uneven lands engraving

[lpreddick]

i'v been loading everything "into the lands" and evaluating how they shoot. i'm an ocd target and varmint shooter and constantly strive for the one hole group with all my rifles. i'v noted several things of interest and have some questions. with neck tensions of .001-.0015 (measured expansion of loaded neck from prepped neck) you can seat a bullet really long, chamber it (no primer/powder) and inspect for engravings. the bullet will seat deeper by a varying amount and have the lands engravings. polish the engravings clean, reseat the bullet .005 and repeat. the bullet no longer self seats and the engravings are shorter and become more so as i repeat this process. in a few of my barrels, as the engravings get really small, there is a difference in thickness and length of one of the four(kreiger barrel). i use a 3.5 power loupe for this inspection. there is no runout in these bullets. it appears that one of the lands is ever so slightly "shorter" than the others. i can see this happening after hundreds of hot loads or a problem during chambering. is what i think i'm seeing real? does this affect the bullet negatively if one is "jumping", esp if jumping a vld. self seating of the bullet may not be to the same level when loaded extra long...like .040+ 'into the lands", unless neck tension is tight. if seating "just at the lands" where this short land is not touching the bullet, will accuracy suffer? the engraving difference i'v seen is not detectable if the bullet is .015+ into the lands. is this much ado about nothing? thanks.

 

[Bob3700]

I soft seat my bullets. Very low neck tension and between .030"-.040" long. My theory is that I don't have to chase throat wear over the life of the bbl. Have not had a bbl that wouldn't shoot using this scheme.

How are you gauging your .001-.0015" neck tension? Do you neck turn your cases? My point here is you may have varying neck tension (and don't know it) that is affecting your engravings.

Is the variation you are seeing having a negative effect on your groups? What calibers are you shooting and seeing this variation in? Many benchrest shooters strive for engravings that are as long as they are wide. That was a goal for many years. You will need a bunch of neck tension to find that spot.

As for one land being shorter than the rest, that can come from a couple of different sources. First would be the actual chamber reamer. I always check the throat of a fresh chamber with a bore scope to ensure that all the lands/throat are the same.

Second (and I suspect your source) is the cleaning rod. If you flex your cleaning rod while running brushes or patches up the bore, the rod rides on the lands and will wear them prematurely, usually in the same spot. Have seen more bbls damaged this way than not, done it myself. That is one of the reasons that some folks cut a new chamber in the bbl after so many rounds. Get a fresh throat and do away with all the wear.

Don't know if that helps any but wanted to respond and give you some thoughts.

RGDS

Bob

 

[shanmoll308]

When repeating this test again, use a fire formed piece of brass that has been neck sized only. Also remove the plunger in your bold head and that will even things right up for you.

Adrian

 

[bigedp51]

When repeating this test again, use a fire formed piece of brass that has been neck sized only. Also remove the plunger in your bold head and that will even things right up for you.

Adrian

Very good point, I was going to say the case was laying in the bottom of the chamber meaning the case was slightly smaller than the chamber diameter and the cartridge was drooping. (very slightly off center with the bore) But the ejector button can push it of center with far more force than just gravity.

 

[gotcha]

Excellent input above. The last time I had this problem I borrowed a F/L die from a friend & problem disappeared. The die I'd been using was distorting the case body and pushing the neck slightly off center. You could check this by making a registration mark on the case head to see if the "short" land changes position. You might also check that shell holder is allowing case to center in die. My answer would fall in the "least likely" column, but you never know

 

[mugadutz]

maybe you are looking for a problem that isn't there i used to talk to Tony Boyer at matches about problems I thought i had. He said what is your problem, you won didn't you Tonys theory is shoot the barrel if it shoots there is no problem
good shooting
Mugadutz

 

[Webster]

In Tony Boyer's book p. 134 he mentions one or two of the lands not engraving the bullet when trying to determine the touch length. He states it's related to the bullet not being perfectly concentric, the ejector pushing the case to one side or an improperly machined barrel.

 

[jo191145]

When you get right down to it mugadutz is correct. The target will tell you everything you need to know.

As for the age old method of using a jammed bullet and light tension to find the lands. Never liked it myself.
Too many variables. It will work as a rough guide.
If you repeatedly polish and reuse the same bullet consistentcy will go out the window. Polishing slightly reduces diameter but worse of all what happens when the rifling lines up on subsequent attempts?

Hornady case guage and comparator gives accurate readings to "just touching". From there its just math.