Three ways to find the lands

[hounddawg]

no matter what method you use is you are doing nothing more than acquiring a datum point for comparison. It would not matter if that datum point is called .012 or .013 or .018 as long as the datum point can be repeated accurately.

Max you ever shot in the 1's with a semi automatic .22 LR off a bipod at 50 yards ? Now for full disclosure is this is the one and only time I had a group that small, it normally shoots in the 2's and 3's

 

[Bart B.]

How far does the throat erode down the barrel for every dozen shots?

To maintain a given bullet jump spread to the lands, you got to seat them incrementally less.

 

[Maxwell Haus]

I don't reload for 22 lr , could not care less.

Datum point ? I thought we were talking about finding the lands ,not the shoulder ?

 

[mehavey]

"Datum" point is any reference point of known or assumed coordinates
from which calculation or measurements may be taken...

In this case, the reference is the start of lands engagement with the bullet curvature -- and the resulting overall length of the cartridge.




"Personally"... I'm either hard into the lands, or standing off at least 5 thou to accommodate manufacturing tolerances

 

[Maxwell Haus]

How many times do you hear someone reference the datum when measuring bullet/land contact ?

 

[hounddawg]

some call it touch, some call it jam. Regardless it is a datum or reference point from which adjustments to the base to ogive for any particular load are made from. The ogive is another datum point which is just a arbitrary reference point since any two inserts for a comparator may or may not be the same diameter. When I first got into 6BR's I used my .22 cal insert for the longest time to measure and adjust my bullet seating.

@ bart - irrelevant unless you subscribe to the belief that you have to know the exact distance down to the .00001 of a inch. As long as the adjustments are made in comparison to the original measurement. The amount of erosion per shot will be determinate on a number of factors such as the temperature of the powder used and more importantly what cartridge. A 22 -250 firing a 55 gn bullet might use 36 gn's of Varget and have a barrel life of 900 - 1000 rounds before burnout while a .223 Rem would only use 25 gns of Varget and have a barrel life of 3000 - 4000 rounds.

Myself when I notice my accuracy eroding I will drop down to 100 yards and test seating again working my way out from the previous seating depth .003 at a time until I find another accuracy point. A trick I learned from watching Erik Cortina videos

 

[Bart B.]

I think accuracy to a couple thousandths inch is good enough.

My 308 Winchester chamber throats advanced about .001 inch every 3 dozen shots. Acceptable accuracy for 3000 to 3500 rounds

 

[hounddawg]

Bart I really do not measure after the first time. I just let the holes in the paper tell me what I need to know. I know what I normally shoot, I know when it drops off time it is to move the seating out a few thousandth.

 

[Bart B.]

I've soft seated bullets for single round loading in bolt action rifles so they seat deeper when loaded. They've all produced best accuracy that way.

 

[hounddawg]

I was prepping for a load test tomorrow and got to thinking about this thread. I used the Horandy to check five 105 6 MM bergers in one of my rifles. Using the .24 comparator insert the numbers were 2.811, 2.812, 2.810, 2.810 and 2.812. I decide to call 2.811 touch, or jam but I think I will call it zero. I seated the bullets for this charge test at 2.800 since my other barrel likes that bullet .010 - .015 from "zero". Is that bullet really .010 - .012 from the throat or lands? No idea but it gives me a reference point so if this barrel does not like the bullet seated .012 from zero and I need to adjusting seating depth I can go out to 2.808 with no fear of a bullet being stuck in the throat in case of a squib, or I can go to 2.797, 2.794, or 2.790 if I want to try more of a jump. To me that is the only purpose of "finding the lands"