6BR Tuning

[Drop Port]

^^^ This is your best advice, start with potential clearance issues and if that is not the problem eliminate and move on.
What is the OD neck measurement of a loaded round?
What is the OD neck measurement of a fired case?
What is the length of your case and do you feel any resistance chambering an empty case when you close the bolt?

 

[Kracken McGraw]

http://bulletin.accurateshooter.com...-chamber-length-with-home-made-modified-case/
Here’s a old but good thread that I remembered to bookmark last time it came up.

 

[jpmimproved]

Fired brass measures .270” at the neck, loaded round is .268”. I don’t know exact freebore, but was chambered as a no turn neck by a reputable smith from this forum specifically for 105-108 gr bullets. I will send a note asking him what freebore/neck diameter. I pulled the ejector from the bolt when measuring cartridge base to ogive a la Alex Wheeler method, and there is visible bearing surface(see image) forward of the neck when the bolt falls closed on an unprimed/uncharged round with the bullet seated. The bolt does NOT fall on its own, there is some force required on a sized case and a bit more on a once fired unsized case. Firm but not a lot, one finger is all it takes.

It appears the case has room to stretch in the chamber, measuring as close to the datum line as possible(if I’m using the term correctly) from a sized vs fired/unsized case shows 0.010” movement forward.

Not sure if there is a way to measure if the case mouth is infringing on the leade/throat area. The case length is 1.556” for new brass after sizing, which is what was used.

Appreciate all the help and suggestions! Learning as I go…

 

[Dolomite]

Are you using a bartlien barrel 8 twist?

 

[jpmimproved]

Case length gauge success! 1.575” it appears. Thanks @Kracken McGraw for the tip. Hurt to cut up a piece of brass and beat up a bullet at current price/availability.

 

[jpmimproved]

Are you using a bartlien barrel 8 twist?

8 krieger

 

[jpmimproved]

Sized, unfired brass is all I have to go off of since this was the first outing, was just wanting to rule out lack of case expansion room. So I am not bumping the shoulder back 0.010”. I usually use the same method in the images above and push about 0.002” from fired.

 

[jpmimproved]

So to be clear you are moving the base to datum .010 when sizing?
The bit of resistance when closing the bolt… could that be the lug engagement your feeling?

Could be lug engagement plus slight drag from the case body? Doesn’t feel like a hard stop. It is slightly more resistance than an empty chamber.

 

[Kracken McGraw]

https://forum.accurateshooter.com/threads/barrel-cleaning-fluid-recommendations.4052133/page-3
This thread has a video of Speedy cleaning barrels where he shows wrapping a brush with a large patch to clean out residue in the chamber as a last step.
I t will be interesting to see where you go from here. I’m sure no expert but after deburring the outside of case necks good, spin the whole neck in steel wool or a extra fine sanding pad to clean necks up good.https://www.accurateshooter.com/cartridge-guides/6mmbr/
This link practically covers it all. I think it suggests start seating bullets at .015 off touch as a starting point on new un-known loads.

 

[jpmimproved]

Anyone have experience with the 108 JLKs? Maybe these just build pressure differently. Still not sure I have figured anything out that would explain the pressure. I know Varget is the go to for 6br, but I’m sure we all know the challenges getting that…

 

[Darwin]

I’m thinking Bart is onto something. I’ve got 2, 26 inch IBI prefits built a year apart. Same Lapua Lot#. .269 Loaded. .271 fired.
Savage/107MK at hard jam, 59 degrees, was in the 2890’s at 29.6 H-4895 with no ejector.
Rem700/108 Elite Hunter, hard jam, 30 below gun, ammo at kept in warm pocket likely 70+, 29.0 no ejector after 5.
Sierra online data shows 26.9 is max at 2620, 24 in barrel.
Some members say H-4985 can be very accurate or something you wanna sell and try a different Lot#.

 

[boltfluter]

Bart has you on the right track! Most likely the lot of H4895 you have. I have seen this with Varget as well. Nuclear pressure and no velocity. Beg, borrow, or steal some H4895 from a different lot number and try it. Good luck.

Paul

 

[jpmimproved]

Been a while since I got out to restest. On the advice y’all gave, I set the bullet back to .030” jump, skimmed the necks and cleaned the chamber, and ran up to 28.8 gr with no real pressure signs. 28.0 - 28.2 gr looks like could be decent node if I’m reading this right. 5 shot groups

 

[Bill Norris]

Might try @ 27.9 as well. U don't have a lot of vertical @ 27.8