6mm ARC (Bolt Action)

Re: my jump issue, I'm starting to think my new 6ARC modified case is screwy. Even if I really jam the bullet with the OAL gauge the best I can get is 0.020" off of the jam with 2.252" factory COAL ... but that does not track with how smoothly the round chambers at all.

Going to get a new modified case and see if measurements hold with that before jumping (pun intended) to conclusions. If they do, the only conclusion is that my chamber is shorter than it should be I assume?

Is there a better method of measuring contact to the lands besides the Hornady OAL gauge? The bullets are 0.2430" exactly so I don't think they're too fat or anything.

EDIT: going to try https://precisionrifleblog.com/how-to-measure-the-distance-to-the-lands-on-your-rifle-barrel/

Dom,
I never bothered testing velocity/temp changes.
I didn't see accuracy swings, and that's all I care about. If my shot was .1 low at 800-1k due to a 20° drop in temp, I just added a click and rocked on.
I no longer do any competitive shooting, so guilt edge performance isn't a requirement.

Keeping records of everyday velocity vs temp is more work than I want to do.

Hell, we went to Arena last month for some LR stuff. (We lost our 2k range 10 months ago)
I took my 300 Win using new ADG brass, F-215, RE-22 and those pulled 215s.
Saturday was tough due to a L-R building and dropping wind. Paired with a little brown bottle flu.
Sunday was magic. I shot 5 rounds at the 1950yd IPSC. 5 hits in a row, including cold bore, so I just stopped and enjoyed the rest of the day.

It's all fun stuff now.

So using the Alex Wheeler method (disassembling the bolt, seeing what no resistance close feels like, and then seating a bullet in 0.001" increments until you hit that no resistance feel) I end up at almost exactly the same measurement as the Hornady OAL gauge indicated. With my ogive comparitor I'm at 1.665" to the lands which is at 2.223" COAL.

So I'm not nuts and factory 2.250" COAL has me well into the lands. The OAL gauge had me at 1.666" to the lands with my ogive comparitor.

I'm shipping this barrel back to have the chamber inspected before shooting it again. That's too much of a coincidence for me to chalk it all up to my gauges or my ability to measure.

EDIT: I reran the Wheeler test because the first run I was comparing a deeply seated Hornady brass to an Alpha brass that I was incrementally kept seating deeper. I figured to be exact I should compare apples to apples and reran comparing Alpha to Alpha and just going until there was no longer a bolt lift "unlock" feeling disengaging the bullet from the rifling as opposed to no friction at all, and that gets me 0.015" off the lands with 2.250" COAL ... still a little close for comfort, but manageable if I drop my powder charge I think.

Just to tie a bow on my 6ARC chamber/freebore adventures, Eric at BMP ran the QuickLoad numbers for me as well, and checked up on their reamer and my chamber is fine.

The second Alpha to Alpha Wheeler test confirmed that as well, and I just did another one to be absolutely sure. The ELD-M is just a long bullet but I'm still off the lands at factory seating depth.

Everything else I saw was because I was _WAY_ over pressure, as I blindly trusted the LeverEvolution data recommended by Hornady in https://static.hornady.media/site/hornady/files/load-data/6mm-arc-bolt.pdf

I learned my lesson and ordered a QuickLoad disc for any future decision making and am glad I didn't grenade my rifle.

Appreciate all the help and feedback here to get it figured out as well!

Note on my ejector marks, I was still a bit bothered that even my lighter loads (29.2 e.g.) were showing ejector marks and was thinking maybe excessive headspace. What really got my gears grinding was that a batch of Hornady brass reloads that were fine in their factory first firing now also had ejection marks at a mild load similar to the factory load.

This was the first time using my new Hornady 6ARC die and ASSUMED it would just headspace to SAAMI spec by default which is what my 6.5CM and 308 dies do. So I was just going to get the first firing out of the way with a standard headspace and then tweak from there on, and again, assumption is the mother of all f-ups.

Measuring today, my 6ARC FL resize die is setting headspace back ~0.010" further than a Hornady factory round so I'm guessing the excessive headspace had my brass slamming into the boltface regardless of charge. I bumped it back up 0.010" using a Redding #12 competition shellholder set. These let you adjust shoulder bump from the shell holder end so you don't have to mess with the seating depth of the die. I'll do that for the rest of the Alpha brass lot's first firings as well and then tweak to the chamber from there.

In the interim I ordered a set of Mighty Armory dies for all the calibers I reload for to replace my hodgepodge of Lee and Hornady dies.

Alright got back out to the range today, I now have my own copy of QuickLoad that I'm checking pressure bounds with, fixed my headspace, and worked up some new loads using the Alpha brass.

Happy to report zero signs of pressure issues, but still getting faster than expected velocities with LVR out of this 20" barrel. As a litmus test I also chrono'd some factory Hornady intended for Gas Guns.

