Delema Quandry LOL. Heres the question Fire formed neck sized brass Not as accurate

[DEZ]

Here is what I'm experiancing in my .243 loaded with 105gr Amax bullets and brand new hornady brass and 33gr of varget . Was getting a very nice group so I thought once the brass was fire formed and just neck sized that my groups would tighten up but the opposite happend with the same load same seating depth my group opened up .I fired a few more of the new brass ones as I had 10 left from the first range trip and the group was smaller . Then I went back to the neck sized once fired brass reloads and the group opened up again . The only thing I could think of that because the cases were now fire formed to my chamber was that they had a little more internal capacity and this was causing the velocity to drop slightly and maybe I lost the sweet spot. any Ideas ?

 

[Alpheus]

That could be it. Another possibility is different neck tension. If the new brass has a different outside diameter then what your neck sizer produces, then that could lead to different peak pressures when fired, leading to different velocities/barrel harmonics/point of impact etc. If you still have some of the new brass, whip out the micrometer and take a look.

To get back to the sweet spot is likely only a few tenths of a grain, plus or minus depending on if the neck sized brass is tighter or looser then the new stuff.

 

[uncledillers]

I think you hit the nail on the head. Same thing happened when I was first starting off. That was due to having a big difference between a spec head spacing and the actual head spacing on my savage in 308. The fire formed brass was at least 5 thou longer then factory sized brass.

 

[DEZ]

Thanks Guys I've switched to a lee Collet neck sizing die insted of the RCBS as neck tention was definitely higher and I guess its back to test loads Dam LOL

 

[Cleftwynd]

My guess is runout, the volume of your chamber isn't changing, and that's what really matters. So it's either runout, or less likely neck tension.

 

[DEZ]

Here is a pic of the new gun well I've had it a few months Rem SPS in a Boyds stock No bedding yet just dropped her in and put in a Rifle basics trigger and a pic of my best 5rnd group. I think with some work it shows promis.

 

[DEZ]

My guess is runout, the volume of your chamber isn't changing, and that's what really matters. So it's either runout, or less likely neck tension.

Yeh guess I better check make sure my case necks are trued thanks forgot about run out .