Hi guys – Same intro as last post.......I’m an older fella and longtime former competitive skeet shooter (well, I competed and maybe leave it at that! haha) who is relatively new to rifles and definitely new to reloading metallic. My good friend @GBMaryland has been helping/advising me and my bringing some questions to the forum in no way indicates a lack of confidence in him. But, I do have some questions that I think would be beneficial to ask of a larger audience. However, I will try not to do a mag dump here and will attempt to limit my questions and try to keep a bit focused.
Background info:
First, I have determined my CBTO to the lands in my gun using a Hornady OAL gauge and their modified case. Yes, I know that this is not 100% consistent and precise but I did find that with a bit of practice, I was getting reasonably consistent numbers and that if a handful of measurements are taken then I feel reasonably confident in the average. That and, at this time, I won’t be pulling the barrel off the gun in order to do the Mark Gordon method to find the lands or similar procedures.
This is my worksheet showing my measurements and resulting CBTO to the lands in this barrel. I have two slightly different math approaches below just as a cross reference but yes, it’s CBTO of 2.2240 to the lands at this point in time and I think this is a pretty good number.
Problem is that if I seat a Berger 140 gr Hybrid at 2.2240 CBTO, then the COAL comes out to be 2.920 which exceeds the max mag length of my MDT binder plate equipped mags which is 2.870. At 2.870 COAL, my CBTO comes out to be 2.174 which is .050 off of the lands. Can’t get any closer. Below is how I came up with this .050 off of the lands at max mag length.
I also measured factory loaded Berger and Hornady ammo COAL which are basically 2.800 which is .0700 less than mag length and should be .0120 jump to the lands of this barrel with this round count.
I’m planning to start my powder charge ladder at .010 less than mag length, which would be .060 jump, just because bullets will vary a bit in length and .010 seems like a reasonable initial cushion.
My questions, if looking at all of this doesn’t hurt your head as much as it does mine (LOL):
Background info:
- 6.5 CM Proof SS barrel
- Hornady brass FL sized with .002 shoulder bump and 1.90 case length after trimming
- Berger 140 gr Hybrid Targets
- H4350
- 1k rounds down this barrel
First, I have determined my CBTO to the lands in my gun using a Hornady OAL gauge and their modified case. Yes, I know that this is not 100% consistent and precise but I did find that with a bit of practice, I was getting reasonably consistent numbers and that if a handful of measurements are taken then I feel reasonably confident in the average. That and, at this time, I won’t be pulling the barrel off the gun in order to do the Mark Gordon method to find the lands or similar procedures.
This is my worksheet showing my measurements and resulting CBTO to the lands in this barrel. I have two slightly different math approaches below just as a cross reference but yes, it’s CBTO of 2.2240 to the lands at this point in time and I think this is a pretty good number.
1 | CBTO - using modified case and Bullet Comparator | ||
#1 | 2.2225 | ||
#2 | 2.2230 | ||
#3 | 2.2225 | ||
#4 | 2.2235 | ||
#5 | 2.2235 | ||
Average = | 2.2230 | ||
2 | Average of CBTO using modified case and Bullet Comparator | ||
Avg w Modified Case = | 2.2230 | ||
3 | Headspace Measurement - using Headspace Comparator | ||
Modified Case = | 1.5575 | ||
4 | Avg Mod Case CBTO minus Modified Case Headspace | ||
Shoulder to Ogive = | 0.6655 | ||
5 | Headspace Measurement - using Headspace Comparator | ||
Sized Fired Case = | 1.5585 | ||
6 | Add #4 to Sized Headspace Measurement = CBTO | ||
CBTO of Cartridge to Lands w Sized Case = | 2.2240 | ||
| Alternative to get CBTO with Sized Case - Delta of Headspace +/- to CBTO | |||
1 | Headspace of Mod Case= | 1.5575 | |
Minus Headspace of Sized Case = | 1.5585 | ||
Delta | 0.001 | ||
2 | Add/subtract Delta from Avg measured CBTO | ||
| If sized headspace is longer, add the delta to measured CBTO, if shorter then subtract | |||
Avg CBTO of modified case = | 2.2230 | ||
Delta = | 0.0010 | ||
CBTO to Lands of Sized Case = | 2.2240 |
Problem is that if I seat a Berger 140 gr Hybrid at 2.2240 CBTO, then the COAL comes out to be 2.920 which exceeds the max mag length of my MDT binder plate equipped mags which is 2.870. At 2.870 COAL, my CBTO comes out to be 2.174 which is .050 off of the lands. Can’t get any closer. Below is how I came up with this .050 off of the lands at max mag length.
COAL vs Mag Length vs Jump | |
MDT max mag length with binder plate | 2.8700 |
CBTO to Lands w Sized Case @1K rounds | 2.2240 |
CBTO at max mag COAL of 2.870 | 2.1740 |
Jump to lands at max mag length of 2.870 | 0.0500 |
COAL to Lands w sized case @1k rounds - too long for mag | 2.9200 |
Jump to lands at 2.860 (.010 less than mag length) | 0.0600 |
Berger Factory 140 gr Hybrid Ammo COAL | 2.800 |
Hornady Factory 140 ELD-M Ammo COAL | 2.810 |
I also measured factory loaded Berger and Hornady ammo COAL which are basically 2.800 which is .0700 less than mag length and should be .0120 jump to the lands of this barrel with this round count.
I’m planning to start my powder charge ladder at .010 less than mag length, which would be .060 jump, just because bullets will vary a bit in length and .010 seems like a reasonable initial cushion.
My questions, if looking at all of this doesn’t hurt your head as much as it does mine (LOL):
- Do my calculations look right and reasonable?
- Since .0500 jump is as close as can get to the lands due to mag length, is it reasonable for me to start with a COAL of 2.860/CBTO of 2.164/jump of .0600
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