Hi there group. I have a Tikka T3 300 win mag and the thing keeps fracturing the shell casings. it happens in the same spot on all shells. I currently reload and by the third shot out of good brass, the bottom of the casing falls off or real close to it.this also happens with all factory ammo i have shot as well. what could be causing such a thing? thanks in advance.
It sounds like you are describing a case head separation. This type of failure is more common in belted magnums when the cases are full length sized such that the they head-space on the belt. The failure happens because when the case head-spaces on the belt, it leaves excess room between the shoulder and the chamber. On firing the case stretches to fill the chamber which thins the brass just in front of the belt. when you resize the case you push the shoulder back and the case gets longer, so you then trim the brass that flowed forward. After a few cycles of this you can end up with a incipient case head separation. While not usually dangerous (since the receiver is vented) it can damage the chamber and magazine.
If this separation is happening on factory ammunition, then you might have a different problem – To start diagnosing I would measure the case with calipers at: 1. Diameter just above the belt. 2. length from base to midpoint on shoulder (use headspace gauge and compare to unfired cartridge). If the measurements are very out of specification, then have a gunsmith check the chamber.
For long case life, belted magnums should be re-sized so that the cartridge head-spaces on the shoulder rather than the belt. You can set up your belted FL Die just like you would for a standard bottleneck case. If (when you head-space on shoulder) you find that you are not getting enough sizing near the belt to allow easy chambering, you may have to use a Belted Magnum Collet Re-sizing Die from Innovative Technologies. http://www.larrywillis.com/
It is not usually a problem when reloading for one rifle since the fired rounds will fit close to the chamber.
Try just necksizing the brass. That way, your brass is “fire formed” to your chamber. Now when you neck size your brass, you can only shoot it in the firearm it was originally fired in. Neck sizing only works the neck and your brass should last a long time. If you do start this, the bushing you will need will depend on the brass and bullets you are using. Load up a round using new brass, mike the round near where the brass meets the bullet, and then subtract .003 from that measurement and that will be the size of the bushing you will need. In semi auto firearms, you will need to full length size your brass every time you reload, because that way you should not have any cycling issues.