Chambering diagnosis help

[Outdoorsman12b]

New 6.5 prc and picked up factory loaded 147 Eld-m to see how it shoots, but usually reload. Day 1 at range had issues with multiple rounds chambering. I thought no big deal I have had the odd box of ammo in the past where this has happened in other rifles. However now 3.5 boxes in and still having issues. Using headspace comparator:

Fired cases 1.647
Factory loaded 1.644-1.647

I thought ok maybe a headspace issue so marked up a case hard to chamber and found no issues with any rub marks on shoulder, but did find some marks on bullet closer to neck. The bullets are measuring .264 in diameter. I am wondering if this is perhaps a carbon ring?

More confusing yet is the rounds headspaced at 1.644 seam to chamber fine, and OAL seems to be consistent at 2.958-2.959 which should be well off lands based on initial measurements I took.

Disregard the clear line at ogive that is from bullet comparator when I was taking some more measurements.

 

[Sadie]

Check neck and bullet run out. Is this a tight clambered custom rifle? How many rounds have been fired in the rifle, if a few boxes I doubt carbon ring.

 

[Outdoorsman12b]

Check neck and bullet run out. Is this a tight clambered custom rifle? How many rounds have been fired in the rifle, if a few boxes I doubt carbon ring.

Thanks! I don’t want to add a concentricity gauge to my bench though. Just shy of 70 rounds. I should also mention this is all same lot of ammo.

 

[steelheadmike]

I went through a similar thing in a 7-08. My chamber is evidently at the low end of SAAMI spec and some of the factory ammo was not quite that low, so I had some issues with it.

What rifle? The hornady ammo was the culprit for me and only the precision hunter. The datum line measurement had 0.003-0.004 variance in it. Also, they had two different brass sizes for different series of ammo. I solved the problem by starting to reload.

 

[Harvey_NW]

Color it at .2" above the case head and chamber one of the sticky rounds, see if it's contacting there. Could be an under spec chamber from a worn reamer, wouldn't be the first I've seen and won't be the last.

If you plan to reload and really want to beat it to the punch, find a gunsmith with an AW2 reamer and have it opened up and throated for whatever you plan to load if needed.

 

[ResearchinStuff]

this sounds a lot like a christensen arms PRC chamber.

 

[Outdoorsman12b]

Color it at .2" above the case head and chamber one of the sticky rounds, see if it's contacting there. Could be an under spec chamber from a worn reamer, wouldn't be the first I've seen and won't be the last.

If you plan to reload and really want to beat it to the punch, find a gunsmith with an AW2 reamer and have it opened up and throated for whatever you plan to load if needed.

Not exactly what you asked but on second look from photo I took last night appears some scratches on brass after chambering

 

[BigNate]

Run a cotton patch in your chamber twisting as you go. A burr will get fuzzy and be visible. If it's out of round you might feel the resistance changing.

 

[TaperPin]

If the bullet shows rub marks around the bullet in front of the neck, there’s only one thing that does that. It’s the easiest thing to trouble shoot - simply clean it and if the bullets no longer drag there’s your answer.

Here’s the chamber print for it and if you look at the freebore in front of the neck it’s listed as .2645”, which is the minimum size and the note says it can be .002” larger and still be within spec. If it’s on the small side, that difference in diameter of .0005” is a .00025” gap all around the bullet and is so small it doesn’t take much fouling to drag on the bullet. Saran Wrap is .0005” and thin aluminum foil is .0007” so that’s at least twice the thickness of the clearance. I don’t even know what to measure that’s only .00025” thick.

If there’s a lot of runout in the neck, the bullet may be trying to go in crooked and will drag on two sides of the bullet, but it would only do that if it’s also quite tight.

If you don’t like cleaning the easiest thing to do is sell it and buy something different.

 

[mtnbound]

Not exactly what you asked but on second look from photo I took last night appears some scratches on brass after chambering

Is that the only spot that you had the black Sharpie get removed? What area of the chamber did that?

 

[Outdoorsman12b]

Thanks for continued responses. Rifle went back to seller and was fixed by smith. She’s feeding without issue now.