AR10 barrel extension options

[bobke]

Have a recently chambered Krieger 6.5CM that is having familiar issue of secant shape bullets ie. 123/136/139 Scenars, Sierra MK’s, BTHP’s of a variety bumping into breech end of barrel. Did not spec the JP extension as I now know I should have, and not having asked which extension would be used and what configuration of chamfer or cone would be, made an incorrect assumption this would be addressed by smith. Cone is 30 degrees, and not broad enough to lift secant shapes up to chamber-which am aware is really the job of the extension and not the cone, though a correct JP would likely function with the narrower cone. The secant shapes bump into backside of barrel, either mashing nose, pushing bullets another .050-.100 further into case or both. Nosler CC’s and 120 Scenar L’s appear they will feed, but also require short loaded OAL, not really offering optimal performance of the longer, more efficient shapes of other Lapuas, which I have accumulated enough of to go well past this new barrel’s lifetime.
All that said, what options do I realistically have to work with? The Krieger shoots single fed really well, but that’s neither the design or option. Can a JP be refitted without misalignment of gas port and extension pin and still headspace correctly, or not a possibility? I can send back to have a broader and shallower chamfer be machined-which seems logical-but without his guarantee rounds will chamber correctly. Is there a spec for proper cone dimensions that do allow for correct feeding, absent the proper extension? Or should I just resolve to shoot the lower BC Nosler CC’s or 120 Scenars and call it a day? Am only about 55 rounds down range to date, and TX weather has kept me off range, but is now warming up again.
Appreciate feedback from those who have resolved this issue. Thx.

 

[toygunner]

I think the chance of the new barrel extension lining up are like winning the lottery. You might get lucky. I don't think a JP or any other barrel extension will correct your issue. It might sound strange but have you tried a different type of mag? The height of the magazine or the slight difference in feed lips could correct your issue. I would try the mags prior to having the lead on the breach changed.

 

[bobke]

Need to work with various mags, but in trying my ASC and Magpuls, same issue. Understand your suggestion, and might play with feed lips on an ASC and see if an improvement. I need about 3-4 meplat diameters to get up on the existing 30d cone. Appreciate your response.

 

[flyfisherman246]

What kind of extension was that?

 

[bobke]

Do not know-appears well made, smooth, but absent the proper dimensions in ramps to mitigate issue noted. Almost surely meant for a 308 application, and in combination with narrow cone, limits my options of bullets to those with more tangent ogive. Barrel is extremely well executed with this exception, and my oversight not to have specified a JP extension. Do know Krieger make one specifically for 6 and 6.5 mm LR/AR large frame, but is proprietary and not available for sale. Options at this point are seeming pretty limited.

 

[Ravenworks]

Need to work with various mags, but in trying my ASC and Magpuls, same issue. Understand your suggestion, and might play with feed lips on an ASC and see if an improvement. I need about 3-4 meplat diameters to get up on the existing 30d cone. Appreciate your response.

I have had nothing but problems with ASC mags.
There are a couple of videos out with good info on tweeking them.

 

[B y r o n]

Do not know-appears well made, smooth, but absent the proper dimensions in ramps to mitigate issue noted. . . ..

Have you bothered to talk this over with whoever made the barrel and if so what was their response?

 

[bobke]

Yes, I have. No 100% certain option to refit an extension, can modify cone, but need a known spec that works and does not leave case head unsupported. Or shoot what does work. Should be out to range in next day or two, but TX weather has been uncooperative of late.

 

[padom]

Pictures would help.... proper feed cone is your answer. I've seen this plenty of times.

Been running KAC and ASC mags in my 6.5 gasser for years without any issues. I have numerous friends that only run ASC mags with zero issues.

 

[lennyo3034]

Had this issue with a JP barrel in 6mm. My solution was to switch to 6.5.

Mine run any 6.5 bullet without problems. JP extension. I don’t know the dimensions of the chamfer. @bohem cut it a few years ago and may know.

If tuning the mags doesn’t work, you can try sending to a smith to remove the extension, recut the chamfer, and reinstall the extension. That way you don’t have to worry about gas port timing or recutting the chamber.

