I've been thinking about neck tension and crimping 5.56x45mm.
My die is setup with a bushing to size LC brass necks to .243" and then pull a carbide expander ball through them to leave an OD of .244". When I seat a bullet, the OD is .0246". This would be commonly referred to as two thou of "neck tension." The amount of tension also depends on how much of the deformation the brass undergoes when the bullet is inserted is plastic and how much is elastic. My brass was brand-new Winchester brand from Lake City, fired and resized only once by me. I haven't messed it up with improper annealing nor have I work-hardened the brass neck by resizing it many times. I'll refer to this two thou delta between sized and seated as two thou of neck tension with the understanding that there's a little more involved there.
Bullet Jump
With two thou of neck tension and no crimp holding the bullet, I measured the overall length and then let the action on the rifle strip it from the magazine and slam it into battery with the muzzle pointed down. I measured it again and saw the bullet had jumped 1 thou. The cartridges I'm using for these tests have no powder or primer. Because the rifle has a floating firing pin, I did not want to be trying to induce a slam fire. I'm not surprised the bullet jumped because when a semi-automatic action accelerates the cartridge toward the chamber and then abruptly stops when the breech closes, it works just like an inertial bullet pulling hammer hit against a hard surface. Newton's first law explains why the bullet, once accelerated, will continue in motion even if the brass is stopped. Friction in the brass case neck is the only thing stopping the bullet from continuing until it hits the lands, unless it is crimped. The friction was enough to allow the bullet to jump only one thou. However, I repeated chambering the same cartridge several times and the bullet continued to jump 1 or 1.5 thou each time.
Crimp Test
Next, I used a Redding micro-adjustable taper crimp die to crimp the case of a different cartridge in the bullet's crimp groove. I repeated the process of measuring the overall length, chambering the round with the full force of the action, and measuring how far the bullet jumped. With the crimp, the bullet still jumped a thou. However, when I repeatedly chambered the same cartridge, the bullet jumped a total of three thou and then stopped moving farther with subsequent chamberings. I suspect the edge of the crimp groove slid forward against the crimp, and that it would take more force to push the crimp open.
More Neck Tension
To get more neck tension, I first removed the carbide expander ball from the die. Now the brass would be .243" before seating the bullet and .246" after -- three thou of neck tension. I also selected a different brand brass, a Speer nickel-plated case, and after sizing it with the .243" bushing without expanding it and then seating a bullet, it measured .247". The Speer case's brass is thicker in the neck -- four thou of neck tension. I repeated the tests on uncrimped cartridges and found the bullets would still slide forward about a thou every time they were slammed into battery. The bullet in the Speer case continued to jump as far forward as eight thou with repeating chamberings.
Crimp + More Neck Tension
I resized cases and seated bullets with 3 and 4 thou neck tension, and this time I also crimped them. I repeated the tests. I found the bullets would jump a thou, and after repeated chamberings they had jumped about three thou. After that, they would not jump farther.
Conclusion
My results suggest to me that increased neck tension will not stop bullet jump. I don't have more bushings to test 5 or 6 thou or more of neck tension, but at some point seating the bullet is going to plastically deform the neck so that it is not actually holding the bullet under any additional spring tension than that which is achieved with a larger bushing.
My results suggest a crimp will not stop all bullet jump, but that a crimp into a crimp groove can limit bullet jump to some smaller distance within the groove provided the inertial force on the bullet does not overcome the crimp. Certainly, my hammer-style bullet puller will overcome the crimp.
Not all bullets have a crimp groove. Many more accurate bullets do not because pressed crimp grooves deform the bullet and result in poor mass concentricity and wobbly bullets. Perhaps crimp grooves cut into a solid bullet on a lathe do not create this problem. Certainly, bullets into which a crimp groove is smashed by a Factory Crimp Die do. For bullets without a crimp groove, it seems best practice not the crimp them and to be aware they can jump some when slammed into battery by a semi-automatic action. If they jump one thou, that doesn't seem to be a big problem to me.
If a bullet has a crimp groove, a taper crimp of the case mouth into the groove can help limit the distance of bullet jump from the cartridge being slammed into battery. However, unless the cartridge is expected to be repeatedly slammed into battery many times, it's not clear that any benefit is derived from the crimp since the crimp is not likely to prevent the 1 or 1.5 thou the bullet is likely to jump upon being chambered the first time, and any ability of the crimp to limit further jump upon additional chamberings won't matter after the cartridge is fired. It could matter more if the recoil spring rate was greater. The M249 uses a piston operated gas system with a greater reciprocating mass, fires from an open-bolt, and has a higher rate of fire that is expected to be sustained for longer. It is highly likely therefore that it has a higher recoil spring rate, and that the cartridge is accelerated to a higher velocity before it is stopped in battery.
Questions
- Do bullets in 5.56x45 cartridges jump when chambered in your semi-automatic rifle? If so, how much?
- How much delta is there between your sized and seated necks?
- Do you crimp your cases when the bullet has a crimp groove? If so, taper or roll?
- If you crimp your cases, why do you do so? Have you done any tests to verify that it is effective?
