Playing with seating depth?s
- Thread starter zr600
- Start date
mbaysinger89
Active member
The closer to the lands you get the higher the pressure. Go forward in small increments looking for access pressure signs as usual. If you start getting signs and want to keep seating even longer reduce the load to safe levels and load longer. So on and so on.
A lot of guys start at the lands or just off and find a load, then play with seating depth so they only go downwards in pressure.
A lot of guys start at the lands or just off and find a load, then play with seating depth so they only go downwards in pressure.
Bearcat 74
New member
I make my jumps of .005" at a time.
Usually two sweet spots, one into or just off the lands and another with some amount of jump. The one near the lands usually isn't super critical - .005 either way doesn't make a real big difference in group size. A difference, but not a big one (usually). Like from .350 to .380. The second spot, away from the lands, sometimes as much as a tenth or more (.100) has almost always shown itself critical - .005 or maybe .010 either way making a big difference in group size, like from .350 to .550.
Way back when I used to live and breath this stuff, my guess, and that's all it was, is that seating into the lands or very near to it was the best bet for concentric bullet entry into the throat and minimizing CG offset leaving the muzzle - which is always good for accuracy. There is a certain amount of bullet dispersion attributable to CG offset that is always present, can be calculated mathematically and proven empirically, minimizing CG offset is always the most important thing in rifle accuracy, all else comes after. It's why bullet jackets matter, it's why good chambering jobs matter, it's why good brass matters, it's why we turn necks, etc., etc., etc.
But my guess was that the second sweet spot with jump was more about dwell time and exiting the barrel around a node - similar to powder tuning. Who knows what was really going on though...
It was always easier and almost always better accuracy to just seat into the lands, where appropriate. Or .005 off where seating into wasn't practical. The oft repeated "just touching" is a nebulous constantly changing relationship between bullet and lands and as such, best avoided entirely. Commit one way or the other. Just say no to "just touching".
It's been about 20 years since I was real serious about tuning with seating depth, but I used to work up hand drawn graphs and charts for all this stuff. Burning up a hundred rounds just testing seating depth. I had fun doing it, or I wouldn't have. Learned things, too. But eventually, I learned to just start with a good barrel and seat either into or just off the lands depending on application and it was going to shoot well without having to sweat the small stuff. Mag length restrictions bring the suck, but they are what they are, just have to deal with them as best you can.
I've got some extremely contrary to popular opinions about pressure and seating depth, too. I've only seen a few sets of actual measured pressure data relative to seating depth. But none of the real data I've ever seen, has supported the popular notion that you'll put your eye out by seating near or even into the lands. In some instances, real measured peak chamber pressure has been LOWER when seated into the lands vs. seated to SAAMI spec COAL. And pressures have gone up when seating deeper than SAAMI. In none of the data I've ever seen has there been a really large spike by going into the lands (or deeper either). Well, let me qualify that, I have seen in the data as much as 8K PSI difference (all strain gage data, no CUP), which some might consider a really large spike. But that's between highest and lowest, not a sudden jump with that last .005 into the lands. But to hear the way it's preached and burned into the lore of the internet you'd think it must be a 20K or 30K spike or something crazy like that. It's not...
- DAA
Way back when I used to live and breath this stuff, my guess, and that's all it was, is that seating into the lands or very near to it was the best bet for concentric bullet entry into the throat and minimizing CG offset leaving the muzzle - which is always good for accuracy. There is a certain amount of bullet dispersion attributable to CG offset that is always present, can be calculated mathematically and proven empirically, minimizing CG offset is always the most important thing in rifle accuracy, all else comes after. It's why bullet jackets matter, it's why good chambering jobs matter, it's why good brass matters, it's why we turn necks, etc., etc., etc.
But my guess was that the second sweet spot with jump was more about dwell time and exiting the barrel around a node - similar to powder tuning. Who knows what was really going on though...
It was always easier and almost always better accuracy to just seat into the lands, where appropriate. Or .005 off where seating into wasn't practical. The oft repeated "just touching" is a nebulous constantly changing relationship between bullet and lands and as such, best avoided entirely. Commit one way or the other. Just say no to "just touching".
It's been about 20 years since I was real serious about tuning with seating depth, but I used to work up hand drawn graphs and charts for all this stuff. Burning up a hundred rounds just testing seating depth. I had fun doing it, or I wouldn't have. Learned things, too. But eventually, I learned to just start with a good barrel and seat either into or just off the lands depending on application and it was going to shoot well without having to sweat the small stuff. Mag length restrictions bring the suck, but they are what they are, just have to deal with them as best you can.
I've got some extremely contrary to popular opinions about pressure and seating depth, too. I've only seen a few sets of actual measured pressure data relative to seating depth. But none of the real data I've ever seen, has supported the popular notion that you'll put your eye out by seating near or even into the lands. In some instances, real measured peak chamber pressure has been LOWER when seated into the lands vs. seated to SAAMI spec COAL. And pressures have gone up when seating deeper than SAAMI. In none of the data I've ever seen has there been a really large spike by going into the lands (or deeper either). Well, let me qualify that, I have seen in the data as much as 8K PSI difference (all strain gage data, no CUP), which some might consider a really large spike. But that's between highest and lowest, not a sudden jump with that last .005 into the lands. But to hear the way it's preached and burned into the lore of the internet you'd think it must be a 20K or 30K spike or something crazy like that. It's not...
- DAA
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