lower, Oly will fix it (for a price).

Don't rule out the ejector. I had mine stick on me and had jamming problems.

Well I'm back and the weekend went great! I fired 400rnds of Guatemalon 55grn FMJ without a malfunction. Before firing I did take a few nicks off the Bolt Release caused from Bolt bounce (?) and I believe that replacing the action spring and buffer probably helped. All I know is that it runs know and I'm happy.
Thanks again for the help!
Russell

Gald to hear that you got it running.