 

[bobke]

Here’s a few, with pics of both a Bartlein and a Lilja that do work, regardless of magazine used. Given existing cone is 30 degrees, smith has requested a ‘spec’ for him to follow.

 

[padom]

Here’s a few, with pics of both a Bartlein and a Lilja that do work, regardless of magazine used. Given existing cone is 30 degrees, smith has requested a ‘spec’ for him to follow.
View attachment 7562712View attachment 7562713View attachment 7562714View attachment 7562715View attachment 7562716

Pic of the new barrel that doesn't work?

 

[bobke]

First two above, plus attached. Last two on Black Rain receivers are a Bartlein redone by Bohem and a Lilja direct from their shop.

 

[flyfisherman246]

Bringing this thread back. Did you ever get this barrel feeding? What feed cone dimensions did you go with?

 

[padom]

I've had Keystone Accuracy do 5 or 10 6.5cm barrels for me over the years.. early years with BAT extension and many with JP extensions.. they put a proper feed cone on the chamber. Never had any problems feeding

 

[SD Carpenter]

I literally had this problem with long bergers. The solution was to shoot Hornady eldms as they aren't as "javeliny" like. I picked up a box of berger 140 otm's which were designed for the AR mag length so I'm optimistic those feed as well. The nosler rdf 140's also shot terrific.

 

[FN15]

i have a 308 with a 22 criterion barrel. it shoots well for a gas gun and is fun with a can on it. it also hammers wild pigs and rings steel on the long range. now I'm thinking I'm going to take out of the safe shoot it this weekend. merry Christmas and enjoy the build

 

[bfoosh006]

I also would like to know the outcome on this.

My first reaction would be to polish the feed cone and the surface around it. An oversized shotgun bore mop , spun via a section of cleaning rod, with polishing material might work.

 

[flyfisherman246]

The reason I ask is because I have a 30 degree (or 60) whatever you want to call it, feed cone cut about .035” deep. It’s a 6 creedmoor and feeds the vmax and Eldm no problem. But with the berger hybrid it slams into the back of the chamber. I’ve read of guys running the 105 hybrids in 6 creed ar10’s. I could push the feed cone deeper which would make it wider, but it can’t go much deeper without loosing chamber support. Does anyone have a feed cone angle and depth they have used reliably with berger hybrid bullets in 6 and 6.5 ar10’s?

 

[r.tenorio671]

The reason I ask is because I have a 30 degree (or 60) whatever you want to call it, feed cone cut about .035” deep. It’s a 6 creedmoor and feeds the vmax and Eldm no problem. But with the berger hybrid it slams into the back of the chamber. I’ve read of guys running the 105 hybrids in 6 creed ar10’s. I could push the feed cone deeper which would make it wider, but it can’t go much deeper without loosing chamber support. Does anyone have a feed cone angle and depth they have used reliably with berger hybrid bullets in 6 and 6.5 ar10’s?

....I would suggest modifying the seating depth in small increments to find out what will feed reliably. You can always adjust your charge weight if necessary to attain the desired speeds. Much cheaper than immediately jumping into machining....

 

[bobke]

I wound up not getting a satisfactory response from original smith after sending it back, so forwarded it to Josh Kunz at PVA and he installed new barrel extension, which reqd a new and shorter base/shoulder dimension. All doable with dedicated handloads, just no interchangeability with std chambers. And after all, and with a large stash of 140 AMax, which continues to produce best accuracy, am good with It as it sits. It’s a Bartlein, and will shoot.
Also have 20” Proof, 20” Krieger by Keystone and a 21” Lilja that are all hammers as well, and feed most everything-Hornady, Nosler, Lapua-reliably, though will only shoot 130 Berger AR Hybrids when called upon.
All said, think for a custom barrel, the JP extension is the answer.

 

[The King]

I've had Keystone Accuracy do 5 or 10 6.5cm barrels for me over the years.. early years with BAT extension and many with JP extensions.. they put a proper feed cone on the chamber. Never had any problems feeding

God I remember those BAT extensions from when we bought an extension from every company that made one to determine who we were going with.

We measured the BAT ones and could not detect any variance from the print of any kind using aerospace grade qc stuff. Nothing. You could use them to calibrate a hardness tester too.