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Seating depth update thoughts

Started by bikemutt, July 26, 2021, 05:16:43 PM

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Where we'd left off last time,  I'd increased powder charge from 41.3gr to 41.7gr H4350. 140gr Hornady BTHP bullets in my X-bolt seated .030 OTL looked encouraging.

Today I ran up from .030 to .035 in 0.001 increments, I think I found a seating node but, it seems quite narrow.

It was fairly windy at the range as well as plenty of mirage, gotta have at least a few excuses, lol.

I'm at a loss on the .032 target other than there was one extra fast round; 2851 fps when the rifle was shooting right at 2820 on average. In any event, it looks like a divergence before a convergence but, it is what it is.

Things look pretty good at .033 and .034 in terms of group size, then a divergence again at .035.

I'm not sure if I give what may be a narrow node another try at this speed, or drop 100 fps where the rifle also shoots well and look for a wider seating node. What do you guys think?



That does seem like a pretty narrow seating node.. heres what i would do with that.. instead of going in .001 increments i would go in .005 increments.. since you went up to .035 off i would load 3 at .040 , .045, .050, .055 and .060 and watch where it closes back up and then opens up again.. then work in the area where it closed up.. some seating nodes are wider than others..


It never ceases to amaze me how guys will wear a barrel out testing powder then when It comes time to test seating they start taking shortcuts.

Getting centered in a seating node will shrink groups by half or more. If everything else is "right" with your set up and bench manners it will shape up those weird looking groups and can pull those "one outs" in within reason. If you run in increments of .003" you will eventually roll into the nodes, if you run bigger increments there's a chance you'll skip over nodes. The bigger the increment the better the chance  you are skipping over them.
I agree you can tune powder very close but to get the absolute most accuracy out of any combination seating depth must be tested. There is a remote possibility you could pick the majic base to ogive measurement tune it with powder and be at the absolute most accurate seating depth but you will never know for sure without thouroghly testing it.

On the subject of nodes how wide of node do we really need? Aren't we looking for the most accurate node so we can find the center or sweet spot of that node where group size can be cut in half again.

I measured .014" of throat erosion in 1400+ rounds on a 6.5x47 Lapua Improved barrel recently. The barrel was still shoog very well even though I didn't do a very good job of keeping up with it. A barrel will favor the "position" of the bullet in the barrel. We are using the distance to the lands as a reference to locate that position in the barrel. Once this position is found many barrels, actually IMO most barrels will still shoot as accurately in spite of throat erosion to a great extent. Maybe not quite as accurate but still "in the node" accurate.

So while a node that's .002 or .003 wide may seem narrow in regards to throat erosion it stays constant by it's position in the chamber as it relates to barrel time I believe.
I've checked distance to the lands many times and found my throat had eroded but the gun was still maintaining the accuracy because of the position in the chamber not the distance to the lands.

Find the node in .003s fine tune it in .001s to find the sweet spot of the node. Keep shooting it in that position regardless of distance to the lands until accuracy degrades. If / when accuracy degrades check distance to the lands and adjust accordingly.




I understand the node (sweet spot) but my question, is it repeatable? Considering the mountain of variables when reloading and at the range, how likely is it preparing the brass, setting up everything in the reloading process, and testing at the range would yield the same results given the accuracy window is only .002?


Quote from: NightForce on December 24, 2021, 05:47:45 AMI understand the node (sweet spot) but my question, is it repeatable? Considering the mountain of variables when reloading and at the range, how likely is it preparing the brass, setting up everything in the reloading process, and testing at the range would yield the same results given the accuracy window is only .002?

If you are referring to the picture above the accuracy window is much larger than .002"

There is over .010" in .5 MOA and .005" of .2 MOA sweet spot within it. This was at 100 yards

Many of the things you mention will open it up to varying degrees but the whole point of all this is to be centered in powder and seating so when we get some condition variation it doesn't completely take us out one end or the other.
We talk a lot about distance from the lands but it's really only a measurement of what we're really looking for, optimum barrel position.

