Reloading Basics - Shoulder Bump

When a case is fired, the chamber pressure generated inside will push the brass in all directions and cause it to stretch. A lot of that stretch will shrink back when the chamber pressure drops to zero but the ejected case will still be ‘bigger’ than it was before it was fired. Before the case can be re-used, it must be resized to remove that stretch.

When you resize a case, the die is squeezing it back down to a predetermined size, commonly to SAAMI dimensions, to ensure it fits and functions correctly in your gun. A full-length die squeezes the whole body of the case back to a standard diameter, while a neck-only resizing die reduces the diameter of just the neck and leaves the body of the case in its ‘fire-formed’ state so that it fits your specific gun precisely.

What a neck-only die misses completely, and a full-length resizing die only addresses if correctly adjusted, is the correction of the length of the case to headspace properly in the gun. Headspace is the distance measured from a closed chamber’s breech face to the chamber feature that limits the insertion depth of a cartridge placed in it (the shoulder on bottlenecked cases). This is a critical dimension because the round has to headspace correctly between the breach face and the shoulder of the chamber. Cases will gradually stretch long ways each time they are fired and if this is not corrected, it will build to a point where the cases are too long. If you have trouble chambering rounds and feel a lot of resistance when you try to close the bolt, it is often this dimension that is the problem. The overall length of cases can be managed by regularly trimming them at the mouth, but this does not address the critical headspace dimension.

Shoulder bump

‘Bumping’ the case’s shoulder simply means using the full-length sizer die to push it back in order to effectively shorten the critical dimension from the shoulder to the rear of the case. Inside the die, there is a sloping surface that is a similar profile to the shoulder of the case and the forward section of your gun’s chamber. When the shoulder of the case is pushed against this surface, it is pushed back or ‘bumped’. There is a lot of very technical and detailed science around bumping, but here we are only considering it in terms of keeping the case length short enough to fit the gun’s headspace and avoid issues with chambering rounds.

How much bump?

When bumping cases, we are only talking about pushing the shoulder back around 0.002”, so just enough to ensure your rounds chamber. However, measuring it can be pretty tricky. There are specialised tools that allow you to measure it precisely, but in the absence of such tools, the easiest way to check if your cases are adequately sized is to simply try them in your gun. If you are having an issue with chambering your rounds, then try cycling a few sized cases to see if they chamber and eject easily. If there is resistance, then you can gradually reduce the headspace of your brass, by adjusting your full-length resizing die further down in the press and retrying the cases to see if it makes any difference. If you find that your cases start to chamber more easily, then you are bumping and it is reducing the length of the cases (to the shoulder) sufficiently. There is no need to go any further with bumping them any shorter. A neck-only sizing die cannot be used for bumping cases and if you try to set it low enough to hit the shoulder, then it will just destroy the case. There are specialist dies that do neck-size cases and bump them, but standard necksizing dies will not do it.

Measuring tools

If you want to measure shoulder bump then there are specific tools for the job. A headspace gauge will allow you to measure the cases before and after you size them and accurately assess just how much you are shortening the dimension by. A shoulder comparator is a tool that fits onto a standard caliper and measures the length to a fixed point on your cartridge shoulder. You can measure a case after it has been fired, adjust your full-length die until it bumps the shoulder, remeasure the case and gradually adjust the die to produce the correct amount of bump. These tools do represent a significant investment but they will allow you to experiment more with headspace.

Bumping your brass

To move the shoulder of a case back, you screw the fulllength die down until you feel it just touching the shoulder of the case. Screwing the die in any further will start the bumping process by pushing the shoulder back. With a ‘normal’ 14-pitch-threaded die, an 1/8th of a rotation will yield approximately 0.009” of downward movement, so it doesn’t take much to give just 0.002” of bump. If you bump too much, then you simply back the die out again and try another fired case. In the absence of a measuring tool, the best way to assess if you are bumping the brass enough is to simply try cycling it in your gun as described above.

Conclusion

Although we have only ‘scratched the surface’ of the subject, shoulder bumping brass is a big subject in its own right and it can have a significant effect on the performance of your ammunition. In terms of overcoming problems with rounds being difficult to chamber, a correct headspace match between the ammunition and the gun’s chamber is often the issue and it is well worth checking to see if your full-length sizing die is doing its job and bumping the shoulder sufficiently. If you do decide to invest in a headspace measuring tool, you can open up another aspect of reloading.

Contacts

Hornady Headspace Gauge Edgar Brothers Ltd www.edgarbrothers.com

L E Wilson Gauges Reloading UK Ltd www.reloading.co.uk