When we want to measure bullet jump, we need to know the length of the chamber from the bolt face to the point on the rifling where the forcing cone diameter is equal to the bore diameter. We often obtain this measurement using a tool similar to the Hornady Lock-N-Load comparator. That tool inserts a modified test case into the chamber until the shoulder of the case contacts the shoulder of the chamber (its for bottle neck cartridges only). Then a sample bullet is extended out the neck of the test case until resistance is felt. The rod used to extend the bullet is locked off, and then the modified test case and bullet are measured from case base to bullet ogive.
This gives a distance that should be repeatable and fairly accurate, but it is far from precise.
The modified test case measurement from base to datum is probably different from that of a fired case (Hornady does offer a service to modify a fired case to account for this), and typically the extractor and ejector combine forces to hold the case, not against the bolt face, but against the extraction groove on the case base, and the extractor claw. Thus there is usually a significant difference between bolt face to bullet ogive as measured and as it actually is at the moment of firing.
NOTE: — Continue ‘bullet jump’ discussion with reference to PRS BLOG and Aberdeen Proving Ground white papers.
NOTE: — Move the paragraphs below to their own post
To dig a bit deeper, let’s take the case of a full length resized, once fired case. Let’s say it’s shoulder has been ‘bumped back’ 0.002″ in the resizing process. That case will come to rest in the chamber with some space at either end, or at both ends. Its outside diameter will be smaller than the chamber inside diameter. When the case is fired, pressure builds to a point where the brass case body expands into contact with the steel chamber walls, and at some point stiction occurs locking the case body to the chamber walls. The case head and the bullet however are not stuck to anything. As pressure increases the case head moves rearward until it is stopped by the bolt face, and the bullet starts moving out of the case neck.
I want to have a bit deeper look at what happens to the case head and primer. The case was just laying there loose in the chamber (because it was full length resized), when the firing pin struck the primer. Under most conditions that forced the case to move forward until it was stopped by the chamber wall in the area between the body and the neck (the area where case datum is located. The firing pin continues (typically firing pins extend about 0.010″ from the bolt face, and detonates the primer, causing the sequence described above with the case sticking to the chamber walls, and now the case head is as far away from the bolt face as it can get.
Several things happen more or less at the same time. One important thing is that the case head will stretch the case walls as it moves into contact with the bolt face, another is that the barrel diameter will expand as pressure peaks (this is how strain gauge transducers obtain chamber pressure readings, buy measuring the degree of stretching and calculating the force based on the thickness of the barrel at a point over the chamber. Often the primer will pop out of the case before the case head moves, it will be stopped by the bolt face, and then when the case head catches up it will be re-seated into the primer pocket.
If the peak chamber pressure is sufficient, the primer shape is affected, as it’s shoulders start to square off, this is not a conclusive sign that chamber pressure was too high, as softer primer cups flow well before harder cups, but neither should flow at the 50,000 psi ‘normal’ operating pressure of most rifle cartridges.
If pressure increases a bit more, the case head may actually flow into the hole that houses the ejector plunger, leaving a recognizable imprint on the case head. This is a sure sign the load was overpressure!
Headspace originally meant the space into with the case head could expand. In the case of our example that would be 0.002″. However establishing a place to measure that distance became difficult when bottleneck cartridges were introduced and over the years, headspace has come to mean the distance from the datum line on the case shoulder to the case head. It’s even called out that way on the SAMMI drawings. The actual headspace is now described as the tolerance between that measurement on the case, CBTO (Case Base To Ogive) and the equivalent measurement in the chamber. Typically that difference is given as about 0.007″ for bottleneck rifle cartridges.
The pressure and heat dissipate very quickly and the steel and brass contact as they cool. The brass exhibits a feature called ‘spring back’. There is an article by AMP Annealing that goes into some detail measuring spring back in rifle cases. For our purposes we will assume the bottleneck rifle case contracts 0.002″ CBTD after firing, and likely about the same amount in body circumference, resulting in the brass releasing from the steel easily after firing with normal pressure.
Should pressure rise to the point where ejectors start leaving impressions on the case head, the chances are the chamber (and case) have expanded significantly more, as temperature and pressure drop the steel contracts so that it returns more or less to its original dimensions, but the brass only contracts about the same amount as the spring back mentioned above. This leaves a case somewhat larger than the chamber stuck inside the chamber. Typically the cam action of the bolt unlocking is sufficient to extract the case, and is often noted as ‘difficult bolt lift’. When pressure increases much further, bolts become very difficult to open, and rifle barrels split.
Back to our ‘normal’ operation of the case. The repetitive firing, resizing and firing of a bottleneck rifle case will eventually cause one of several case failure types to occur.
Case Head Separation – When cases are full length resized, the brass ‘flows’ from the head toward the neck. The softer and thinner brass migrates more easily, leaving an increasingly thinner case wall somewhere just above the ‘web’. This can be felt by using a light touch and a pick, or measured by using a long anvil to reach inside the case body.
Annealing every reloading helps to prevent much of the problem. Additionally, neck sizing only will remove most of the force causing brass migration in the first place, however after several firing cycles, the case may not be able to easily enter the chamber as it becomes ‘perfectly’ sized.
Case Neck Constriction – When full length resizing the displaced brass from the body of the case flows toward the softer thinner neck area, increasing the neck length. Should the neck be allowed to grow long enough to reach the end of the chamber, the cam action of the bolt closing may wedge the barrel, case neck and bullet tightly. If the situation is severe enough (and it only takes a few extra thousandths of an inch to do so), peak pressure may exceed the yield strength of the chamber, causing a catastrophic failure and posing a serious risk to the shooter. Case lengths and chamber dimensions are designed to allow a bit of neck length growth, and there is a tolerance between maximum case length and the minimum chamber neck length. Trimming to recommended case length will keep constriction from occurring.
Case Neck Splitting – Repeated ‘working’ of the case neck brass will increase its hardness. Within just a few firings case neck can become so brittle that they crack. A cracked case neck can not hold a bullet, and there is no way to fix a cracked neck. Cases with cracked necks are trash. You can prevent case neck cracking for a near infinite number of loadings by annealing the case properly between each firing, before resizing.