I have been reading the posts regarding the piston operated Stag Model 8 and it seems that everyone is quite satisfied with their rifle.  I however have had to return two model 8s because of severe gouging in the upper receiver in the area where the charging handle rides when the bolt is in battery.  Neither of the rifles had been modified and both had fired 120~150 rounds of American Eagle Tactical XM193 5.56 x 45mm 55 grain FMJ.

What follows is the scenario for both rifles.

First time out to the range.  Less than 70 rounds.
Field stripped the rifle and cleaned it.  This included the gas piston.  No problems noticed.
Returned to the range a few days later.  Another 60 rounds.
Noticed misfires and "delayed" firing.  The firing pin detents on the primers for the misfires/delayed firing rounds was ever so slight.  
Returned home and cleaned the rifle.  Only this time I noticed that there were metal flakes near the ejection port and sever gouging in the upper receiver.  

Returned the rifle to Stag.  They replaced the upper and returned the rifle to me.

The scenario for the second rifle was the same as for the first only this time the gouging was not as severe.  Stag issued a return UPS label and the rifle is in transit.


I am curious if anyone else has experieced a similar issue with their model 8?

Here are some pics.  Sorry about the quality, but the area in question is the shiny silver area below the sring to the right of the machined out rounded area.  The is about an 1/8 inch deep.

http://i848.photobucket.com/albums/ab46/sky_6/DSCN0147.jpg http://i848.photobucket.com/albums/ab46/sky_6/DSCN0148.jpg

Pics?

Cam pin drag, welcome to the piston club.

If you upper is doing this, consider the POF roller cam pin.  Cheap $20 insurance.

The POF roller cam pin just drags into the upper receiver differently. Instead of surface wear, it made a rather large dent. Even with the DI system, I consider the upper receiver to be a disposable wear item due to their low cost.

Was this with a Stag Model 8?

No similar wear on my Stag Arms Model 8 after 700 rounds down the pipe. In fact I've had no issues at all with my Model 8.

Yes.  This occurred on a Stag Model 8.  I have returned two for the same reason.  The second rifle was not as bad as the first.  I do not believe that this is acceptable wear.  I have read here and on other forums that "cam pin drag" seems to be quite common.  This is not abrasion but a gouging of the receiver. Metal flakes can be seen.  To say this falls within acceptable wear parameters blows my mind.

NWCP:

what ammunition are you using?  5.56 or 223?  As stated in my original post I am using 5.56.    I really want to get a handle on what is causing this problem.

I fire both 5.56 55 grain XM193 and ..223 55 grain HSM down the pipe. I have also put quite a bit of 5.56 M855 military surplus 62 grain green tip through it. I've experienced no issues with any of the ammo.  Mine is scoped and used primarily as a coyote rifle. I use Tetra Gun Grease rather than my old standby Break Free CLP. So far so good. With their lifetime warranty I'm pretty comfortable with my Stag 8.

Thanks for the reply.  It seems that we are shooting the same stuff so this tells me it is not my choice of ammo that could be causing this problem.  I am cautiously optimistic that this problem will be rememdied by Stag.

My M8 is running great.  I have about 1500 rounds in her now, and have what I would call "normal" use marks.  No flakes or cuts in my upper, that I have not seen in DI guns.  I love my M8, and would like to add a SBR version to my safe...

My dod issued colt m16 a3 had the same wear, most of the rifles in the armory racks had these marks. They are due to the rof, and not really a problem. Keep an eye on the bolt face, and look for early locking. If there is no sign of early lock up, fire when ready. The olny real issue, is premature lock up, which can chip, break, or shatter bolts. Since this happens on the return stroke, there are no real saftey issues. The (gouging) must be real severe to have any unintended affects.

I have had the exact same problem with my model 8.  First time at the range every 4th round resulted in a very light tap on the round with no fire.  Took it home, cleaned it, next time every other round same result.  The RO told me my factory mag was at fault.  Took it home, very thoroughly cleaned it, no more light taps, rifle worked perfect.  Since then I haven't had a single misfire but I have the gouging pretty bad right where you do.  Contacted STAG, sent them pics, and they told me its normal and not to worry about it.  I also took it to the shop where I bought it and he said it was normal and I will get plenty more marks like that as I put more rounds through it.  I'm no where near an expert but it just doesn't seem normal.  I have just shy of 1000 rds through it and the gouge has gotten larger since I contacted STAG.  Is this really not harming the rifle?

I just bought the Stag 8H piston upper the other day and tried it for the first time this morning on the range.  

Very accurate, very reliable.  Fired about 150 rounds through it after breaking the barrel in.

I noticed the cam pin wear to the rear of the half moon indent.  

This appears to be common with piston guns.

