Seating Depth Change Due to Recoil

[Overwire]

Rifle is a Tikka 6.5cm

I have noticed the CBTO change on ammo that is in the magazine when the rifle is fired. I am seeing approximately .010" reduction in CTBO due to recoil. New Lapua brass, 124g Hammer Hunters, .0024" neck tension.

Has anyone else experienced this? Mitigations? Are there any Tikka mags that prevent the bullet tips from slamming into the front of the mag under recoil?

 

[willfrye027]

A lot of guys like to crimp the hammers. Lee crimp die is pretty cheap. Otherwise seat them deeper in the case or increase neck tension if using a bushing die or expander mandrel to set neck tension

 

[longrange13]

Rifle is a Tikka 6.5cm

I have noticed the CBTO change on ammo that is in the magazine when the rifle is fired. I am seeing approximately .010" reduction in CTBO due to recoil. New Lapua brass, 124g Hammer Hunters, .0024" neck tension.

Has anyone else experienced this? Mitigations? Are there any Tikka mags that prevent the bullet tips from slamming into the front of the mag under recoil?

The solution is running more neck tension. This light neck tension has been pushed for a while, but there is a lot of data coming out now suggesting that heavier neck tension gives you a more consistent bullet release. It also prevents this change of seating depth under recoil which is a win win.

 

[Overwire]

The solution is running more neck tension. This light neck tension has been pushed for a while, but there is a lot of data coming out now suggesting that heavier neck tension gives you a more consistent bullet release. It also prevents this change of seating depth under recoil which is a win.

Well the .0024" interference is already enough to cause the necks to take a permanent set (i.e. the material is already plastically deforming from .0024 inch "neck tension"). Therefore, increasing the interference wont actually increase the clamping pressure on the bullet. I'm already at max.

I've started crimping with the Lee factory crimp die. My brass is now once fired and I did not anneal so any work hardening of the brass should also help in increasing bullet clamping pressure. So far seems good.

 

[longrange13]

Well the .0024" interference is already enough to cause the necks to take a permanent set (i.e. the material is already plastically deforming from .0024 inch "neck tension"). Therefore, increasing the interference wont actually increase the clamping pressure on the bullet. I'm already at max.

I've started crimping with the Lee factory crimp die. My brass is now once fired and I did not anneal so any work hardening of the brass should also help in increasing bullet clamping pressure. So far seems good.

I’m not following, what do you mean by you are already at max clamping force? If you use a smaller bushing and smaller expander mandrel before seating the bullet, it should impart more clamping force on the bullet.

The crimp is a good idea tho, glad it’s working. I’ve seen guys blow match bullets apart in the air using too much crimp, but done correctly I’ve seen it really help accuracy. It also guarantees a consistent bullet release in theory.

 

[wapitibob]

The brass neck is stretched to the point it starts to "flow" at the grain level. Once that happens more stretching won't impart more tension, it's lost some ability to return to the pre stretched state and opening up the necks even more will just cause more flow or permanent deformation of the mtl.

 

[Overwire]

^Yep, what wapitibob said. In my case, I am loading the bullets at .0024" interference but if I pull the bullet back out of the brass the neck only springs back about .0015". The additional .0009" is the material yielding (what bob called "flow") and doesn't contribute appreciably to the clamping pressure.

Whenever you stretch a linear elastic material, such as brass or most metals, the resistive force (spring back force) will be proportional to the distance it is stretched until you stretch it enough that it can no longer spring back completely. At that point, the resistive force plateaus and more stretch will not result in appreciably more resistive (spring back) force. As the brass is work hardened the amount it can be stretched and fully spring back will increase so theoretically the more the necks are worked without annealing the more bullet clamping force that can be achieved.

 

[longrange13]

The brass neck is stretched to the point it starts to "flow" at the grain level. Once that happens more stretching won't impart more tension, it's lost some ability to return to the pre stretched state and opening up the necks even more will just cause more flow or permanent deformation of the mtl.

I’m not a metal expert so I can’t argue the point. All I know is if I size a case using a neck bushing 2 thousands under diameter and size a case using a bushing 8 thousands under diameter and then try and pull the bullets with an impact hammer, the bullets seated in the brass sized with the smaller bushing take quite a few more hits to pull versus the cases sized with the larger bushing.

 

[shtrbc]

I use the Lee Factory Crimp dies on all my hunting rifles. Work great. And.....sometimes you see better SD and ES numbers too.

 

[Novashooter]

I’m not a metal expert so I can’t argue the point. All I know is if I size a case using a neck bushing 2 thousands under diameter and size a case using a bushing 8 thousands under diameter and then try and pull the bullets with an impact hammer, the bullets seated in the brass sized with the smaller bushing take quite a few more hits to pull versus the cases sized with the larger bushing.

That is going to be dependent on your case, neck thickness, annealing, etc. In most instances a .003" interference fit will get you all of the neck tension that a brass case can provide. It isn't a hard and fast rule, only a general rule of thumb.

Crimping can be a solution and often works really well on a grooved bullet like this. The Lee collet crimp die is one of the best out there. Some other possible causes and cures that I've run into. If you are lubing your cases and not washing them after, you can get lube on the inside of your case necks which does not help. Another one is if you dry tumble, and don't brush your case necks, that soot can serve as a dry lubricant which doesn't grip a bullet well.