Shoulder bump for AR10

[jepp2]

Just to share my experience with my LR308. It has a Krieger barrel and I am converting to Peterson SRP brass.
I use a Whidden case gauge to measure my shoulder for sizing. On the new Peterson brass it measured 0.000 and after firing them they measured 0.0015" to 0.002". When I size I will size them back to the 0.000, same as factory new. Over time I will determine at what length I get resistance on bolt closing and record that, and I may adjust what I size to after that.

Here is the Whidden gauge measurement with a Go gauge in it.

 

[Woofie]

Why not buy a Wilson case gauge, re-size to SAAMI specs initially and take it from there? Has worked for me in various gas guns. We can make everything complicated if we try hard enough

 

[Jeff Porter]

When I load for a gas gun I want it to drop in the chamber like a hot dog down a hallway. No more than that but I will error on the side of overworking the brass in a gas gun versus mortaring a round out of the chamber.

 

[RegionRat]

@Woofie , @jepp2 isn't making things complicated, in fact he is pointing out a very simple solution.

There is nothing wrong with the LE Wilson Case Gage, but unless you are just using your fingers and eyes, it is more complicated than the Whidden Gage because you would need to introduce other instruments to get a number you can use to manage your brass prep.

LE Wilson MSRP is $38 and the Whidden for 308 is $59, but if you buy the Wilson instrument to get a measurement ($120 MSRP), or introduce your calipers or a depth mic, then the Whidden tool is far less expensive and less complicated.

The old RCBS tool is similar to the Whidden, but I like the Whidden better since they are always dead on with a Go-Gage and only some of the RCBS versions hit the zero. The Whidden is also far better marked and readable. YMMV

 

[jlorenzo]

2nd the suggestion 1.535, set the die off LONGEST brass. Base shouldn't be an issue if all the brass is out off the same chamber. Spent primer method works with AR platforms,too.

What is the spent primer method?

 

[RegionRat]

What is the spent primer method?

You can use a spent primer that sits proud of the case, or even layers of tape on the case head, to determine the shoulder datum setting for an AR or bolt gun.

By using the height of the tape, or the distance a primer cup sticks up to fill the gap between the case touching at the shoulder versus the bolt face, you can estimate the shoulder datum dimension when the bolt just barely closes.

Pieces of tape have a thickness that forces the results to reflect those thickness increments.

Spent primers can also be used to get an estimate, but they can also be a little tricky to measure to a fine degree. Both these methods take a little skill in finesse, so you should use several samples to get confidence.

If you dedicate a Go-Gage to a caliber you will use in the future, you can use the tape trick with the Go-Gage to estimate the shoulder datum length as well.

 

[Woofie]

@Woofie , @jepp2 isn't making things complicated, in fact he is pointing out a very simple solution.

There is nothing wrong with the LE Wilson Case Gage, but unless you are just using your fingers and eyes, it is more complicated than the Whidden Gage because you would need to introduce other instruments to get a number you can use to manage your brass prep.

LE Wilson MSRP is $38 and the Whidden for 308 is $59, but if you buy the Wilson instrument to get a measurement ($120 MSRP), or introduce your calipers or a depth mic, then the Whidden tool is far less expensive and less complicated.

The old RCBS tool is similar to the Whidden, but I like the Whidden better since they are always dead on with a Go-Gage and only some of the RCBS versions hit the zero. The Whidden is also far better marked and readable. YMMV

The Wilson gauge in about 9 calibres from .223 to .50 BMG is what I'm used to so it's what I use. I'm sure John Whidden's stuff is top notch, I use his bump gauges.
Loading for gas guns I just size to the (SAAMI) notch on the Wilson gauges, although most of my dies are set and locked to a particular rifle. (I nearly always double check with a Wilson gauge; I can get length at the same time) Some of my gas guns are pretty precise builds, others have a bit of slack. Most of my gas guns have pretty catholic tastes when it comes to sizing. I really only spend time on measuring bump sizes on my bolt guns these days.
As a chef I read many years ago said: "Life's too short to peel a mushroom." Sometimes' for gas guns, life's too short to bump size for a gas gun. Just size to SAAMI and get on with it.
I will note: My Rock River "AR10" 26" Varminter has one of the most forgiving, i.e., roomy chambers of my gas guns. It will shoot just about anything you throw at it. It is still one of the most accurate though.