Data points to consider:

- Hornady book max LVR load for a gas gun is 29.7gr on a 2.245 COAL with a 108gr eld-m, based on 52000PSI PMAX
- Hornady book max LVR load for a bolt gun is 31.2gr on a 2.245 COAL with a 108gr eld-m, based on 62000PSI PMAX
- Hornady fired 6ARC brass has a h20 volume weight of ~34.5gr
- Alpha fired 6ARC brass also has a h20 volume weight of ~34.5gr
- Hornady 24" barrel reference loads: https://static.hornady.media/site/hornady/files/load-data/6mm-arc-bolt.pdf

Rifle data:

- Zermatt Origin Action, 20" 1:75 benchmark cut with the following reamer print: https://terminalcurves.com/6MM_ARC_BMP.pdf

Base info for my loads (pretty much factory spec in terms of seating depth):

COAL: 2.250
CBTO: 1.699
Primer: CCI BR4
Brass: Alpha
Powder: LeverEvolution, loaded at ~77 degrees, shot at ~75degrees

Velocities and SD over 5 shot groups vs closest Hornady book load data:

My 20": 29.2gr LVR Chrono'd 2730fps sd 6 ... 24" Hornady book data: 29.1gr LVR 2650fps
My 20": 29.7gr LVR Chrono'd 2784fps sd 6 ... 24" Hornady book data: 29.6gr LVR 2700fps
My 20": 30.2gr LVR Chrono'd 2803fps sd 6 ... 24" Hornady book data: 30.2gr LVR 2750fps

QuickLoad Predictions based on 34.500gr H20 vs my loads and Hornady book data:

20": 29.2gr LVR 2621fps ... 24": 29.1gr LVR 2711fps
20": 29.7gr LVR 2669fps ... 24": 29.6gr LVR 2759fps
20": 30.2gr LVR 2717fps ... 24": 30.2gr LVR 2816fps

Hornady factory 6ARC 108gr eld-m ammo notes:

- Posted box velocity out of 24" barrel is 2750fps
- Chrono'd velocity out of my 20" barrel over a 10 shot group is 2665fps which at about 20fps per inch slower tracks pretty darn close to what I'd expect to see out of the 20" with factory ammo. If my barrel was mechanically causing some sort of pressure issue I'd expect to see a velocity increase with factory ammo as well.

I don't know which powder Hornady is using for their factory rounds, but I assume something temp stable.

With LVR, assuming their book data is out of a 24" barrel I'm still running considerably faster than their book data, to the tune of 130-160fps when also taking into account the 80fps I _SHOULD_ be giving up on barrel length vs their data. Half of that is likely explained by LVR just being a powder that gets better velocities than most I suppose. But even with that my barrel seems to be running LVR very fast for a 20".

But it MUST be the powder itself, because again, Hornady factory ammo loaded to the same measurements with the same bullet is running exactly what I'd expect to see out of the 20".

So beyond the primer and using LVR, not sure how to explain that large of a velocity increase.

At 29.7gr LVR QuickLoad has me in an acceptable PSI range but thinks I should be around 2669fps out of the 20" and 2621fps at 29.2gr of LVR. I am consistently 110fps faster than what QuickLoad predicts.

What's interesting, when setting a 24" barrel length QuickLoad fairly closely tracks to the Hornady book 24" barrel data but is consistently 60fps high (which you can chalk up to minor burn rate and temperature diffs probably).

For me 29.7gr hits the 2750fps mark I was aiming for as a baseline and I don't see any pressure signs like ejector marks anymore with the correct headspacing. Primers also look fine, so I _THINK_ I'm going to call that good and chalk up the difference to LVR gremlins!? I mean clearly my velocities are still high to the point where you'd think _something_ is causing a pressure difference but I'm scratching my head on what it might be if not just my lot of LVR and the CCI primer.

EDIT: to be on the safe side rooted on real world chrono data, since ~2800-2850fps indicates max pressure of around 62000psi in a 24" barrel based on both QuickLoad and Hornady data, in a 20" that should be around ~2700-2750fps so any charges that keep me comfortably at or below that velocity line _SHOULD_ be fine in my 20" and I reckon that should account for whatever burn rate variance is going on with my lot of LVR.

Groups are still 0.3-0.5MOA on average with each load. SD was surprisingly consistent amongst all of the loads (~6 for all on 5 round groups).

@padom here's what I'm thinking to stay on the safe side of things: since both Hornady and QuickLoad with LVR loads out of a 24" barrel project max loads (i.e. approaching pmax of ~62000PSI) around charges that push the 2800-2850fps mark, for a 20" I should stay at or below ~2700-2750fps, which in my case is the 29.2gr charge of LVR for the 8lbs lot I currently have. I reckon that's a sane approach based on the actual chrono data I have?

padom said:

My fired starline 6arc brass has h2o volume of 24.95gr

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I'm guessing you meant 34.95gr?

padom said:

Guys dont even run these bullets in 6BR because of case capacity... So why would you want to run these bullets in a 6 ARC bolt gun with even less case capacity???

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I run it because I get big 6mm bullets running at BR speeds with single digit SD and sub 30 ES while using Starline brass, lower cost ball powder dispensed with a volumetric thrower. Can't do that with any of the 6BR variants.

If I do a second barrel, I'll either have the smith throat it or call up Manson or JGS and get a reamer made up with more FB.

I have BMP cutting me a 24" 6ARC barrel throated for a 2.40" COAL on a 108gr eld-m as we discussed earlier in this thread to get it seated above "the donut". Opens up case capacity quite a bit and drops PSI accordingly so will have to up the charge a bit but at least will have much more flexibility to play with seating depths. Will report back on how it shoots.

Rifle is a 25" X-Caliber blank on an Origin action with a SAAMI spec 6MM ARC chamber. Through a baffled suppressor.

padom said:

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What's funny?