Are there more than one optimal seating depth fore a 223 bullet?

[jlow]

So let's just say i found a nice accuracy node for a 77 grain 223 bullet via OCW and optimized seating depth that is close to the lands. This would of course shoot great but its COAL would of course be long to work in a mag and the rounds would have to be single loaded.

The question is for the same bullet would there be another optimal seating depth that would fit in a mag? Charge weight most likely will have to be different as with the deeper seating, chamber pressure will be higher for the same charge but still the question remains - are there other charge/seating depth combos out there for the same bullet/powder/case/primer combo that would be as equally accurate/precise?

The other way to ask this question is what would happen if one do the seating depth optimization first say with a low charge weight (AKA as per what Berger suggest), before optimizing charg weight? My understanding is seating depth once optimized remains optimized when charge weight is changed. So if one was to do the seating depth optimization, would one fined one single optimized seating depth or would one find a bunch of different ones like barrel time?

Just seating here and wondering if anyone has looked into this in depth?

 

[Sniper338]

I find my seating depth before powder charge... usually ive found when you find the seating depth... thats where it likes that bullet... if you want a different seating depth itd take a different bullet...

 

[jlow]

That's my understanding i.e. when you find the good seating depth, the gun will like that depth even when powder weight changes.

The question though is when you find the good seating depth, do you keep looking? So my guess is most people starts close to the lands and work away from the lands and when they find a good seating depth, they stop and use that together with charge weight adjustment. That makes ultimate sense as you are trying to get there ASAP and not waste any more component.

The question though is what if you keep looking deeper? Would there be others?

 

[Sniper338]

Sounds like a brian litz question..... but my GUESS is no... i tend not to worry about hypothetical stuff though... i find what shoots then go shoot as much as i can... if it aint broke i dont fix it...

 

['Freak]

Thinkin’ it is possible that with some bullets in some loads shot from some rifles there may well exist more than one ‘ideal’ seating depth where that rifle will shoot that bullet as well or nearly so, or even better than what you initially thought was 'ideal'. Only one way to find out.

 

[6BRinNZ]

That's my understanding i.e. when you find the good seating depth, the gun will like that depth even when powder weight changes.

The question though is when you find the good seating depth, do you keep looking? So my guess is most people starts close to the lands and work away from the lands and when they find a good seating depth, they stop and use that together with charge weight adjustment. That makes ultimate sense as you are trying to get there ASAP and not waste any more component.

The question though is what if you keep looking deeper? Would there be others?

I definitely keep going for at least another couple of depths as I want to know how wide the node is (POI, shape and size changes). I don't necessarily look for smallest.

 

[jlow]

Thanks guys. It’s not really a theoretical question since I do have a 223 that shoots well when the 77 grainers are seated close to the lands. I of course would also like to also have loads for the same gun that fits in a mag and so was wondering if it was worth seating down deeper to see another optimal seating depth exists.

Looking for someone that has done this and their experience.

 

[amlevin]

With my .223 I find that seating depth variations yield the same results as one will see in an OCW test.

There can be different "nodes". I find that one load/seating depth combination works well with the bullet in or very near the lands and I can also have one that performs very well in the "Mag Length" range of COAL. Only difference is that I can get more powder in the case when I "Load long" so I also see greater speed than when I have to shorten it up. An exception to that is when I'm using powders like 2000MR or CFE223 that "leave some bullet room".

 

[jlow]

Thanks amlevin - that's what I was hoping to hear!

 

[mikecr]

That's my understanding i.e. when you find the good seating depth, the gun will like that depth even when powder weight changes.
The question though is when you find the good seating depth, do you keep looking? So my guess is most people starts close to the lands and work away from the lands and when they find a good seating depth, they stop and use that together with charge weight adjustment. That makes ultimate sense as you are trying to get there ASAP and not waste any more component.

I agree with this and with Sniper338. But I believe most(by far) don't actually test seating at all.

The question though is what if you keep looking deeper? Would there be others?