 

[granite1]

Why would you size 0.003" shorter than factory new brass that functioned just fine in your upper?

just asking?

 

[Old Navy]

I finally had time to read this whole thread. Never saw any value in a bump die or a neck die. I don't remember the OP mentioning using a stand alone de-primer before measuring his cases. Been my experience that to get the best measurement you need to remove the primer first. This is especially true when shooting factory ammo. I bought a few boxes of factory ammo for my Aero M5 in 6.5 CM to get shooting while waiting on dies. One box was Hornady Match ammo. I measured a few of them and they were all generous on the base the datum. Like .004" to .007". After one firing they still did not all measure to .002". They were perfect after the second firings. I think Whidden likes them twice fired and Harrell's wants them fired three times to make you the best die. He also did not specify that he used a bump die or FL die.

 

[jepp2]

just asking?

Some folks feel it is appropriate to FL size new factory brass. I measure the case head to datum on my new brass and compare it with the length where I have bolt contact. "If" the factory brass is shorter than my bolt contact length, I see no benefit in FL sizing and some detriments. 1. it puts the case neck through and extra work cycle (and some sizing dies undersize the case neck by as much as 0.008") then the expander when opening it back up pulls if off center 2. as much as I read about folks having issues setting their sizing die up, there are some that will set the shoulder back excessively.

I do run an expander mandrel through the case neck to make it round and open it to the proper size if necessary.

So with that background, sizing once fired brass shorter than it was when new and functioned just fine makes absolutely no sense to me. I may actually move the shoulder LONGER by not having the die come all the way down to get the brass to it chamber length faster.

Hope I addressed what you were asking.

 

[jepp2]

After firing, they measured 1.533-1.538.

Were they still primed with the fired primers? That is an unusually large range.

 

[Coyotefurharvester]

For AR's especially when running common ammo between multiple rifles, an ammo checker is the most time efficient way to produce and safely check it. I just checked several different factory brands of 22-250 ammo through my Lyman ammo checker. Measurements were from 0.0025 to 0.0075 below top deck. When I set the sizing die I will adjust until the brass drops in 0.0035- 0.0045 below the top deck for AR's, I hunt with AR's in the winter and don't want to need a FA to go into battery. You need to be measuring deprimed brass and setting up sizing die, gutted.
Measure the length of a sized, deprimed case. Seat a spent primer .010 proud, chamber into battery(I drop into chamber and let the bolt drop from at least half). Measure the length, the difference in length from No primer case length to bolt seated primer case length will give you a close value on the distance from bolt face to cartridge base of your sized brass. Repeat a couple times on a different case each time, an average will be more accurate assessment of the setting of your die.

 

[granite1]

Some folks feel it is appropriate to FL size new factory brass. I measure the case head to datum on my new brass and compare it with the length where I have bolt contact. "If" the factory brass is shorter than my bolt contact length, I see no benefit in FL sizing and some detriments. 1. it puts the case neck through and extra work cycle (and some sizing dies undersize the case neck by as much as 0.008") then the expander when opening it back up pulls if off center 2. as much as I read about folks having issues setting their sizing die up, there are some that will set the shoulder back excessively.

I do run an expander mandrel through the case neck to make it round and open it to the proper size if necessary.

So with that background, sizing once fired brass shorter than it was when new and functioned just fine makes absolutely no sense to me. I may actually move the shoulder LONGER by not having the die come all the way down to get the brass to it chamber length faster.

Hope I addressed what you were asking.

I do the same. I was agreeing with your question, in a kind of a smartass way.