Seating Depth

Brian in Montana

Brian in Montana

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What is your method when playing with the seating depth of a load. I'm thinking of just doing groups of test loads with the same charge, brass, primer, bullet, but with the only variable being 2 or 3 thousandths of an inch in OAL as measured from the ogive using a comparator. Any advice?
 
Since seating depth affects pressure I would start with a minimum or near minimum load. If you haven't measured your chamber with the specific bullet, do so. Depending on the bullet, the manufacturer may have a recommendation as to the seating depth. Of course, your magazine may limit you as well. If I have enough mag length and I'm using a projectile given to higher accuracy seated closer to the lands, I'll start at .020" off the lands and go from there. I've found a lot of standard cup and core and other lead core bullets shoot better seated closer to the rifling than standard length. But, some shoot just fine at standard length. Sold copper, such as barnes, shoot well with a little jump, so it depends on what you're using. You're on the right track isolating variables. Good luck.
 
Again, a relative beginner here, but here is what I have learned. Before beginning any load development, I measure max CBTO of my particular rifle and compare the COAL of that CBTO max load and compare it to the max COAL of my magazine minus 10 thou. If the magazine limit puts me more than 0.075" off the lands to begin with I haven't bothered optimizing for seating depth. If I can actually work in the 0.010"-0.070" range with my magazine, I will test the options. I start with minimum recommended jump to lands (often 10 or 15 thousandths) to begin working up my load. [In the case of Barnes TTSX which is what I have been primarily working with this summer I start at Barnes recommended 50 thou]. After finding preferred powder type and charge weight according to usual process, then I use that specific provisionally selected load and test the load adding 5 or 10 thou to the jump over 4 or 5 increments, so for example 0.010", 0.020", 0.030", 0.040" and 0.050" jumps. [for barnes I go forward to 0.040" and then go back to 0.050", 0.060" and 0.070" and I haven't seen any pressure indications across the range but your milage may vary]. I look at the data and call it a day. Looking forward to hearing thoughts on OP's question from HT veterans.
 
What dies are you using? I use RCBS dies and it would be difficult for me to consistently measure to 2 thousandths. I would say do 10 or 20 thousandths to start with, at the lowest consistent measurement.
 
I did a test on here a while ago showing my results at .005 increments. If I were to do it again I would of done .010 and at 200 yards. Then I would of repeated the best bracket at .005. In the end I got the best load for the gun, but I shot more than what I needed to in getting the same results.
 
Brian
You are correct in using the same charge,bullet, primer and brass. Not sure how far from the lands you are loaded currently or what bullet you're working with but here's what I do.
Load 3-4 rounds at .010,.020,.030,.040,.050,.060. You most likely will see your groups open in size and then start to tighten and you might actually see them open again. Whatever node they tighten and stay tight let's say that .030-.040 were both relatively same group size and POI.
Now go back and load a finer test at your .003 around those depths like .027,.030,.033,.036,.039,.042. Then study those results and pick your best group. I usually test seating depth at 200 and once I find my depth I test it at 500 to be sure.
 
Seems we're talking more hundredths than thousandths. I'm using Hornady Custom grade dies. I can make pretty small adjustments if I'm patient and careful. I get the feeling I'm on the right track, but was perhaps thinking a bit too small.
 
Brian
Once you find your window you will fine tune by a few thousandths.

What's your current load? How far from the lands? What's the ES and SD of the load? Also did you do any seating depth tests previously?
 
I'm using 140grn federal Trophy Bonded Tips in the 7mm-08 with H4895. I got decent accuracy with a couple of loads in a recent test batch I did. The one I'm primarily interested in is 40.5grns. Average MV was 2824 fps with an extreme spread of 28. My 5 shot group was about 1.5 inches. In an effort to close that up half-inch or so I thought about working up some follow up loads in increments of .2 grains higher and lower, but I've never really done much testing of seating depth and how that effects accuracy. I thought I'd give that a try. My COL is 2.8.

I've read that the Fed TBT's are a pretty dense bullet and tend to do well with a jump. I'll probably start at 2.82" and work down to like 2.74" or something along those lines.
 
VikingsGuy said:
Is that basis mag max or rifle specific distance to lands?
Click to expand...

+1. What's your max mag length? Also what is your OAL touching the lands with the 140gr?
Once we get a little more info I'd be happy to give a few suggestions.
 
I start load development at a length that only leaves me one direction to go. Increments start gross and then get finer based on results.
 
I haven't used my OAL gauge with the TBT's. I've used it with Accubonds and Interlocks. The rifle is a Remington 700 SPS, and the magazine is too short for me to get loads close to the lands like I see some of you guys talk about. I largely just default to loading most things 2.8" over all. The Hornady manual lists the COL for 139grn Interlocks as 2.775, so that's what I do on those.
 
Brian in Montana said:
Seems we're talking more hundredths than thousandths. I'm using Hornady Custom grade dies. I can make pretty small adjustments if I'm patient and careful. I get the feeling I'm on the right track, but was perhaps thinking a bit too small.
Click to expand...

You should look into the Microjust seating stem for your seating die. It works with all Hornady's dies and makes the process of adjusting seating depth a lot less time consuming. Once you've found the seating depth you're happy with, you can replace it with the regular seating stem and set your die to your handloads. The difference in seating depth if you do it right will be negligible at worst.
 
I figured you were loading to mag length. You might be able to load a touch longer and still cycle. I would try to load 2.90,2.85,2.80,2.75,2.70,2.65 and test those depths with your 40.5gr powder charge. Post a pic of the results.
 
cahunter805 said:
I figured you were loading to mag length. You might be able to load a touch longer and still cycle. I would try to load 2.90,2.85,2.80,2.75,2.70,2.65 and test those depths with your 40.5gr powder charge. Post a pic of the results.
Click to expand...

When I am limited by mag and that is more than .100” off lands I have assumed no point in messing with seating depth and loaded to 0.015” short of mag max and called it a day - am I missing an opportunity?
 
Use a fired case. Crimp/size neck only enough that a bullet fits snugly, as in you can still slide it with your hand.
Chamber. Do NOT pull the trigger!
Gently remove dummy cartridge and measure with calipers. This will be your COAL for that brand/model/weight of bullet.

Now if it's not a VLD, or monolith, subtract 0.020". Check to see if this will fit your magazine.
If it does, then load to that length and shoot a group.
VLD usually, not always like it about 0.005" off the lands.
Monolith start at 0.050" off the lands, and crimp the casing.
 
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