Seating depth question


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CJK8
October 18, 2011, 12:25 AM
I have found a couple of loads that are pretty accurate for my bolt action .223. I didn’t worry about trying to figure out seating depth off the lands while doing that. I now have found that on one bullet I am about .055” of the lands and on another bullet I am about .025” off the lands. Here are a couple of questions I have.

If I move the bullet closer to the lands but stay off the lands by at least .005” will pressure stay the same or will it increase even though the bullet is not touching the lands? Along the same lines, if I change my seating depth, do I need to start over and work my way up on the load?

Is it likely I could find a reduced load that is just as accurate or more accurate as I have now with a different bullet seating depth?

Bottom line question is, does the load have more to do with accuracy than the seating depth or vice versa? The few folks I spoke to said find the right load then tweak the seating depth but I am wondering if the opposite is also as good an approach and what the pressure issues may or may not be.

Thanks.

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rcmodel
October 18, 2011, 11:46 AM
Factory Match & Varmint ammo is loaded to standard length that will work in any magazine and rifle.
With no regard to where the lands might or might not be in any particular rifle brand.

Myself?
I think seating to the lands is highly over-rated in some circles.

As two examples, I load:
75 grain varmint bullets in my custom 25-06.
The bullets are too short to reach the lands.
It shoots about 3/4 MOA on a good day.

I load all my .223 varmint ammo to standard .223 length.
It shoots under 1 MOA in my Colt AR, and bug hole groups in my CZ-527.

Seating to the lands in each of my rifles might improve accuracy.
But I can't shoot well enough to find out.

rc

brickeyee
October 18, 2011, 11:54 AM
The bullets are too short to reach the lands.
It shoots about 3/4 MOA on a good day.

So you have no idea if closer would reduce the group.

rcmodel
October 18, 2011, 12:03 PM
Nope.

Not without the bullets falling out of the cases.

rc

MtnCreek
October 18, 2011, 12:38 PM
You need to keep in mind your equipment, components and procedures have tolerances. Just because one bullet is seated at x.xxx” ogive to bolt face does not mean the next loaded cartridge will be. Pressures will change as you change seating depth; it should decrease as you seat longer because you’re increasing volume of the case. As you get close to the lands (or seated to the lands), you reduce the bullet jump causing a potential pressure spike because the bullet meets the resistance of the lands as the powder has had little room to expand. If you want to seat close to the lands, you need to figure out what your tolerances are. If your goal is 0.005 ogive to lands but your loads produce +/- 0.005”, you will not end up with the best loads. Some will have a short jump and some will be against the lands, causing inconsistent pressure and inconstant groups. IMHO, that +/- will have less of an effect if you’re 0.015 off the lands. If you’re at max charge, I would back it off a little before approaching the lands, but IMHO, you’re probably just looking at getting a sticky bolt if you get a little over pressure.

I think the folks you talked to that said to just tweak the seating depth gave good advice. I tried to seat really close to the lands on a couple of rifles and my loading tolerances were not good enough to make it work.

beatledog7
October 18, 2011, 12:42 PM
For 99% of us, of what practical use would a smaller than 3/4 MOA group be?

MtnCreek
October 18, 2011, 12:45 PM
For 99% of us, of what practical use would a smaller than 3/4 MOA group be?
It can determine who’s buying…:)

rcmodel
October 18, 2011, 01:02 PM
For 99% of us, of what practical use would a smaller than 3/4 MOA group be? It's the continuing quest for accuracy & load testing that drives many folks.

By the time they finish testing 4 case brands, 5 types of primers, 6 kinds of powder, 7 different bullets, and try several different seating depths with each one?
They finally find that ultimate load!

But then the barrels half burned out from all the testing, and the rifle will need a new barrel soon.

Which is good!
They can start the quest for the ultimate load all over again in the new barrel. :D

rc

Walkalong
October 18, 2011, 01:53 PM
I learned that the hard way early on, but at least over the years I figured out what makes a difference, what didn't, and to what levels.

Barrel

Bullet

Action

Powder

Primer

Everything else.

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