Shoulder bumping how does that work

[Alpha1]

Why would you want to bump a case shoulder what's the benefit. Can some one describe the process.

[snayperskaya]

Do you mean the way a 7.62x39 Saiga puts a raised ring around a spent case?, if so its so Russian law enforcement can differentiate between a case used in a civilian AK rifle and a military one.

[Laurie]

'Bump' is shoulder set-back during the sizing process. Adjusting the amount determines the longitudinal case to chamber fit. Full length sizing automatically 'bumps' the shoulder in the final part of press handle operation / press-ram + case movement.

Bump dies and bushing-bump dies (as per the Forster variety and custom examples from Neil Jones and suchlike) either reset the shoulder position alone, or size the neck plus reset the shoulder. Reasons are either as part of case reforming from one design to another or in the example of the bushing-bump type to ensure easy chambering and a good case to chamber fit. Size the neck alone and you soon find that the case becomes a slight crush fit in the chamber because the shoulder moves forward during firing. Eventually this may cause real chambering problems. However, precision shooters have a second reason - avoiding any crush fit and ensuring case consistency. If case A has a thou' clearance at the shoulder when chambered as a live round, case B just touches the chamber shoulder, and case C needs noticeable bolt-closure effort, the rounds will likely perform inconsistently when fired. Use a bushing-bump die set to push the shoulder back 0.001-0.002" from its fired position and every round sits with its case marginally away from contact at this point - result, better performance.

Shoulder bumping, normally as part of the FL size cycle is often critical in semi-auto and straight-pull rifles. The norm there is to set shoulders back four or five thou' from their fired positions to ensure 100% bolt lock-up, as there is no bolt-handle feedback on closure that a turnbolt type provides. If shoulder are insufficiently set-back, the rifle may become unreliable - or worse, the firing pin falls onto the primer with partial lug lock-up. This saw many American custom M1A match rifles with minimum dimension chamber specs destroyed, and the occasional shooter hurt in the days when the US M14 / M1A was the primary High Power Service Rifle tool. A weakness in the design saw ignition take place on quarter / half-locked bolts. It wasn't just handloads - occasional out of spec factory lots caused failures too.

Conversley, adjusting the die too hard down in the press may (not guaranteed) push shoulders much further back than is either needed or desired causing excessive case-wall stretch and early failure through separation. This is a particular issue with rimmed and belted cases whose rifles are often 'generously' chambered in the shoulder area, correct 'headpsacing' or fit vis a vis the closed bolt achieved through the rim or belt fit in its recess.

There are various ways of measuring / setting the sizer die up to get the correct amount of 'bump', but the easiest way is to measure a selection of fired and unsized cases from the rifle using callipers, a Hornady comparator body, and appropriate Hornady 'headspace gauge'. This Youtube video explains its use:

http://www.youtube.com/watch?v=P-UrMTyJ1_E

(You can buy the comparator body and single gauges as needed. The gauge needed for each cartridge is listed on the Hornady company website.)

[Alpha1]

Thanks for that Laurie interesting. All these years I have been shooting reloading and still so much to learn.

[6.5x55]

I had to do this recently, with some S&B 6.5X55SE factory loads I was given. They would not chamer in my rifle, so I pulled the bullets & full length resized the brass, problem solved. The bullets were to deaply seated for my liking & OAL was within spec. , so I knew it had to be the shoulder.

[Maggot]

I do this.

The brass from my Dolphin hardly moves (A thou or so) and I dont get feed issues as I occasionally got with the old neck only sizer.

Also, as I dont swap cases between rifles, and dont eject them with any real force, I tend not to use a sizing ball/central spindle.

Another advantage of the bush/bump die (although not a bumping function) is that you can reset the shoulders (bump) while only selectively sizing a portion of the neck.

I generally do about 80% but have just set a 2 & 15 batch at about 50% to see what (if any) difference it makes.

Not surprisingly I had to back the seater off a bit as there was less neck tension so the bullet had less resistance to seating.

[Alpha1]

So these bumping body/dies are adjustable yes.

[The Gun Pimp]

So these bumping body/dies are adjustable yes.

Only in the way that any normal FL sizing die is adjustable - i.e. by the lock-ring.

And this can be a problem - you may find that even with the die screwed-down so it's touching the shell-holder on full 'bump' of the press handle, it's still not pushing (bumping) the shoulder back sufficiently for your chamber.

In this case, the only option is to skim a few thou. off the bottom of the die or grind-down the shell holder.

[rox]

So these bumping body/dies are adjustable yes.

A bump die such as the Forster bushing bump die can adjust neck tension using bushings and the length of neck sizing using the thread on top of the die. The position of the shoulder (i.e. amount of bump) is adjusted by screwing the die into or out of the press. A body die is a one-piece afair, so you can only adjust it in and out of the press. Bear in mind that if you use a body die to move the shoulder you are also sizing the body - once you have sized the neck to hold a bullet the result is: a full length sized case.

..

[ovenpaa]

Also with the Forester example you can change/remove/machine you own bushings so you are only setting the shoulders back which means you can then do the neck sizing as a separate stage if you so wish.

Forster dies are easier to machine than Lee which is always handy.