Seating is two changes at once. Coarse affect with seating, fine affect to powder. So you gotta recognize that changing seating will collapse your OCW(which will also be coarse),, and with this it gets difficult to see which is what.
I would back way away from any OCW to run deeper seating testing, so that I can see more purely what seating is doing for you. Then re-run OCW at best deeper seating. There is a chance you'll identify a deeper seating that is clearly best in that area, even if not best overall. Then OCW there 'might' be better than prior OCW -resulting in performance that is just as good. No predicting it, just try it.

 

[jlow]

Thanks mikecr - that's my plan!

 

[expiper]

some guns shoot good no matter where you put the bullet,,,some guns wont shoot good no matter where you put the bullet,,,sorta like women,,,Roger

 

[amlevin]

I would back way away from any OCW to run deeper seating testing, so that I can see more purely what seating is doing for you. Then re-run OCW at best deeper seating. There is a chance you'll identify a deeper seating that is clearly best in that area, even if not best overall. Then OCW there 'might' be better than prior OCW -resulting in performance that is just as good. No predicting it, just try it.

This is much like the process a "Ham" radio operator goes through when tuning to a new frequency. Between radio settings and antenna "Loading" there's a lot of "back and forth" before the perfect "tune" is reached.

I did an OCW test on an .223 bullet/powder conbination and when when I had the charge weight that gave me the best group size I then went to testing seating depth. Just because the weather that day was more conducive to a chrono setup (no rain for a change), I shot every different CBTO group over the chrono and then graphed the results. There was a clear relationship to the SD as well as speed changes as the CBTO was altered. I also found a similar correlation between the lower SD's and smaller group sizes. For this "test" only CBTO measurement was changed, nothing else. My conclusion was that as the ideal relationship to powder/case volume was reached, the "burn" became more uniform with the resulting accuracy I observed.


Here's what I've been able to get, using the above method, from my .223 Bolt Action shot from a bipod with rear sandbag (25.2 gr PP2000 MR, 77 gr SMK, Lapua Match, Wolf .223 Primer, 2.022" CBTO)

Here's a "test" I did, changing merely the seating depth of 75 gr A-max bullets, leaving the powder charge weight exactly the same. Top left was "at the lands" and then each group was .005" CBTO shorter. It was pretty graphic as to which seating depth was preferred by the rifle (same setup as above.

Note, all groups 5-shots ea.

 

[Dusty Stevens]

What happens on day #3 when your seating depth needs to be changed to dial it on back in?

 

[jlow]

Nice group amlevin! Would love to see your graph.

I too see good correlation between tight groups and small SDs.

 

[mikecr]

What happens on day #3 when your seating depth needs to be changed to dial it on back in?

We're talking about finding best seating from that of an initial range of depths.
If you're concerned with chasing the lands, this is may not be needed.
If best seating chosen with a particular bullet is off the lands(OTL), then this should remain with no chasing needed.
If in the lands(ITL), well I've yet to find this as best so I don't know for sure. But I would imagine tweaking would follow erosion to reshape/restore grouping. I just haven't seen it needed with OTL seating I've been running with.

 

[Dusty Stevens]

On short range guns we may change the seating depth 3 times a match to stay on top of it and its not chasing erosion. Temp and humidity changes it just as much as powder charges change sometimes. Some days we have to change it for every group. What im saying is seating depth is not the same forever or even day to day if you want to stay in tune. At least it doesnt on guns where we can tell when a change is needed

 

[Mozella]

Nice group amlevin! Would love to see your graph.

I too see good correlation between tight groups and small SDs.

I don't. Take a look at this chart (unfortunately, I can't figure out how to make it show up larger on the forum but if you click on it you can see a larger version):
SD vs group size

It shows MOA and Mean Radius vs SD for a series of 5 shot groups made at 100 yards with my .223 Howa 1500. I consider Mean Radius, also called "Average to Center", a more significant measure of precision than group size. Unfortunately, it's a real PItA to calculate by hand, but a scoring program makes it real easy.

What this chart shows is low correlation between SD and precision, at least in this day's test of 105 rounds, all shot in groups if five. Notice that an SD down around 13 or so shows some of the worst performance. An SD of around 15 or 16 looks pretty good, but then look at the very worst SD. It shows darn good performance shooting an MOA of around .300 or so. WTF?

