Seating Depth

[Deleted noremaximus]

Good morning, I'm about to conduct a seating death test and I need a bit of clarification. I'm starting 20 thousands off Jam, and going in a 3 thousand increments. I have eight, three shot groups ready to test. How many groups do I need to find a seating depth node? Is eight groups a little excessive? Thank you.

 

[Uncle Ed]

Long range load development at 100 yards.​

Erik Cortina

Some of you have asked for more detailed instructions on how I do this. Here they are:

1. Find Jam by seating a bullet long on a dummy piece of brass (no primer nor powder) and apply die wax to the bullet ogive and record it's base to ogive length.
2. Chamber the round and close the bolt.
3. Snap the bolt open and measure the base to ogive measurement. If it is shorter than previous measurement, this is your jam. Do it a few times with different cases to make sure.
4. Load a known powder/primer/bullet combination. I load 4 of each powder charge in 0.5 gr. increments and seat bullets at jam - .020". I use one shot of each to get barrel fouled up and also keep an eye for max pressure at the same time. You can also use these rounds to break in a barrel if you are inclined to. If I encounter pressure on the hotter rounds, I will not shoot groups with the other loaded rounds and will pull bullets when I get back home. Do not shoot in round robin style because position and natural point of aim will be compromised.
5. Shoot 3 shot groups starting from lowest to highest. All groups are shot over a chronograph.
6. Examine target and find the place where consecutive groups line up vertically and ES is the lowest and speed increases the least from one group to the next.
7. Load to the middle of the powder node and do a seating depth test.
8. Load 3 shot groups starting from Jam - 0.005" all the way out to Jam - .040" in .003" increments.
9. When you find the seating depth test that shoots the best, load towards the longest side of the node to allow more room for throat erosion.
10. Final step is to load the new seating depth and load 5 shot groups in 0.1 gr. increments 0.5 gr. on each side of node (if pressure limits are not reached). This will cover an entire grain of powder and you will be able to pinpoint where the powder node starts and ends. In the summer, load towards the low end of the node, and do the opposite in the winter.

 

[Bill Norris]

According to Eric, you should clearly see a node or maybe even more than one node within the 8 different groups.

 

[K22]

What kind of groups are you achieving with the current seating depth?

What is your shooting discipline?

What is your group size requirements?

I personally never seat closer than .010" to the lands due to variability in bullet ogives and the need to not have a bullet stick in the throat when I eject a loaded round. Also, I seat so that there is sufficient bullet depth in the case to provide for adequate neck tension. The old "one bullet diameter" minimum in the case neck is a good rule of thumb to follow in my opinion.

Personally, I wouldn't use eight different seating depths, maybe two or three. Personally I've never found seating depth to be a major accuracy factor whereas bullet selection for me had the most significant impact on group size assuming you're using a suitable powder for the cartridge you're loading for.

Also, in some of my rifles with some bullets, a greater jump produced some improvement in group size versus a smaller jump. The possibilities can be numerous and in this day of component storages I take a careful look at what I need relative to group size before I use up a lot of components in testing that may not produce the results you are hoping to achieve.

 

[Deleted noremaximus]

I am shooting F-class, My current group is about .29 Inch. three bullet holes are converging. I'm right at the powder charge node of .5 grains. So my load is 39.7gr of R17 with a 140 RDF bullet. I don't mind investing more rounds past the eight groups If need be. I honestly thought eight groups was a lot already. What should I do? Go all the way to 40 thousands off jam? Thanks again for all the great info.

 

[Straightshooter1]

Good morning, I'm about to conduct a seating death test and I need a bit of clarification. I'm starting 20 thousands off Jam, and going in a 3 thousand increments. I have eight, three shot groups ready to test. How many groups do I need to find a seating depth node? Is eight groups a little excessive? Thank you.

How many??? Only need to fire them until you find the tight group(s) and it begins to open up so you know the size of the window you're dealing with. Like one I did this last weekend where I find I'll have to retest a load/seating-depth (like the 43..4 and 43.6 in the pic below).

I like to use 3 shot groups, though sometime I feel 2 is enough), but I'll load up 4 or 5 of each load/seating-depth just in case I pull a bad shot so I can fire another to be sure to get a result I have confidence in.

 

[cw308]

I always shot a 5 shot group with 3/10 of a grain increase for load development. Alot of my benchrest shooters use the chronograph , I go by my group size.

 

[Hoot]

I like the 43gr load not only because of its size but because the dispersion is almost entirely horizontal.

 

[Tim Singleton]

Tune out horizontal with seating depth and vertical with powder

 

[Deleted noremaximus]

Final question, do you do round robin with seat depth test or regular at 100 yards? Thanks.

 

[Kracken McGraw]

Tune out horizontal with seating depth and vertical with powder

Does it alway take more powder to cure the vertical stringing, or can it go the other way too?

 

[Tim Singleton]

Does it alway take more powder to cure the vertical stringing, or can it go the other way too?

Well if you are already at the upper end of your load window then yes going down will most times take out the vertical

 

[RegionRat]

I don't want to saturate you with more different input than you have already heard, and the articles in the following links have spent more words than I will.

Take a scan of the related articles in the seating depth node series if you like.

The real take-away is two main points. One is Jam versus Jump and the other is How Much Jump

The Jam versus Jump discussion isn't what we are discussing here so I will assume we don't need to worry about that philosophy. How large a jump is worth discussing since the idea of how many rounds it takes to find the best jump zone is what you are asking.

Don't bother reading those at all if you don't like the idea of a single sweep of a large range before you start group testing. On a match gun, I usually only need one sweep for charge and one sweep for depth before I follow up with one or two groups to verify those results.

By not doing those sweeps, you risk not knowing how wide each node is or how far they are. YMMV.

ETA: An example of a wide sweep for vertical versus seating depth for a Berger 6mm 105 Hybrid in 6Dasher








https://precisionrifleblog.com/2020/03/21/bullet-jump-and-seating-depth-reloading-best-practices/

 

[JimSC]

I load 50 at touch and take a portable press with a seating die to the range. Shoot two at touch and evaluate. If the two are .5 or better I continue seating at one at a time until I get a uncalled flyer. If the two rounds are .6 I just move out another .003 with two more and repeat. Continue with that process until I can get five or six rounds into a bughole.

The way I see it if the first two are 3/4 inch apart why bother with a third. When I get a satisfactory group I take whatever I have left and seat them at that depth then test them at mid and long range. Saves me from preloading lot of rounds that won't group worth a darn. I only need one good node and could give a darn less about how many bad nodes are between that good one and the next good one

 

[Shinbone]

I'm just a rookie, but here are the results of a jump test with a .22-250 Kimber Montana in .005" increments.