I am building a 6mm BenchRest Norma. I have the Barrel, Lapua brass (new), and a reamer from PTG that has a .272 neck. Before I cut the chamber and make the reamer non-returnable, I am hoping someone can help me to understand the neck sizing situation, as I have never reloaded at this precision/accurate level.

Placing a .242 bullet (I don't have the new ones in yet) into the new case and measuring the neck dia, I get .2675, and that makes me think that I am a long way from the chamber neck dia of .272 and will be working the brass too hard.

With the above info, should I be calling PTG for a smaller dia reamer? It seems that I will be needing to neck size down to about .265-6 to get proper hold on the bullet, every shot will blow the neck back out to .272, and that seems like a lot. But I have never done this, so I don't know what to think.

Suggestions appreciated.

Cole

ColeC: I just miked several of my Lapua 6mm BR cases. They were .o130" to just under .0127". But most averaged .013". Now .013 + .013 = .026". Add .026" to .243" = .269". That leaves .003" for bullet release. This is quite enough if you keep your chamber clean as well as your brass.
Double check your case neck wall thickness and the diameter of your bullets. Carefull measuring now will pay off in the future.
I am not aware of any bullets that are .242" dia. all 6mm bullets I have seen are .243". But then I haven't seen everything.
Any way a .003" difference between chamber neck and loaded cartridge neck is pretty close to minimum. So a .0045" difference is not too great. And if you get .243" bullets the difference goes to .0035". You are a lot closer to "just right" than you think.

muck

Thank you Muck:

The bullet was an old (?) laying around my friend's reloading bench that I used just to have something inside the neck so I could measure the outside dia.

I had no idea what 'just right' would be, so your information is very helpful. I am assuming that the bullets that I eventually use (planning on Speer 70gr TNT for varmints and probably Tubb DTAC for target) will be .243, and a caliper check of the case neck walls shows .0125-.013 (ish, as the tool isn't the correct type) so as you state I should end up at .2685-.269

With the above info, can you advise the correct neck sizing bushing for minimum bullet grip? I'm guessing .267-.268, but have no idea what spring-back will be. I assume .269 would leave me with no bullet grip?

Thank you for your time,

Cole

quote:

Originally posted by muck:
ColeC: I just miked several of my Lapua 6mm BR cases. They were .o130" to just under .0127". But most averaged .013". Now .013 + .013 = .026". Add .026" to .243" = .269". That leaves .003" for bullet release. This is quite enough if you keep your chamber clean as well as your brass.
Double check your case neck wall thickness and the diameter of your bullets. Carefull measuring now will pay off in the future.
I am not aware of any bullets that are .242" dia. all 6mm bullets I have seen are .243". But then I haven't seen everything.
Any way a .003" difference between chamber neck and loaded cartridge neck is pretty close to minimum. So a .0045" difference is not too great. And if you get .243" bullets the difference goes to .0035". You are a lot closer to "just right" than you think.

muck

Are you building a benchrest or competition rifle or a hunting rifle? Bench rest rifles are usually built with a closer fit at the neck.

quote:

Originally posted by SR4759:
Are you building a benchrest or competition rifle or a hunting rifle? Bench rest rifles are usually built with a closer fit at the neck.

Primarily - shoot at a nearby 560 & 1000 yd range. Benchrest style stock, 3" wide forend, etc. Never done it before.

But since I am going with my sons to shoot pd in eastern Colorado this summer, I plan to set it on a bench and take some 500+ yd shots at them. How's that for confusing the issue?

Advice is appreciated.

Cole

Some custom bullets with a pressure ring can measure .2432" to .2438" over the pressure ring.

A loaded round neck diameter of .2675" when using a .243" dia. bullet will need a bushing of about .265" to allow for brass spring back and variation of neck wall thickness since you are not outside neck turning the brass. You could cut you chamber neck on the smaller side ONLY if you are outside neck turning the brass or taking exact measurements of wall thickness of each and every case with a mic made special to measure wall thickness.

quote:

Originally posted by ColeC:

quote:

Originally posted by SR4759:
Are you building a benchrest or competition rifle or a hunting rifle? Bench rest rifles are usually built with a closer fit at the neck.

Primarily - shoot at a nearby 560 & 1000 yd range. Benchrest style stock, 3" wide forend, etc. Never done it before.

But since I am going with my sons to shoot pd in eastern Colorado this summer, I plan to set it on a bench and take some 500+ yd shots at them. How's that for confusing the issue?

Advice is appreciated.

Cole

If you are not going to compete with it I think your current choice is pretty good. I have a couple of rifles that are several benchrest generations older. The original 40X-BR barrel was not a tight neck barrel and it shoots way better than I can without the tight neck. It is a 6X47. I figure that when I have really learned to shoot I can try the 2 tight neck Hart barrels. I am still learning.

Good reading here http://www.6mmbr.com/6mmbr.html I dont have a 6br, shooting a Rem 40x 243win. standard factory chamber, but i do neck turn and use bushing dies.

Thank you all.

Interesting how much you can get out of re-reading things. I had read the 6mmbr.com 6mm section, actually several times, and it was a major factor in my choosing to shoot the 6BR.
This time I better understand the neck sizing part.

I hope you all have a great day,

Cole

You'd probably get more accurate info on benchrest.com or 6mmbr.com than a general reloading forum. I shoot a few 6mm rifles on the bench, one of which is a fairly new 6mmBR Norma. When I was having it built the gunsmith and barrel maker both recommended the looser neck diameter and less neck turning, especially in view of the fact that I like to shoot jam seated.

I went with their advice and was not disappointed. You should also be aware that there are differences in the brass that Lapua makes for this cartridge and the Brass Norma makes. There is a good article on 6mmbr.com in the archives concerning this and other features of the 6mm BR cartridges.
http://www.benchrest.com/
http://www.6mmbr.com/index.html