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Bullet seating, are we doing it wrong?


Fox Tales

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 The standard method to calculate seating depth , would be to first measure the max OAL to the lands , using a OAL gauge. We can then apply this measurement to calculate bullet jump .All pretty standard stuff , I know. 

 Supposing we make the initial measurement using a SAMMI spec case and apply to new brass . Following load development we arrive at jump of 10thou' for best accuracy. All's good so far .

 Now we take a different tack and decide to neck size only. We know that the cases have gained length and it's measurable using a headspace gauge. If the case has gained 10thou' then the bullet seating has proportionately changed and the jump increased. The reason behind this is that during the mechanics of the firing cycle the case is pushed forward in the chamber and using the shoulders as a stop .

Applying this should we increase COAL to accommodate and if this is the case would calculating shoulder to tip length be of more use ?If this is the case then seating dies are fundamentally flawed and seating should be in relation to the shoulder .

 Comments please .

 Cheers

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Take a case, that’s been fired a couple of times in that’s been fired a couple of times in that chamber, to a gunsmith or anyone with a lathe, and get it modified for the tool you use. You could send the case to HPS. They do it regularly. Because that case is fully headspaced against the shoulder all your measurements will be spot on from then on.

I suggest the best and only really accurate way to measure your COAL and BTCO fir any bullet is to use the empty bolt method.

Remove the firing pin, roughly seat the bullet too long and try to close the bolt. Keep doing this in 2 thou intervals until eventually the bolt handle drops without assistance. That’s your dead accurate measurement.

There’s a YouTube video showing how it’s done if you care to search for it.

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5 hours ago, Fox Tales said:

 Now we take a different tack and decide to neck size only. We know that the cases have gained length and it's measurable using a headspace gauge. If the case has gained 10thou' then the bullet seating has proportionately changed and the jump increased. The reason behind this is that during the mechanics of the firing cycle the case is pushed forward in the chamber and using the shoulders as a stop .

Applying this should we increase COAL to accommodate and if this is the case would calculating shoulder to tip length be of more use ?If this is the case then seating dies are fundamentally flawed and seating should be in relation to the shoulder .

1 hour ago, Brillo said:

I suggest the best and only really accurate way to measure your COAL and BTCO fir any bullet is to use the empty bolt method.

Remove the firing pin, roughly seat the bullet too long and try to close the bolt. Keep doing this in 2 thou intervals until eventually the bolt handle drops without assistance. That’s your dead accurate measurement.

There’s a YouTube video showing how it’s done if you care to search for it.

It's a comparative tool so as long as the parts of your comparator, including the case, are kept the same then just allow an offset for the measured length of the non-fired and fire-formed cases.  Once you have a starting position it's down to development until you get an optimised load - just measure the real-world optimised load with your ogive comparator and duplicate going forward.

Measuring COAL to the tip is not the way to go for accuracy (though you need to be aware of it for magazine fit).

As I have the lathe and tools I do my own fire-formed cases just to avoid adding offsets:  5/16 x 36 UNS is the thread for Hornady tool.  Tapping drill is a letter "L" drill.  The tap is an unusual size - Tracy Tools have them but they're not cheap!

The drop bolt method does apply more force to the bullet even when dropping without any detectable resistance so it will seat deeper in the lands by a few thou.  I find either method (done properly) is repeatable so either will give you a comparison to start with and you can work from there to optimise seating.

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At the end of the day it’s all relative. What ever method you use , it makes no real world difference if your measurement is a few thou different between methods as long as you always use the same method every time.

When you find the jump that works for you, what difference does the measurement make. Once I’ve got my particular load tuned I record my a Redding Competition seating die setting and that’s it done.

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 I may have put my point across poorly .Apologies .

 Using a headspace gauge measure a new case (head to datum point on the shoulder ). Now if we subtract this figure from the COAL we will get a figure that relates the  shoulder to bullet . If we then apply the same to a neck only sized , fire formed case and load to the same COAL the relationship between shoulder and bullet will be shorter by comparison. 

 Since the shoulder forms the stop during the firing sequence this means the bullet is further from the lands . Admittedly if you always FL or only do load development after fire forming and NS then this relationship will always be constant. 

 Which brings me to the question of why we don't measure seating depth in relation to the shoulder ,rather than OAL ? 

Yes I know I'm probably over thinking this , sometimes it's just good to question conventional wisdom. 

 Just putting it out there.

 As for measuring I like to measure to Ogive and use a bullet of average length after measuring a dozen or so from the box . Whichever method is used the relationship will change .

 

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3 hours ago, Catch-22 said:

I would ask...does it really matter?

Whatever method you use to do your measuring, as long as it’s consistent, repeatable and the results on target make you smile... then it doesn’t really matter does it?

👍 🔫

👍 exactly

 

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I see what Fox Tales is getting at, and it's inherently correct - but the claimed effect is exaggerated. 10 thou' difference in case-head to shoulder datum points between new brass and fireformed then properly resized fired cases is huge, at least in modern rimless commercial cartridges and firearms. (Early 20th century rimmed military designs such as 303 and 7.62X54R as werll as belted magnums deliberately ran with large amounts of case to chamber clearance at the shoulder, but they don't headspace on this point.)

10 thou' clearance is in effect 'excess headspace'. it means the first fireforming use risks thinning the case walls just above the web. Oddly enough, I've just seen something similar with new brass for a particular cartridge desdigned mainly for use in AR type rifles and it caused me some vexation and head scratching when shot in a turnbolt rifle until I twigged what was going on during fireforming.

The difference between a good quality new case and one fired / resized in a decent chamber is much smaller usually, under 5 thou' and often at around 3. Of course, the modified case used in the OAL gauge tool might produce such a variation. This tool is a boon, but it's nothing like as precise as many believe - hence the very sound advice that unless very precise measuring methods are employed, jump shouldn't be set at 5 thou' or less and 10 is a lot better. This avoids the possibility of an over-measurement producing a COAL that is right on the lands but only just - not in itself necessarily undesirable, but what is to be avoided is the situation where small variances in bullet ogive positions may see one round with its bullet a thou' or two 'in' and the next a similar amount 'out', a set-up that usually produces fliers.

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 Thanks Laurie,  I was using 10thou for ease rather than accuracy. After reading your reply I can see it would have little to no effect, especially when variation of Ogive shape is taken into consideration.

 I doubt I would see a difference of 3 thou in seating down range . 

 Cheers 

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