brass: mixed range brass limited to S&B, Fiocchi and Geco. (for development I stick with S&B headstamp because they're 80% of my cases)
bullet: geco 124gr FMJ RN
powder VV N320
measurements for push test (5 cases and 5 bullets tested)
cases 0.744"
bullet 0.616"
push test in barrel 1.21" (without subtraction for safety margin)
My first step would be to make 5 dummy rounds to test feeding function and plunk test with an OAL of 1.12".
This would give me a seating depth of 0.24". And if I recall correctly it was Mr. Wobbly who said to seat the bullet at least 0.200" and as close to, but not exceeding, 0.250".
• I think your scores will improve with 124gr bullets over the prior 147gr bullets. So that's a very good choice.
• Next thing you'll want to do is do the "push test" to find your "Max Useable OAL". This test tells how far
OUT the barrel will allow bullet to be.
https://czfirearms.us/index.php?topic=103620.0• You might also want to set your caliper to 9.0mm and then slide the INSIDE measuring tines into several brands of brass cases. That will give you a really good idea of what the "Max Seating Depth" will be. This because the tines will at some point strike the thickening case wall, which limits bullet seating depth. You want to stay well clear of that internal "ramp" built into the brass. This test tells how far IN the brass case will allow the bullet to be.
[ Before you play soccer you want to know where the
boundaries are so that you know what's allowable. Correct ?? These 2 simple tests give you some numbers that become your upper and lower bullet insertion "boundaries", or limits. ]
• You are also correct to proceed with the Seating Depth as your
primary goal. And your calculations are correct. An OAL (if the Max OAL allows its use) of 1.120" (28.45 to 28.50mm) will yield an excellent (probably even your
BEST) chance of success.
Hopefully an OAL of 1.120" lies well between your Max and Min OAL. Adjust as required.
• You'll want to load your N320 probably starting at 4.0gr. You may want to try 4.1gr. Definitely change at 0.1gr increments in this load area. The Vit book will warn you this is over Max Load, but the velocities will prove it is not quite there. With a Jacketed RN (following the Lapua data) you'll instead want to watch for a max bullet velocity of ~326mps (1070fps). You should end up well below that.
The Loading
• Now with this data you are ready to reload. All mechanical powder measures need time to "settle in" and even then they still wander. So the best process starts with NEVER using the first 10-20 drops. Back into the hopper they go. Allow the powder time to settle.
Then the best setup method is to use an
averaging scheme. If you want 4.1gr per load, then adjust your powder measure until 10 dumps weighs exactly 41.0gr. Some loads will be 4.09gr and some will be 4.11gr, but each will be as close to 4.10gr as possible.
Then check
each cartridge by dropping it into your barrel, or a trusted case gauge. I like the ones from L.E.Wilson the best.
Finally, stack the ammo in plastic bin boxes with the bullet end DOWN. This will allow you to run your finger tip over each case. Most people can actually "feel" poor primer seating before they can see it.
Then comes the hard part. Your performance !!
Best of luck.