Find your sweet spot but don't chase it to the .001"th unless testing shows it needs it. More and more I'm finding it's the position in the  barrel not some magical distance from the lands which are continuously eroding.

When we are talking about maintaing the sweet spot in 1000 or 600 yard Benchrest it truly is/can be .001" or .002" wide. The farther out you go the narrower it gets.
Shooting steel or even F Class as I understand it requires "less" accuracy  as i understand it but it still pays to find the node and center in it.


Interesting for sure, I set my depth on my bolt guns at around 4k usually, but I may try this method when I get a chance to load some again,


Quote from: Mark611 on December 26, 2021, 02:59:31 AMInteresting for sure, I set my depth on my bolt guns at around 4k usually, but I may try this method when I get a chance to load some again,

In the distance from the lands approach. It's nothing more than a repeatable starting point to maintain a seating depth either in or out of the lands. There seems to be a lot of accuracy for many bullets just off or just into the lands.

To be fair there is another group that believes you seat the bullet where you want it in relation to the pressure ring and neck shoulder junction. In this approach a chamber is typically throat reamed to to put the pressure ring of a specific bullet just above the neck shoulder junction and will typically put the ogive out of the lands. From there powder is tuned to dial the load in. What most don't tell you is after they get it as small as they can with powder they go to adjusting seating depth. This approach can be more beneficial to chasing eroding lands which ironically is what these guys claim they are NOT doing LOL 

The distance to the lands approach is much the same but uses a different approach to a starting point that may or may not be similar to the other as far as barrel position. Typically you start at a position in relation to the land and test powder then fine tune the best charge weight with seating depth.

I'm not saying one school of thought is better than the other but suggesting they are the same but use different starting points. Regardless of which method anyone prefers there is no denying the real accuracy is in the seating depth. If we pick a position we may get to by educated guess from past experience load the perfect/smallest load by powder tuning alone but we can then seating depth adjust that load right out of tune to prove the point. If we're centered in a wide node it will take more adjustment and a narrow node will take less.


Seating depth modes are interesting in that some are wider than others and they don't all have the same degree of precision (small group size). The widest nodes are definitely the most consistent on target, but not necessarily the most precise.

 Considering our inability to control conditions during a match, I tend to select a seating node first on its consistentcy (width) and then on its precision second.  As I'm learning to modify my rifle's precision using a tuner, I may become less concerned about the width of the seating node used.


At the risk of jacking my own thread, I have a question about playing in the jam zone.

What, if any, are the risk factors with seating to jam, or more?


If we are talking about a shallow jam - say 0.015" or less, there is no increased risk over a jump, provided you practice safe reloading protocol. Start with a low charge and work up incrementally to your desired node.
Think about the bullet and throat dimensions. At even 0.015" there is very little interaction between them.


Light neck tension and the ability to eject a live round are a concern. However I do run light NT and have without incident ejected live rounds seated +.010 before but usually in the +.004-.006" range more frequently. It makes me pucker up to do it but it's never pulled a bullet on me.

Some time seat a bullet +.100" long then put it in your rifle and chamber it. I bet it will seat down into the +.020" range from chambering and will not stick in the lands. I've done this many times


Dave's point regarding ejection of a live round is a good one. I may be off base, but I believe jamming bullets is a more common practice among benchrest shooters who are using a single round platform with no magazine constraints involved. All of my magazine fed rifle's get reloads with bullets seated for a jump. I didn't start looking at jammed bullet seating until I got into bench rest competition. The particular bullet I use in my 6mm competition rifles has been shown to do well either jumped or jammed. In my particular rifles, the jammed reloads outperform the jumped versions.


Like Jerry I prefer a jam in a bench gun but my hunting guns all jump.


Actually I qualified my prior statement by singling out my 6mms. I have a 284 Shehane BR rifle that just does not like 180 Hybrid Bergers jammed. It's much happier with 80 off which is the largest jump I've used in any of my reloads. I could not get it to print under 2" at 600 until I went to that extreme jump range.

Bottom line: let your rifle tell you what it wants with whatever particular bullet you are feeding it. But, don't assume a jam is either risky or less beneficial than a jump.