How far can a shoulder be bumped back ?

[hounddawg]

The reason I am asking is I have a 6MM ARC barrel on order and a ton of 6.5 Grendel brass both new and used. The case is almost the same except the shoulder on the ARC is .030 farther back than on the Grendel. Half the people on the neet swear you can convert, the other half say no way to get that shoulder bumped back that far.

Opinions ? I plan on buying some stock loaded ammo any way to get my measurements from I was just wondering how many boxes I should buy. I never had much luck with Hornady brass and really prefer my Starline

 

[Bart B.]

I've set 30 caliber shoulders back about 1/4th inch.

Made 308 cases from 30-06 cases, 30-338 Keele mag from 300 Winchester magnum cases.

 

[HiBC]

One way to find out is to try.

After many fruitless discussions on the topic with Mr G.,I think I finally figured out what his "secret knowledge" was about setting shoulders back.

It came to me while forming 300 BLK from 5.56. Think of it as necking the case body down to neck dia.

If your first attempts don't work out,you may need an intermediate step.

Like 30 cal or something. Work hardened brass can be a problem. Annealing may be necessary.

 

[mehavey]

30-thou is barely standard paperclip thickness.
No big deal. (300 Savage from 308 example here)

Anneal the shoulder. Run through die (good lube). Trim
(might have to turn the new neck/base)

 

[hounddawg]

thanks guys, there are videos of guys doing it but just wanted a few opinions from the people here. I have necked down .308 down to .260 in one step and 6.5 grendel up to .30 Major in one step but never knocked a shoulder back like that. Most of the guys who have reported success do talk about having to do a trimming afterward but that makes sense. That extra brass has to go somewhere.

 

[hounddawg]

well cancel on the buying loaded ammo to get a set measurement, just looked at online prices on ammoseek. 3 - 4 dollars per round. Instead I will use a Hornady 6MM ARC modified case. More than one way to skin a chicken. Stupid prices on ammo these days, I figured 40 bucks for 20

 

[Unclenick]

The Grendel should go. If it is difficult, I would take a spare 6.5×55 sizing die and grind it down until it sets the outside corner of its should where the ARC's is supposed to be. The idea is the Swede's 25° shoulder slope will not move the whole shoulder at once (starts on the outside and moves inward) and will leave the neck alone, then the 6mm ARC die would resize the neck starting at the top of the shoulder until it comes down to form the 30° shoulder, also narrowing any excess shoulder width at the shoulder's departure from the body that was left by the shortened 6.5×55 die. Once the sizing is accomplished and the neck trimmed, you'll need to check for a donut on the inside of the neck and ream it out if it does. Check the neck OD with a bullet seated to make sure there is an adequate release gap between it and the neck.

 

[stagpanther]

I've made plenty of 6mm predator/243 lbc (which hornady made dies for for years before "poof" along came the arc ) brass out of plain Jane Grendel brass using a Redding type S Grendel full-length bushing die with a selection of bushings. I do remember having to sometimes turn the base of neck if a donut formed.

 

[FrankenMauser]

You can do it. It will be fine.

Among other things well beyond moving a shoulder 0.030"...
I form .577-450 from 24 ga shot shells. To do so, you have to create a shoulder on a straight-walled case, and then push said shoulder down the body more than an inch.

 

[hounddawg]

I have a set of Lee dies on the way in 6mm ARC, I plan on annealing first, applying a thin coat of Imperial before using the Lee minus the expander bushing to size the neck down and bump the shoulder back. Then use a .240 expander mandrel to bring the necks back out. That should push any doughnuts to the outside where a skim turn will take care of any irregularities. Once I trim the neck to length and they should be good to go.

The Hornady modified case is just to compare head to shoulder measurement when setting up the sizing die. I would have bought a GO gage for that but they are not in stock anywhere I looked.

 

[FrankenMauser]

Sounds like a good method.
I usually outside turn when the 'forbidden donut' is going to come into play, as well.

 

[hounddawg]

Put my 6ARC barrel on my 6.5 Grendel upper yesterday and USPS brought me a set of dies for it. I don't have any factory ammo or a GO gage to set headspace with so I decided to experiment a bit. I looked up the SAAAMI drawing and SAAMI calls for 1.190 BTO using a .350 comparator with a overall length of 1.480. I sized the first case by knocking the shoulder back to 1.195, then trimming to length. I chambered it and let the bolt close. Rifle went into battery but I needed to use a small brass hammer to get it open again. I remeasured the case and the bolt closing had knocked the shoulder back .004 to 1.191. I repeated the test being real careful with my measurements and had the same result. I adjusted the die to put the shoulder at 1.187. Cases now chambered easily and ejected easily. I am going to call it good and prep 25 the same way and add a skim cut on the necks for doing a small initial load test next week

What amazed me was how close that little aluminum insert was to .350,
thumbs up to Hornady