80GN projectiles touching lands

bolster55 · Post by **bolster55** » Tue Feb 17, 2009 6:10 pm

Hello All,
Why is it so conflicting to get a consistant reading when a projectile is toucing the lands
Sierra 80GN MK's. New case, new projectile, firing pin removed and ejector removed. Insert projectile into case a short distance and insert into action, close dismantled bolt and check projectile for witness marks from riflling. Measure case to theoretical ogive (gauge made and mounted on verniers for readings), 3 assembled rounds (without powder, primer etc.) with readings within 0.07mm.
Hornady 80gn A-Max same procedure but very different readings, a variation of upto 0.29, that's close to 0.012".
If cartridges were assembled with a variation of 0.012" in bullet seating depth you would think a lot of verticles would result, yes?
Why such a variation between projectiles? Am I doing it wrong?
The new cases were neck sized, throat opened and then run through neck bushing to get consistant neck size.

Regards

bully_eye · Post by **bully_eye** » Tue Feb 17, 2009 7:50 pm

Could easily be that the difference is caused by the differing bearing surface of the bullet. I have a tool that measures bearing surface but you can do it with two stoney point comparitors or similar tools and batch bullets accordingly.

Michael

bully_eye · Post by **bully_eye** » Tue Feb 17, 2009 7:52 pm

ps while .012" would be a big variation in bearing surface it isn't impossible.

Post by **AlanF** » Tue Feb 17, 2009 8:18 pm

Bolster,

In my experience the method you're using is suspect because as the projectile is pulled back out of the lands it can pull back a tad out of the case, and hence show a longer head to ogive measurement. There was a tip on 6mmBR.com recently using a case with slits in the neck, and putting oil on the ogive of projectile. Apparently the combination of very light neck tension and a lubricated ogive makes for consistent readings.

Alan

bolster55 · Post by **bolster55** » Wed Feb 18, 2009 7:20 am

Thank you for the replies, I will change my method to see if I get better results.
Cheers

johnk · Post by **johnk** » Wed Feb 18, 2009 7:52 am

Check out the plunger on your seating die. They're all a compromise, but if it's addressing the projectile closer to the point, there's more chance it will generate variations; if it's too close to the ogive, it won't have the mechanical strength to do the job without distorting. More important, does it have clearance for the long point, or was it made for stubby field projectiles?

If your loads are compressed, that's a likely cause - projectiles backing out after seating.

Anyway, IMHO, just touching is not worth chasing. Even if you seat all your projectiles a few thou long, then mike them individually & adjust each according to its particular variations (I tried once), the throat will have moved on by the time you've shot 100 or so rounds, so where will you be at the end if the Queens? Stuff 'em up the throat & wear the consequences if one doesn't unload or give 'em a serious jump.

Post by **AlanF** » Wed Feb 18, 2009 8:24 am

Bolster,

If you do want to try the gadget I mentioned, here is a link to the article :

http://accurateshooter.wordpress.com/2008/12/28/tool-tip-make-your-own-length-to-lands-gauge/

John,

I agree with what you say and don't (now) try to get the projectile just touching in a loaded round, but merely use it as a reference point, so if I decide on a 15 thou jump, I can get the touching head to ogive measurement, then adjust the seater to give a measurement shorter by 15 thou, thus maintaining the jump figure throughout the rapidly changing life of a 6.5-284 barrel!

Alan

johnk · Post by **johnk** » Wed Feb 18, 2009 9:10 am

AlanF wrote:I can get the touching head to ogive measurement, then adjust the seater to give a measurement shorter by 15 thou, thus maintaining the jump figure throughout the rapidly changing life of a 6.5-284 barrel!

Alan,

Is it true that they use carborundum as an inhibitor in 6.5/284 powder? :roll:

John

Post by **AlanF** » Wed Feb 18, 2009 9:59 am

johnk wrote:Is it true that they use carborundum as an inhibitor in 6.5/284 powder?

I think its more likely some sort of drying agent, because the throat assumes the appearance of cracked mud in a dry lake bed. Maybe its climate change?? :-k

Alan

bolster55 · Post by **bolster55** » Wed Feb 18, 2009 5:47 pm

I think AlanF may have the answer.
I changed the neck tension, a lot less, when I then inserted the cartridges into the chamber they were a lot more consistent, primarily because the projectile was not being jammed into the lands and in turn the projectile was not being pulled back out of the case when opening the bolt, which gave me a variation in length.
Thank you all for the replies, I hope a few more are forth coming with different ideas that will make us all think about what and how we do things in our sport, and, hopefully improve our scores.

Cheers

Malcolm Hill · Post by **Malcolm Hill** » Wed Feb 18, 2009 9:14 pm

Bolster
I have found the most consistent way of checking length to the lands is to use a tightly necked case and seat a bullet longer than what it should be. Then as you say use the bolt with no firing pin to give good feel and keep seating the bullet in a few thou at a time until the bolt will just close. When you get close to the just touching length the bolt will start to engage it's locking cams and begin to close.Go little by little and it will eventually just close with minimal pressure.The tight fitting bullet gets rid of the inaccurasies of it being pushed into the case(and by how much?) or being withdrawn a bit as the case is removed. To check for actual contact of the rifling as opposed to the bullet being a tight fit in the throat (which I have seen catch people with a new barrel) I coat the bullet with a dark colored whiteboard marker when doing the final checks. This easily shows up the light rifling marks with the bullet "just" touching or shows up as a full ring around the bullet if it is touching or jamming in a tight throat. Hope this helps.
Regards Malcolm.

bolster55 · Post by **bolster55** » Thu Feb 19, 2009 5:08 am

Thanks Malcolm, that seems a simple way to do it, I'll give it a try and see which will give me the best results.
Cheers

Malcolm Hill · Post by **Malcolm Hill** » Thu Feb 19, 2009 5:48 am

Bolster
I forgot to mention that you should check the empty sized case in the rifle first to make sure the bolt closes by itself without any resistance at all so as not to give a false feel when you are doing your checking. I just use old cases for this job and run them through a full length sizer without an expander plug to get the tight neck. Make sure to remove the old primer though as they can sometimes sit a few thou proud of the case head and give an incorrect end result.
Regards Malcolm.

bolster55 · Post by **bolster55** » Thu Feb 19, 2009 5:43 pm

Hello All,
I did check that Malcolm and found the bolt closed freely on both fired and new cases without projectiles inserted and after primer removal/neck sizing.
Cheers

Woody_rod · Post by **Woody_rod** » Mon Feb 23, 2009 9:03 am

There are other possibilities for depth variations.

One might be your reloading press. Going by what I have learnt about machinery in my own engineering business, a basic underlying principle is the heavier a machine is for its given work area, the better it is. Our CNC machining center gives very good performance, but weighs 5.5 tonnes. We are talking micron changes, which is smaller than 0.0001" (one ten thousandths of an inch).

I use a Lee cast press, which seems ok, but has not been measured for either flex or inconsitancies in seating particularly. 0.0001" tolerances in chambers can be attained, but with lightweight presses, this may be questionable.

To answer my own question, I will go next door and measure the play and flex on my press to see if it moves. Will post the outcome when I have it.