I'd have to check my data to get the exact numbers, but I think the chart speaks for itself. This is pretty carefully gathered and measured data. I scan all my targets and use On Target software to measure the hits. I find it a little surprising and a bit disappointing at the same time.

Back to the OPs question. Yes, seating depth is meaningful, no surprise there. But I think the only way to find out the answer he's looking for is to do some very careful testing. Here's a chart of three different depths, .005", .010", and .015". Actually, the number references "bullet jump". It does show a more orderly relationship of how the variable performs, but the results are not exactly what you might expect.

You can see that the smallest jump produces the best average results and the best single group. However, it also produces the worst Mean Average and the worst group isn't so great compared with the rest. Interestingly, the largest jump I tested that day, .015" produces the most consistency and it's worst Mean Average was better than the worst Mean Average of the .005" jump.

The rounds were all within a thou. of the intended CBTO measurement and most within half a thou.

Chart Seating depth vs Group Size

One of the things about hand loading that interests me is looking at the data and trying to discover what it really means. I can say one thing for sure. If you shoot 30 five shot groups and then eyeball the targets at the range in an attempt to learn something, you're kidding yourself. You must carefully prepare your rounds, shoot them with care, and then (most importantly) take your targets home, score them very carefully and accurately, and then sort through all the data to make sense of it. Putting various parameters in graph form really helps give you the big picture. M.S. Excl or some other spread sheet is your friend when it comes to searching for the truth.

 

[amlevin]

What this chart shows is low correlation between SD and precision, at least in this day's test of 105 rounds, all shot in groups if five. Notice that an SD down around 13 or so shows some of the worst performance. An SD of around 15 or 16 looks pretty good, but then look at the very worst SD. It shows darn good performance shooting an MOA of around .300 or so. WTF?

I think what gets lost in the SD vs Group Size discussion is the role the shooter is playing throughout the "testing".

I one were to set up a rail gun and evaluate their ammunition, shooting in a perfect environment, there would more likely than not be a perfect correlation between low SD (or Mean Absolute Deviation if you prefer), and group size (or again Average to Center if preferred). Any other method of testing allows for "shooter influence" as they adjust for each shot and just become another variable in the equation.

I kind of like to play around with what I call a "probability factor". If you divide the smaller of the MAD obtained from your Chronograph (mine is a Pact XP Pro) or the ATC obtained from On Target measurements, the smaller of the two into the other, the smaller the resulting number is, the greater the probability that your rounds will hit the same hole. It also gives one the ability to show a wide range of results rather than what they get on a good day when they're shooting well.

I also don't hold much credence in small sample groups over the chrono. 25 to 50 are what I consider to be adequate sample sizes but I'm not shooting "Barrel Burning" calibers either where every round over a chrono is one less accurate round in competition.

 

[jlow]

Thanks for the informative and interesting post. To show some of my own data, here is something I shot this morning with the gun on a rest at 100 yards. The is from an AR with a new 20” Kreiger barrel shooting 77 grain Nosler CC out of Lapua 4x fired cases, Wolf SRM primers, and 25.4 grain of TAC. This is a continuation of a load development effort I started last week where I shot an abbrivated OCW using 24.2 to 25.6 grain of TAC with bullets seated 10 thousands off. Looking at POI shift, I found little shift between 25.2 to 25.6 grains and so picked the middle charge i.e. 25.4 grains. MV was 2871 fps with a SDEV of 17 fps. BTW, QL predicted an accuracy node at 25.3 grain so very close.

Today, I loaded up a bunch using the 25.4 grain charge but seating bullet deeper in 3 thousand increment. The chart shows the groups I shot. Starting from the extreme top right “1” which are the three fouling rounds, then “2” which are rounds with identical seating depth but shot again. From “3” onwards, the bullets were seated 3, 6, 9, 12… deeper. You can see that 1 & 2 were almost identical in terms of POI and SD. The best groups were #5 (0.754 MOA) and the second “1” (o.405 MOA) on the extreme left. They also had the best SD which are 6 and 5 fps respectively.

So group size and SD both increased then decreased, increased again, then decreased in what look like a cyclic fashion?