Distance to the lans

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fclassNrugby
Posts: 3
Joined: Mon Feb 27, 2023 6:42 am

Distance to the lans

#1 Postby fclassNrugby » Sun Mar 26, 2023 7:09 am

OK, need some help here with a new rifle. The measurements i get in the Barnard (s series) 308cal is 59.11mm to the lans.
This seems excessive for me as the jump is almost 5mm? am i missing something here?
Standard ADI Sierra match ammo is 54.3ish.

tachyon
Posts: 157
Joined: Mon Jul 15, 2019 1:44 pm

Re: Distance to the lands

#2 Postby tachyon » Sun Mar 26, 2023 8:37 am

It’s not entirely clear how you are measuring the position of the lands. 5mm is 200 thou and, while this is on the longer side it is not at all an unusual freebore. Factory ammo is typically loaded to SAAMI spec, to fit magazine length and whatever random chamber it finds itself used in.

Most target rifle handload ammo is well over spec for length, because 1) we like to get close to/into the lands and 2) longer = more room for more powder.
Last edited by tachyon on Sun Mar 26, 2023 4:12 pm, edited 1 time in total.

Cliff Austen
Posts: 173
Joined: Wed Jul 06, 2005 11:18 am
Location: Sydney NSW Australia

Re: Distance to the lans

#3 Postby Cliff Austen » Sun Mar 26, 2023 9:28 am

First spend some time with someone with experience on reloading.
Watch the attached. Three methods of finding distance to the lands.
https://www.google.com/search?q=finding ... 0DOVa2cFNs
Cheers
Cliff

tachyon
Posts: 157
Joined: Mon Jul 15, 2019 1:44 pm

Re: Distance to the lands

#4 Postby tachyon » Sun Mar 26, 2023 3:54 pm

So I think I've worked out what you're saying.

NB 1 mm = 40 thou(sandth’s) of an inch. Most shooters talk imperial measurements…

Here is my measurement on a factory ADI 155 Sierra - 54.34mm

Factory.jpg


Having pulled the projectile from one of these, seated it long in an empty case, lubricated the ogive, and then jammed the bolt closed on it and extracted it we can see hard jam is at 58.35mm

Jam.jpg


On this rifle, with my neck tension, there is around 20 thou (0.5mm) between touch and hard jam, so touch would be at 57.85mm,

Also on this rifle the freebore is 125 thou, so if your 59.11 is touch, your freebore is 1.25mm more than mine which makes it 175 thou. If 59.11 is jam your freebore is 155 thou. 308's typically run freebore between 50 thou on the low side and 220 thou on the high side. If you want to run light = short fat projectiles you want something on the lower side of freebore. If you want to run heavy = long skinny projectiles you want something on the higher side.

Here is your freebore problem, and also why that problem exists...

Bullets.jpg


At the top is the factory ADI 155. Just below is the OAL required to jam it with a 125 thou freebore. You can see the scratches from pulling it. Below that is a pulled 155 Sierra where you can see the bearing surface is just barely in the neck.

Now, moving on to why I run 125 for a freebore - below the Siena is a Berger 200.20x that is seated for 12 thou of jam (ie it is 8 thou LESS jammed than the 155 Sierra above). It's a little hard to see where the bearing surface ends and boat tail begins but it's about mid neck. This is nice compromise between powder capacity, keeping clear of the donut at the neck/shoulder junction, and having enough neck/bullet contact to hold it in place.

Now the 200.20x is exactly the same nose as a 215 hybrid, it simply has less bearing length. A 215 can be seated although the boat tail bearing transition is (dangerously) close to the neck shoulder junction donut. For that reason, most people who run these run with ~200 freebore to provide and extra 2mm OAL.

The bottom projectile is a Warner 198 flatline solid. If it was lined up better you would see the boat tail/bearing area is near the neck junction but it has driving bands above this making it irrelevant.

Anyway, in terms of practical stuff - Sierra 155s don't mind jumping, but you will need a fair jump to keep them in the neck. Many people struggle to get the 155 Lapua Scenar's going because they are longer, so need a longer freebore. They might be a good option for your rifle.
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