Is there that much in seating depth?

[rummer]

Guys,

I'd like to know how much difference a little difference in seating depth makes.

Common opinion seems to be 10 thousandths off the lands is 'right'.

So how much are you losing if your 20 though off? 30 etc.

Seems guys get reeeeal particular about a thou of an inch here or there.

How much diff can it really make

Cheers.

[Westy]

The deeper the seating the greater the pressure in the case,now depending on what your shooting and what CUP your creating you could be infore one Hell of a BANG????Just something to think about

[brett1868]

Firstly by having the projectile almost on the lands of the rifling helps prevent any "wobble" of the projectile when leaving the case or at least that's the theory.

Secondly by seating the projectile long you can gain some case volume for more powder which means more velocity.

Precision is all about removing the variables where possible and seating depth is one of them.Tuning of the C.O.A.L. to chamber is a trial and test approach that the precision boys spend a lot of time on developing.

That said I think Weatherby used to have a small amount of free bore which was supposed to allow the projectile to accelerate a fraction before hitting the rifling which according to them gave a little more velocity.

The Weatherby setup is based on a 27 year old memory so don't quote me on it

[Baldrick314]

Consistency is key with seating depth. For hunting use it probably won't matter so much if it varies a couple thou here or there but where you should be careful is if you've seated it close enough to the lands that those extra couple thou will make it engage the lands, then you'll end up with a nasty pressure spike.

So far I haven't done any testing to see whether accuracy in my rifle would improve by playing with seating depth but it's something I've thought about down the track.

Other factors that effect seating depth will be you're magazine size if you plan to use one and the length of your projectile because you still want enough of the projectile seated in the case so there's a firm grip on it.

And as for the weatherby's mentioned above, they have a fair amount of freebore to mitigate some of the pressure while still achieving high velocity

[bigfellascott]

Where's Apollo - he's right into this sort of thing, he'd know the ins and outs of how to explain this better.

[tripletwo]

Firstly by having the projectile almost on the lands of the rifling helps prevent any "wobble" of the projectile when leaving the case or at least that's the theory.

Not always the case either.

Commonly the way, but I've read a few comments from guys saying their rifles like a little jump and seated their stuff deeper.

They were shooting better than me so I wasn't going to argue.

[Aussier]

For hunting use it probably won't matter so much if it varies a couple thou here or there but where you should be careful is if you've seated it close enough to the lands that those extra couple thou will make it engage the lands, then you'll end up with a nasty pressure spike.

Pressure issues aside, for hunting it's basically no concern at all.

It's not F-Class, you don't need to be shooting 1/4 MOA.

[pip]

Hi my 2c worth,
For my two sakos .223 and .308 I have given them both a full run on the load data.
The .223 I tested 3 projectiles 3 powders 3 powder weights each and 3 different seating depths. In the end it preferred 20th off the lands.
Oh and vmax and BM2

The .308 same deal: it also liked 20th , It shot 150 SST 's , the best ,for interest I seated them to the canelure and gave a light crimp, they both shot the same, .250 moa at 100. With a 30 fps higher speed.
From memory at the canelure it at 120 odd th off the lands.

The seating differences showed more on the .223

[yoshie]

Free bore is when the bullet has left the case before it engages the land of the rifling. It was a safety measure that reduced the pressure spike because the weatherby cases were running pretty hot back then. Weatherby rifles were hunting rifles not target rifles but they were regarded as being quite accurate.

[salate]

It was a safety measure that reduced the pressure spike because the weatherby cases were running pretty hot back then.

Weatherby was all about velocity then right? Just pushing them as fast as they could.

[nords]

Free bore is when the bullet has left the case before it engages the land of the rifling.

I bet that makes the chamber and throat lots of fun to clean.

[handofcod]

I found this very informative!

http://www.bergerbullets.com/wp-content ... 3/COAL.pdf

[1290]

Weatherby chamberings still have that freebore.... greater than 19mm in cases! so depending on the specific chambering, the issue of freebore and seating depth is irrelevant...

Some chamberings have zero freebore, some lots, some freebore is overbore some is underbore, therefore some bullets when leaving the case neck are jumping an air gap being a zone of no-contact, before engaging the rifling, some bullets are swaged down as they hit the 'freebore'.... there are so many combinations its ridiculous to generalise.....

Therefore I'd say for accuracy its most import to be consistent with powder load, consistent with seating position, it will then follow that the velocity will be consistent and the issues with engaging the rifling will also be consistent (chamber pressure ).

There are also variations with rifling leade angle, some are sharp and abrupt, others are ridiculously 'leisurely'.....

Now, dont say I dont do anything for you noobs....If you look at the ol trusty 308 winchester, based on the CIP chamber and cartridge with the addition of the NATO spec M80 ball round, 147 gr projectile, you get something looking like this;



Theres a lot of detail there, so have patience.... but the red is the chamber spec, the blue is the case spec and the green is the NATO spec..... blue has double lines in areas representing the min / max....while the thick red is the rifling.. more to come .

[5Tom]

Seating to the lands and tuning a powder charge to suit will produce consistent pressure as long as your powder is consistent. You will most likely turn your repeater rifle into a single shot because the cartridge length is too long to fit in the mag. You also risk getting your projectile stuck in the lands if you cycle a round without firing it, causing gun powder to spill all over inside your action.

[1290]

Thisis the cartridge max (blue) and Chamber Min (red). The double blue lines represent the tolerance in the cartridge.


Following, you can start to see the detail in the chamber (red) with the freebore zone and where the rifling starts... that bullet is the m80 fmj to scale while the blue bullet outline being the cip spec 'shape' to compare...


Now you might start to hurt your eyes.. but again red is chamber, blue is cartridge, green is the m80 bullet jammed in there...here you'll notice to the right the smallest diameter part of the chmber is the freebore with the bullet sitting in the 'air'...;


Here.... words added you can start to understand lands, off the lands and what it means, what it doesnt mean too! This bullet (from c. 1960) was, at least according to my interpretation here, designed to sit 26 thou 'off the lands'; and note the bullet is underbore to the freebore....

[Broom]

Now you might start to hurt your eyes..

Check

[rummer]

Thanks for the pics and info 1290.

[barrafishing0]

I did the coal test last weekend to find that .1 mm from lands with reduction in length by .3 mm back to 1.2 mm from lands reduced my group by almost half. New to working up a load but I was amazed, worth the effort to experiment and the ammo needed every day of the week

[rummer]

What was the original group size, Barra?

[barrafishing0]

55 mm and each group came down by 5 - 10 mm. Ended up 1.1/1.2 mm from lands. Sloped there but maybe there is more accuracy below 70.0 mm coal but didn't take enough pills with on the day to try. Once I get powder right it will be better than any thing I will put the cross on. Happy as

[mausermate]

55 mm and each group came down by 5 - 10 mm. Ended up 1.1/1.2 mm from lands. Sloped there but maybe there is more accuracy below 70.0 mm coal but didn't take enough pills with on the day to try. Once I get powder right it will be better than any thing I will put the cross on. Happy as

nice to see someone using metric on this Aussie forum. On ya barra.

[kritch]

Go the metric