I did a test and eval on the new HK416 when it came out a couple of years ago and found the same problem with it.  

In a true gas gun, the gas forces the cam pin to unlock into the 12 o'clock position before rearward travel.

In a piston gun, the cam pin, and bolt appear to be spring loaded, so I'm betting the spring cannot keep up with the blow back, timing wise, and the cam pin scrapes past the half moon indent.

Go ahead and install the POF Roller Cam Pin. It is a drop in part that works as advertised. For $20 it's cheap insurance. I ordered one myself to see what it's all about. Fit and finish are good. The rifle funtions fine with it. We'll see how it wears down the road. I can always drop the stock key back into the rifle if needed.

Thanks, I'm looking at it now.

The roller cam pin does not eliminate cam pin drag but it does change its pattern, as others have said.  I've played with two of them in three different uppers and all three uppers show the same amount/type of wear after using the roller cam pin.  One upper has been used exclusively with the POF pin and it's got wear just like the other two uppers that were used with standard cam pins prior to the swap.

This might be a better option––it's cheaper anyway––if you're looking to stop cam pin drag.  I haven't used one personally and hopefully another member can comment:

http://www.lwrci.com/p-89-advanced-cam-pin.aspx

I thought that bolt spring looked like a joke...
Think I might look into a little stronger one, I'll post if I find one.

party sub:  If you are shooting factory ammo and experiencing misfires then I believe you need to contact Stag and return that rifle.  My first rifle misfired, delayed fired and on some occassions failed to lock up properly.  This is unacceptable and dangerous.  As for the gouging I believe it is not acceptable even though I have heard here and elsewhere that this is expected behavior on pistons.  Any weapon that retools itself during normal operation is dangerous.  I received my 3rd partial upper Monday from stag and will be testing this rifle next week.  If I again have problems with gouging then it will be my belief that AR15s outfitted with gas pistons pose an unacceptable risk and I would not consider another one.  

I don't believe that Stag is the only piston manufacturer that is having these problems on an ARs that were "converted" from DI to piston.  

sky, my Colt M4 upper had wear in that area, albeit not as severe.  Mine looked like the first pic BEFORE I put the piston system on.  How much do you work the action with the charging handle?  The reason I ask is the dynamics of the DI system.  Forces come from within the bolt not using the cam pin to cause the locking action.  Working the action with the charging handle will make the cam pin do the work.  Which causes the wear on the upper in DI mode.  IMHO



I think the term "remachining" is actually pretty good.  I foresee someone coming out with new geometry on the inside of the upper for ARs (eventually)  We try to forestall it with the POF and LWRC cam pins.  Doesn't mean the gun is non-functional, it just means I think people (myself included) like to buy this stuff and someone will build it.

sky6 - yeah all Remington ammo from dicks.  I told STAG about it and they said it was no big deal since it hasn't happened again since i gave it a real good cleaning.  Im going to shoot a couple hundred rounds next week and ill see if there is any misfire or more gouging.

It has been a while since I posted a  comment regarding my experience with the Stag Model 8.  On my last post I was returning the Stag Model 8 for the 2nd time to correct the issue I was having with gouging in the upper receiver.  This time I was given a partial upper.  It took almost a month after receiving this fix before I could get back out on the range.  I have been out 3 times with this correction and it appears that the heavy gouging experienced earlier has been corrected.  I am now seeing what I consider to be normal abrasion wear which I consider acceptable.  So far I have put about 200 rounds through this receiver without experiencing the gouging I had seen earlier.  Only time will tell.  

My experience with this AR15 piston will prevent me from considering another AR 15 piston.  It has been an unpleasant experience, and one that I do not wish to experience again.

Sounds like Stag has taken care of your issue. I have no intention of buying another piston AR unless HK produces a civilian version and markets it here in the States. The I'd only be getting it because I collect HK firearms. I'm quite pleased with my Stag Model 8 so there is no real need to add another piston rifle to the gun safe. Any addition would be just a pure want. My scoped Stag 8 is living up to my expectations both at the range and in the field.

this  pic of the locking lugs on my bolt.  you will notice the 45 degree cut on the right hand side of the lugs.  Is this normal wear?

http://www.usefulramblings.org/wp-content/uploads/DSCN0296_forum.jpg

I dont think I have ever seen a piston gun without this type of wear in the upper. I have a barrett rec7 with the same wear....called about it and they just said it was normal. They also stated they had a rec7 with over 50000 rounds through it with a slice worn in the upper right through there...call em and ask they will explain.

Well I have to give credit to Stag Arms.  The model 8 is now behaving as expected.  And regarding the pic of the bolt lugs above:   I looked at new bolts and they are machined at a slight angle.  After locking and unlocking the finish wears and it becomes more noticable.  On to other things.