6 ARC and concentricity issues

[hounddawg]

Anyone here do concentricity checks on their AR ammo? My 6ARC seems to be doing a number on the case necks. Bolt gun brass less than .001, 6 ARC .005. The cases are old Hornady 6.5 Grendel brass I formed into 6 ARC. I am using a Hornady FL die with expander removed then expanding to .240 with a mandrel, same procedure I use with bolt gun ammo. This one has me scratching my head

 

[stagpanther]

Dang--that's how I made my arc brass too, bit foggy on what I did but I think it also involved knocking the shoulder back and trimming the case length which upon necking down is too long IIRC. So, I'm wondering if in the process of reforming the shoulder maybe an asymmetry was created at the neck/shoulder junction?

 

[hounddawg]

No clue Stag, I have some never been fired Starline that I am planning to form into ARC brass to see how it turns out. This time I am going to size, use the mandrel to expand, then do a skim cut on the necks before forming. I will probably make them tomorrow, 115 heat index so a nice indoor project.

The other stuff had a half dozen firings through my Grendel before it was formed. It shot OK though. I got .75 MOA groups at 100, 1.5 MOA at 300 and considering the scope has telephone poles for crosshairs I can live with that.

 

[stagpanther]

I believe I trimmed the length before sizing down--that's why I think maybe the neck might be pressed down into the shoulder, it's fairly easy to crush it. I bought the ARC and lost interest pretty quickly since I didn't see much difference between it and the 6mm Predator I already had. I'lll revisit it one day. I got too many projects going at once. Mel80 on this forum is a big fan of the ARC and she did early conversions of Grendel brass too--you might ask her for her thoughts.

 

[Unclenick]

I have no experience with this one, but I would be checking concentricity on the brass first, to determine whether this is coming from the brass or from how the bullet is getting pushed down into the case.

Years ago I sorted a heaping pile of new Winchester .308 W brass using the NECO gauge and found neck wall runout was consistently half that of the body wall thickness runout back close to the head. This makes me suspicious that setting the Grendel shoulder back much is getting you into territory where there is more wall thickness variation that winds up in the new neck. Your idea to outside turn the neck is a good one, then see how good your concentricity is on the first reloading after you've fired them once with the turned neck wall.

 

[Nathan]

I deal like this could be fake news for you. Mark the brass where the runout is occurring. Then look at it under magnification. I’ll bet a small dent is in the area where measured or supported. That is throwing the Gage off but not necessarily the fit to chamber. Make sense?

 

[hounddawg]

Made 10 rounds using new unfired Starline 6.5 Grendel yesterday. Sized, expanded, neck turned, trimmed. Same issue. I am suspecting the turret press is allowing the base to tilt a bit when setting the shoulder back that far. The Hornady concentricity unit uses the case head or support so if the head is angled a bit that could do it

I will dig the Rock Chucker out and mount it in the next couple of days and form another ten on it to see if the problem persists. Also considering getting a Sinclair concentricity base just to check case body and neck.

Curious thing is the ammo shoots decent considering I suck accuracy wise with AR's in general. Five shot groups are always under a MOA and about half are down in the .5's. I got the barrel on sale for less than $200 ( Faxxon) so that is better than what I expected from the rifle. The scope is a Sig BDX which is way up there on the cool factor but not exactly a target scope. I am halfway considering shooting it as a match rifle or in the prone tactical class but F class and small bore are eating time and money so right now it is just a fun gun

I'll update the thread as I try some stuff and figure it out. Thanks for the suggestions all

 

[jetinteriorguy]

I’d say doing this on a turret press sounds like a likely culprit. But then again I have loaded some pretty good ammo for my AR on my Lee Classic Cast turret so you never know.

 

[hounddawg]

that's what I am leaning toward Jet. I think it has to do with pushing that shoulder back .030. I clamped my Lee Classic cast to the bench and made 5 ARC rounds this afternoon and the runout was down to .003. I am going back to the range one day this week, shoot what I have loaded and go from there.

On the Lee Turret, that is what I use for everything, and rarely get more than .001 doing normal loading using the same procedure.

I will have to mount the RC temporarily just to form some cases it looks like.

 

[jetinteriorguy]

I too am a huge fan of the Lee Turret, I do all my pistol loading on it and my .223 and 7.62x39 plinking ammo on it with great success. All of my more accurate rifle stuff is all done single stage on my Orange Crusher.

 

[hounddawg]

What will be interesting for me is if I find I get even less runout forming the cases with the RC. Like I said earlier, using my normal processing procedure of .223 through .308 on the Lee turret I have runout of less than .001. Which is by any measure pretty damn good.

 

[Bart B.]

Has anyone ever put a runout gauge indicator on the case body at the pressure ring and just behind the shoulder to see if they're egg shaped?

 

[hounddawg]

I was disgusted with the groups today, decided to put the 6.5 Grendel barrel back on. It was fun but the Grendel shoots better, is less hassle to make ammo for and I have an abundance of 6.5 bullets and several pounds of suitable powder