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Posted: 11/11/2013 2:22:32 PM EDT
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https://www.flickr.com/photos/67153874@N03/10808095814/lightbox/I I have a custom built 6.8spc AR that is having FTF issues. It looks like the feed ramp portion of the upper is set back farther than the innermost feed ramp and that is causing the projectile to hang up. I'm told the barrel is pressed not threaded. It has a SI defence matched upper and lower with a custom nitrided barrel.
Normally I would take it back to the guy that built it but he's telling me he won't have time to look at it until dec 1. I don't want to void the warranty but I would also like to enjoy the rifle. Is this something I can fix or have fixed without the original builder knowing it was worked on? Most of my experience is with AK's and this is my first AR so please excuse my ignorance when it comes to parts and their correct names. NormallyI would figure this out with a search but I don't have a lot of time right now. I'm very busy and any time I have to research has been used to try to find 150gr nosler projectiles for my g20. I have a pic but don't know how to paste a link so you will have to copy and paste the link below. Sorry http://www.flickr.com/photos/67153874@N03/10808095814/lightbox/ http://farm3.staticflickr.com/2872/10808095814_e8f2165d37_o.jpg |
The barrel with barrel extension slips into the upper receiver socket, and is retained in the upper receiver via a barrel nut.
So on that note, the barrel nut needs to be removed, the barrel slipped out of the upper receiver barrel socket (read removed), them using a chop stick with 400 sandpaper wrapped around it, sand/polish the very ends of the barrel extension feed ramps ends so they are flush with the extended ramps on the upper receiver. This will solve the lip that you now have (silver spot on the middle of the left feed ramp), with the lip catching the tip of the bullet as it goes up the ram to cause the jam. Again, barrel needs to be pull and you don't want to go sanding on the barrel feed ramps when the barrel is still in the receiver since it will end up with you removing the anodizing from the upper barrel ramps (anodizing has a 70RC, while raw aluminum is as soft as butter. And its just the tips of the barrel extension ramps that you want to polish down so they match, or every so lightly below the extended feed ramps on the upper receiver. |
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Quoted:
Dano523 just helped me with this same problem. It's a very easy job. I followed mine up with 690 grit cuz I had some. Got to love when everyone jumps on the Colt M-4 extended ramp band wagon, but can't get either the upper receiver ramps nor barrel extension ramps right to work with each other (read neither to colt spec, or to each other for that matter, since you have different manufacturers making each non colt part). 
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