Shoulder bump for AR15

[Okie_Poke]

I have a question I feel like I should already know the answer to but have never thought to ask before: how much do you bump the shoulder when resizing brass for an ar15, or is that even a valid concept in a gas gun (or is the case being ejected “too soon” for this concept to work)?

Here’s the impetus for my question. I have a 6.5 Grendel AR that I use for hunting and occasionally shoot in a local 300 yard “AR Tactical” match. I plan to shoot that match more next year. This weekend, I cleaned my Grendel dies up and then set them back up. To ensure reliable feeding, I have always just used the factory instructions when setting up dies for my ARs: 1/4 turn past the shell holder. I did that again here, but I also took some measurements using a Hornady headspace comparator.

As currently set, my dies are bumping the shoulder back 0.015.” That seems like a lot to me. I measured some factory ammo with the same comparator, and it matches what my resized brass measures. I also have a case gauge, and my resized brass is sitting right at the minimum headspace measurement in the gauge—right where I’d think it should be for a semi auto. So the die is setup by the book.

I should probably leave it alone, but Grendel brass isn’t quite as common as some other calibers and I’d like to prolong my case life as best I can. I’ve read things on the internet about bumping the shoulder on brass for AR15s 0.003” - 0.005,” which would be substantially less than I’m currently doing and I don’t want to lose reliable feeding. I’d like to hear your thoughts and experience. Thank you in advance.

 

[Walkalong]

I set up my sizer for my .223/5.56 cases with a Wilson case gauge and I get 8 to 12 firings with no issues before the primer pockets start to loosen up and the case gets scrapped. No problem at all with case stretch or internal ruts leading to case head separation.

I set my sizer to move the shoulder back .003 from where they are on fired cases for my 300 BLK and have no issues either. Both being fired in ARs of course.

.005 is too much, IMHO, and unnecessary for function.

 

[Blue68f100]

I set my dies for 0.003" shoulder bump back with 0.004" neck tension. Any thing over 0.003" is going to reduce case life. I don't crimp either since the 0.004" neck tension holds the bullet.

 

[W.E.G.]

You don't know how much you are "bumping" your fired brass until you know the actual base-to-shoulder dimension of the chamber of YOUR rifle.

You are shooting an oddball caliber, so its not likely you will be able to purchase an incremental headspace gage set for that caliber. So you can't reliably collect the data you need with a headspace gage.

If you want to know the actual base-to-shoulder dimension of YOUR chamber, you can remove the gas tube from the rifle, or you can use a modified bolt carrier (something like the Kali-Key system https://kalikey.com/ ). This will allow you to fire live ammo, and allow the brass to expand to chamber dimension. Then you can use your comparator tool to measure the fired brass in order to learn what is the true base-to-shoulder dimension of your chamber.

At that point, you will have actual data for YOUR chamber. Use this data to determine what is the amount of shoulder-setback your rifle requires. I like 0.005" setback in an AR.

That shellholder/quarter-turn stuff mentioned in the instructions that come with your dies only gets you in the ballpark. You have to use some system of ACTUALLY MEASURING in order to know for sure where you are.

 

[Okie_Poke]

Thanks for the responses. This is helpful.

If you want to know the actual base-to-shoulder dimension of YOUR chamber, you can remove the gas tube from the rifle, or you can use a modified bolt carrier (something like the Kali-Key system https://kalikey.com/ ). This will allow you to fire live ammo, and allow the brass to expand to chamber dimension. Then you can use your comparator tool to measure the fired brass in order to learn what is the true base-to-shoulder dimension of your chamber.

I understand what this is doing—essentially turning the AR into a manually operated action so that the case isn’t ejected by operation of the gas system. But why do this? Will the brass measure smaller than it does when the gas system operates because it cools down prior to extraction? Suppose I set my die to bump 0.003” or 0.005” off what my ejected cases measure now, and that both fits in a case gauge and chambers in my rifle. Do I gain anything by doing the extra measurement other than knowing my “actual” bump?

Relatedly, do I need to know my “actual” bump? Are you other folks reporting your bump from cases ejected by the gas system or based on a measurement like what WEG has explained?

Thank you for explaining!

 

[Blue68f100]

It's not necessary unless you have an over gassed system. Over gassed system ejects the brass under higher pressure while the neck is still stuck to the chamber. If you have an adjustable gas block, just close it off. You will be very surprised as to little velocity is loss in a AR. Since the bullet has to pass the gas port to work. All the powder is already burnt, so very little velocity is lost. All mine have rifle length gas system, have not tested on shorter ones.

 

[W.E.G.]

Most semi-auto guns spit the spent brass out with it at a longer base-to-shoulder dimension than what you would get if you disabled the gas system.

My M1A blows the shoulder out quite a bit.

For fun, when you are at the range next time, grab a spent case that came out of your rifle, and insert that case in the chamber of your rifle. Now slam the bolt on that spent case.

When you get done fighting that spent case out of your chamber, you tell us whether you think the spent cases from your rifle accurately represent the dimensions of the chamber.

And truth be told, the Kali-Key system can be a little balky even on the extraction.
I'm not a jacked muscle-dude like the guy who does the Kali-Key videos. So I gotta work that handle pretty hard to get the cases out usually. Lubrication and a clean chamber helps.

 

[Kaldor]

I bump general use AR blaster ammo to full cam over. I want that stuff to fit any gun I put it in.

For brass like my 6 Grendel, I have 500 pieces of Starline that only gets run in that gun, I bump that 5 thou. I like to leave just a little more space as ARs do get dirty, and I still want reliable chambering, so I make that trade off.

 

[Okie_Poke]

Thank you all for your considered responses. I’ve got several rounds loaded at various bumps down to .003” off how they get ejected with gas system working. All pass case gauge and seem okay in my shop, but I’ll test fire them at the range to see how it goes. Hopefully I don’t need a muscled friend to help me extract any stuck cases. Thanks again!

 

[jmorris]

Relatedly, do I need to know my “actual” bump?

No. The rifle doesn’t require quantification just brass that will fit the chamber. That said, a perfect sized case and one that’s way oversized and is destined for a split will both fit…

I have a number of AR’s and generally do use a case gauge to cover all of them I own in the same caliber but if I don’t have a gauge or am only loading for one of them in particular this works.



You can size more and it will still work, the case just has a shorter life with more trimming of excess material after sizing.

Hopefully I don’t need a muscled friend to help me extract any stuck cases.

I have always found the “mortar” technique to work best on AR style rifles. Unlock charging handle and pull as you impact the butt on a hard surface.