Seating depth and pressure

[Tuby27]

I’m a little confused if someone would clarify. When people say seating the projectile deeper, do they mean in the case or closer to the lands? And which will cause more pressure, I would assume a projectile seated deeper in a case would cause higher pressure due to less burning space. So of my picture below of my 270win with Barnes LRX. Which one would have higher pressure signs, left or right?

second question, I may be using the Hornady OAL gauge incorrectly but what I measured my tikka t3x superlite with it showed my lands at about 2.698”. When I use a sized/split neck case and bullet and pushed in and locked with my bolt, it comes up at about 2.827”. Which should I trust? In the picture, the right is my rigged case shown at 2.777” (0.050” off the lands using the sized split neck case)

anyone’s reloaded 270win LRX look like the longer one, with the projectile groove shown?

thanks.

 

[Miflatlander91]

All things being equal same charge weight the one on the left will hit pressure first. When guys talk about deeper they are talking about further into the case.

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[bsnedeker]

Frankly I don't trust either of those methods. I remove my firing pin and extractor and seat a bullet that I know is too long to chamber. When you try to chamber it the bolt will not drop at all... don't try to force it. Seat the bullet a bit deeper, try again and again until the bolt drops with no resistance. As you get closer you should only seat 1 thou deeper at a time. Once the bolt drops with no resistance you have your exact distance to the lands. Measure your CBTO and you are ready to load that bullet at whatever you want to start at.

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[JRMiller]

All things being equal same charge weight the one on the left will hit pressure first. When guys talk about deeper they are talking about further into the case.

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^This to a point for similar loads, but on light loads with an extremely long CBTO the void in the case can actually (mostly with certain powders) cause an increase in pressure.
I generally don't worry about it, its rare, and as long as your not at the extremes of powder charge and CBTO length your fine.

As far as the split case OAL test, your really looking for a CBTO test ( CBTO=cartridge's-base-to-ogive, which is the same as your chambers length, not a cartridges OAL).

And the test is only valid if you do it in this order:
1) make sure the bullet is very tight in the split case neck, you don't want it loose.
2) remove the bolt from the rifle, place it aside
3) insert the case firmly into the chamber, push in with a wooden dowel if need be
4) using the wooden dowel down the barrel (from the muzzle end), gently, gently push the case out of the chamber via the wooden dowel acting on the bullet tip.
5) using a CBTO gauge, measure your CBTO

It must be done like this because with the bolt in and using the bolt to both insert and extract the case, the bullet, which is not crimped in the case, you will drag the bullet slightly back and forth in the case and you will not get an accurate reading.
hope that makes sense

 

[N2TRKYS]

I use the cleaning rod method to find my COAL. From my experience, that measure has been a moot point for me. My magazines have never allowed me to get close to it. The listed COAL in the load data info has been spot on for accuracy for me.

 

[Harvey_NW]

All things being equal same charge weight the one on the left will hit pressure first. When guys talk about deeper they are talking about further into the case.

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I disagree, and would argue that with tons of published compressed loads at a SAAMI COAL. If you were to do a charge/pressure ladder with a COAL that is touching or jamming the lands you'll likely reach pressure earlier than a load with jump.

Seating deeper is pressing the bullet deeper into the case, and has a negligible effect on chamber pressure. Reducing the amount of jump and getting close to the lands is what causes pressure spikes. The load on the right could produce pressure signs at the same charge as the load on the left because it's much closer to the lands and has less alleviation for pressure when firing. It's best to work in small steps once you start narrowing down where you need to be, or just tailor your load starting with a spot you need to be, i.e. mag length.

As far as finding the exact OAL the methods in answers above should get you dialed in. I think it's nice measurement to know but the more important factors are mag length, being seated at least a calibers depth into the neck, or having the bottom edge of the bearing surface near the neck/shoulder junction to try and optimize load density for certain powders.

 

[JFK]

There are people that know more about reloading than me on this forum, but I’ll just add that the ONLY time I’ve ever had a pressure spike was with a Barnes bullet seated long like the cartridge on the right. Copper doesn’t like being jammed into or even close to the lands. My most accurate Barnes load for my 270 is more than .080 off, seated deep like the cartridge on the left in your pic, and near max charge. No pressure signs to speak of.


If you are new to reloading I’d load some up at published COL. If you are not happy with accuracy seat some deeper (shorten COL) in .010 increments and you’ll probably find one that shoots. I wouldn’t even bother with the hornady gauge.

 

[Vandy321]

Frankly I don't trust either of those methods. I remove my firing pin and extractor and seat a bullet that I know is too long to chamber. When you try to chamber it the bolt will not drop at all... don't try to force it. Seat the bullet a bit deeper, try again and again until the bolt drops with no resistance. As you get closer you should only seat 1 thou deeper at a time. Once the bolt drops with no resistance you have your exact distance to the lands. Measure your CBTO and you are ready to load that bullet at whatever you want to start at.

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That's the wheeler method...OP find a video on youtube.

I found it best to feel using the bolt lift vs fall. It's a bit more sensitive that way..obviously, don't close it if it won't close. But that super clean bolt lift with just a hint of a stick is when you know you have it dialed.

Can also pull that case out everytime and hit the ogive with a black marker...it rubs off in a circle when you find the lands.

 

[zr600]

I have had good luck with the hornady tool. Deeper means seating the bullet so it will measure shorter. This should decrease pressure because you are further from the lands. But if you are compressed that could also change things too.