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Posted: 10/2/2011 5:14:47 PM EST
Built a new ar15 flattop. My third.

getting frequent jams, dents, and heavy scratches. Not from ejection.

METHOD
bcg fully cocked and held back
load one shell in the mag
insert mag
release bcg

6 jams out of 50 shells, failure to get completely into battery

Dents are at the 8 o'clock position, a quarter of the way down the shell. The black line is at 12 o'clock in the mag.

https://www.sugarsync.com/pf/D984499_7601439_472659

I can replicate the problem on the bench by slowly allowing the bcg forward. The tail of the shell gets forced down and left.

Any thoughts on what could be causing this?

thanks, doug

How do I insert an image?

Link Posted: 10/2/2011 5:40:29 PM EST
[Last Edit: 10/2/2011 5:41:14 PM EST by Dano523]
Link Posted: 10/2/2011 7:58:12 PM EST
This has M4 grooves. I'm sure its not hitting the upper under them.

the mags are: AR Stoner new, PMAG - also new.

Here are more photos. I placed the cartridge where it dents. Half inch or so into the chamber.

ONE

two

three

Link Posted: 10/2/2011 9:48:36 PM EST
What brand of barrel?

DI/Piston?
Link Posted: 10/3/2011 1:19:12 AM EST
[Last Edit: 10/3/2011 1:20:35 AM EST by Banditman]
Looks like m4 cut receiver and non m4 barrel extension.

In fact that barrel extension looks plum nasty,
Link Posted: 10/3/2011 3:25:03 AM EST
Link Posted: 10/3/2011 9:18:53 AM EST
The receiver was ground by hand.

This is a Schilen barrel and matched bolt. The barrel shoots very, very well.

I have another Schilen just like this one, (this one's a gift). That barrelt looks exactly the same.

The bullet tip is NOT hitting the receiver, it's an optical illusion in the photo. I can watch it being fed, and it hits the middle of the ramp. The binding doesn't occur until the shell is part way into the chamber. Maybe a half inch or so.

I think the cartridge is being fed at too steep an angle. Then hitting inside the chamber at 2o'clock, the bolt continues forward and places the dent at 8 0'clock on back edge of the chamber. Or not sliding across the bolt face as easily as it should.

I'm going to load a mag with more shells and see if the extra pressure from the mag will force the butt of the cartridge up more quickly.

thanks for everyone's help.

doug
Link Posted: 10/3/2011 9:31:07 AM EST
What brand of lower?

How does the distance of the mag catch slot from the top of the lower to the slot compare to your other lowers?
Link Posted: 10/3/2011 9:47:52 AM EST
Spoke with Schilen. The feed ramps need to be adjusted.

Sending the upper assembly to them.

Should just take a week.
Link Posted: 10/3/2011 2:35:57 PM EST
Op in the future if you are building it correctly there should be no need to
modify parts.
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