ARC Nucleus 2.0 Primary Extraction Difficulties

Llamas Waffles

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Jan 29, 2022
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Hello all, I'll try to keep this short but detailed enough to try and get some help. I searched the forums and the interwebs but found nothing similar, so here goes:

I have a newly built ARC Nucleus (2.0) that I put together in January (used a pre-fit barrel with the Bar-Loc setup) in 6.5 CM. It has a Trigger-Tech primary trigger and I put in a 16lb spring in place of the 25lb.

The rifle has just shy of 500 rounds on it and, for the most part, cycles and runs beautifully. A few times though, both in matches and practice, I've had an intermittent issue with difficult primary extraction. I can lift the bolt 90% of the way very smooth them it just seems to lock up to the point I have to smack the bolt upwards to finish unlocking it the last 5*of rotation. Once unlocked it cycles perfectly through ejection and chambering the next round.

I have only shot factory ammo (S&B and Hornady), none of the brass has any signs of overpressure or marks that would indicate a tight or poorly machined chamber.

It doesn't seem to have any correlation to how many rounds are in the mag or hot vs cold; had it happen on both full and near empty mags and beginning/end of strings.

With all that said, what can/should I check that may be causing the action to bind during primary extraction? Any thoughts or suggestions on what may be causing it? Thanks in advance for any help!

- Niko
 

spife7980

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Sounds like its having to use the primary extraction which means its a bit high in pressure causing the base to swell and stick itself in the chamber. I say thats just what you get with factory ammo where they use a garden spade to weigh powder.

Clean your barrel really good recently? If not you should.
 

Steel head

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    Sounds like its having to use the primary extraction which means its a bit high in pressure causing the base to swell and stick itself in the chamber. I say thats just what you get with factory ammo where they use a garden spade to weigh powder.

    Clean your barrel really good recently? If not you should.

    This
    Sounds like pressure not action issues.
     
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    BLKWLFK9

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    Llamas Waffles

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    Jan 29, 2022
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    Pressure issues were my first thought, which is why I checked all the brass, especially the ones that locked up. I didn't see any visible signs of over pressure (cratering of or flattened primers, overpressure rings/bulges, etc.) but maybe there's something I'm not seeing? Other things to check for pressure signs?

    Would over pressure brass cause sticking all the way through the bolt lift and not just the last few degrees?

    As far as ammo specs, most issues have been with S&B 140gr FMJBT.
    S&B spec velocity says 2657 from 21.5" barrel, Chrono from my rifle says 2600-2625 (ES) from 22", so below factory spec. I know a few years ago there were reports of S&B loading 6.5 hot, but those also had flat/damaged primers.

    If overpressure is the answer then it is what it is and I can change ammo, just want to make sure I'm not missing something else.

    Thanks!