6mmBR throat and neck

[CWPINST]

I am interested in building a 6mm BR and would like to know opinions about the various throat and neck dimensions. My goal is to have a good shooting rifle to at least 600 yards. Donor action will most likely be a Remington. I would like to be able to shoot 90-107gr. bullets. I would assume a 8 twist in about a #7 LV taper would be about right for the barrel.

Thanks

 

[Bob3700]

CWP,

I have just had some very long discussions with various 6BR and 6BR improved gurus.

Dave Kiff from Pacific recommends .104" freebore. Several champion level shooters like .110" - .112".

Once concept is that if you start with a .104" freebore,shooting 107 SMK type bullets) you shoot into the sweetspot of .110" - .112" which is the ideal freebore for this family of cartridges.

These freebore dimension will keep you out of the donut area with a standard 6 BR.

Hope that helps.

Bob

 

[billmo]

Bob, Is there any method of establishing a COL to bullet tip by using the freebore lengths that you listed? Say for instance a 105 gr. Berger VLD. I am in the process of making a decision on
freebore length. Thanks, Bill

 

[Bob3700]

Bill,

Try as I might, I could never get any reamer grinder to walk me through the math on how they establish the freebore dimension. In the end, they will tell you that they are going by what the big dogs are winning with in their guns.

That said, here is the closest that I have come to doing the math.

Take the length of the neck of your cartridge, let's say it is .250". Now subtract say .020" to stay above the shoulder neck junction. That gives you .230" . You add .010" to that for the space for your trim to length on the brass. Add another .015" for the 45 degree angle in the chamber. Now you have approx. .230 +.010 + .015 = .255" The bearing surface of a 107 SMK is .350". So you need approx. .095" freebore

I am sure that there are those who know more about this, but that is what I have uncovered while studing several diff. 6MM cartridge reamer prints and talking to a couple of reamer grinders.

Most grinders will spec a freebore dimension anywhere from .100" to .120" for shooting the 105 - 107 gr bullets.

HTH

Bob

 

[billmo]

Hey Thanks, Bob. Yea, I read about that freebore and how some guys want it long. A fellow member sent me a drawing of a reamer and according to the info from him, app. .040 of freebore is what is reccomended by that particular reamer manf.
I was wanting to shoot some of the lighter bullets too and therefore wanting to be lean on freebore. I may be trying to have my cake and eat it too. I will let you know what I finally wind up doing. Thanks, Bill

 

[FroggyOne2]

Bill,

From what I have been reading on here doing a lot of forum searches.. I have found, that it seems to be to have the cake and eat it too.. one should think about a .073 freebore for a chamber where you would like to shoot the long pills as well as the little ones.. like the SIE 70gr. YMMV, but this is what I have found from the things that I have read on this site.

Frog

 

[dreever]

I have two 6BR's both with .269 necks. One has .060 free bore the other a .104. I use Berger 108 BT and 105VLD's exclusively. One rifle prefers the 105's the other the 108's. Both have Kreiger .237 bore barrels. Same powder charge,primer, etc. for both. Perhaps the answer lies in the type of Ogive. The 105VLD has a Secant Ogive and works the best in my .060 freebore rifle. The 108BT has a Tangent Ogive and works the best for me in the .104 free bore rifle. Either rifle will shoot both bullets, but when eeking out the last bit of accuracy each has it's prefered bullet.
On any given day one can out shoot the other. The best group I have ever shot at 600 yards is with the .060 free bore rifle and Berger 105VLD's, but this is only by a few hundreths of an inch over the rifle shooting the 108's. Again on any given day one can outshoot the other which I'm sure comes down to "Driver error".
Sure, with the .104 you could get more powder in, but why if you load has an E.S. in the single digits and is capable of shooting under an inch at 600 yards.
One benefit of the .060 is that you could use just about any bullet and get into the lands. Shorter throat's can also possibly give you longer barrel life as you chase the lands.
Jackie Schmidt's test of the 6BR on this site confirms what an .060 free bore and Berger 105's are capable of.
For me the bottom line is that when it comes time to re-barrel the rifle with the .104 free bore I'll probably just use my .060 reamer since I really see no difference in the performance of the two and perhaps then I can stock just one bullet instead of two. Would simplify life a bit when buying bullets.

Danny

 

[Bob3700]

Danny,

The freebore issue indeed is dependant on the bullets you want to use. Light short vs heavy long.

That said, the bullet you use will impact whether you will have issues with donuts at the neck/shoulder junction. Long bullets seated deep into the case will reach the donut area before light bullets.

It will also affect your case capacity. Longer bullets getting down past the neck/shoulder junction impact how much powder you can squeeze into that case.

At 600 yds, your case capacity may not be a factor, but when you head to 1000yds, you need all the boiler room you can get in that small case. So with increased freebore, you get that extra room. Increased freebore,up to a point) also keeps the base of your bullet away from the donut.

Different games require different freebore dimensions.

Bob

 

[Skeeter]

Danny,

I just finished my dasher and fire formed all the brass. Tonight I am testing out my loads and bullet selection.

when I built my gun Dave recommended for my gun to have a tight neck,.269) and a freebore of .075 for the specific weight and style of bullet I wanted to shoot.

I had mine setup for Berger 88 LD's and am testing it with those as well as Combined 95's and 95 VLD's.

The 88's dont come anywhere near the doughnut region but the VLD's do get right to it... although they are boat tails so I dont think it's close enough yet to be an issue.

I'll post back later this evening after I test fire them and let you know which ones proved to be the winner.

Skeet