6BR Freebore Pictures

[Boatschool02]

All,

My Borden/Brux 6mmBR was recently completed. (Pics and load testing feedback in the coming weeks.) As this was my first custom build, I spent WAY, WAY too much time trying to make sense of bullets, barrel twists, and freebore dimensions.

BLUF: There is NO one perfect, middle of the road freebore/twist combo. Optimizing for one class of bullet results in unavoidable freebore or twist trade-offs. IMHO, freebore generalizations should NOT be made based on bullet weight as this ignores the difference between flat backs and boat tails. (If you remove Berger's 87 & 95 VLD from your consideration, the picture clears up considerably... of course, those are precisely the ones I wanted to use for informal paper punching at 300 yards. If building specifically for those bullets again, I'd go even shorter on the freebore.) When I rebarrel, I will take one of two diametrically opposed paths based on actual intended usage instead of the middle of the road "do-it-all" illusion I held previously.

1. 7.75 Twist 0.080-.120" Freebore
- Ideal for 90-105 class. (sans 95 VLD) Option to shoot almost everything except light/short boat tails.
2. 12-14 Twist Zero Freebore
- Ideal for almost everything under 70 grains.

Baseline info:
JGS custom 0.055 freebore, 1-30 degree reamer
Brux 0.236 bore, 8.8 Twist
Hornady comparator case length: 1.554"
Bullets seated to HARD JAM. (Cortina load development method will yield 0.005 to 0.040" MORE bearing surface in the neck than reflected in these photos. Yes, a couple of these would be at or behind the NSJ and I won't be shooting them.)

Big shout out to Berger Bullets for leading the industry in access to information:

http://www.bergerbullets.com/pdf/Quick-Reference-Sheets.pdf

I was limited to 10 photos. Can send pics of the 95 classic, 77 Lapua HP, 69 High BC, and the 70 SMK if anyone is interested.

95 VLD-Hunting (0.006" shorter BTO-BT than 87VLD)



95 SMK



90 Scenar



88 High BC Varmint



80 Flat Back Varmint



87 VMAX



75 VMAX



70 BLITZKING



105 VLD Target (Little parallax on this one. More forward of NSJ than it looks.)



68 Flat Back Target (65 BT & 69 FB are 0.135 and 0.052" BTO-BT shorter respectively.)

 

[500Stroker]

Great pic's, very good info and insight.
Keep up the good work and please keep the rest of us informed on your progress.

 

[GT Accuracy]

Luke and I have spoken at length through out his build. This information he is about to put forth for everyone's education is going to be really fun to read and add to your notes. It had me rethink a lot of the common "best free bore" lengths for the 1-3 bullets that people know they are going to stick with.

 

[alleycat]

If I understand correctly this chamber has .055 freebore?

 

[Boatschool02]

That is correct.

 

[Boatschool02]

Continued:

65 VMAX



105 AMAX



77 Lapua HP



69 High BC FB



70 SMK



95 Classic



87 VLD

 

[pat fulghum]

holy cow.. tooooo many bullets choices....

John Whidden Gunworks did a 30" 6mm Bartlien 7.5 twist (I leave in Seattle heavy air at sea level).
.120 freebore for Berger 105 Hybrids

Got the barrel screwed on... and loaded up some rounds based on previous Bartlien barrel.

29.5 Varget over CCI 450, Berger 105, 2.405 COAL

2825 FPS, ES 13, 6.5 SD



Here are two shot groups. at 100 yards...

This was 2 five round groups.... rounds 4-8 and 11-15 down the barrel. (cleaned after 8 and 2 fouler shots)

I decided that was good enough for me and called it a day...Took the might 6mmbr to Long Range Parma shoot (8/9/1000) 3 days later and won one of the days.

 

[Phil]

Boatschool02,

That 65 amax looks sexy in that .055 freebore, any chance you could do a 55 Nosler B Tip for all to see????


Phil.

 

[Alex Wheeler]

Boatschool02,

That 65 amax looks sexy in that .055 freebore, any chance you could do a 55 Nosler B Tip for all to see????


Phil.

55 nos bt has a super short bearing. You need a zero fb for those. The 60 grain sierra hp will work though.

 

[tonysnoo]

Good morning Boat...

Did you happen to note the OAL of each of those combos? The reason I ask is that we are going to build a BR repeater using AR mag bottom metal. Im looking for 87-108 candidates in the sub 2.29 oal . Thanks and sorry for the side track.

 

[Boatschool02]

Phil,
Unfortunately, the 69 Berger and 70 Blitzking are the shortest (Bast to Ogive minus Boattail) bullets I bought.
You are right to notice just how much bearing surface remains on the 65 VMAX vs the 75. I was pleasantly surprised and will definitely be giving it a try in the coming weeks.
My "calibrated eyeball" measurements (all normal caveats apply) with a Hornady comparator are as follows: (Hornady's technical services department numbers provided in parenthesis for comparison):

65 VMAX BT: 0.067 (0.0675) BTO: 0.416 (0.386) BTO-BT= 0.349" (0.318)
75 VMAX BT: 0.120 (0.130) BTO: 0.511 (0.487) BTO-BT= 0.391" (0.357)

What does all that mean? In theory, the 65 should have about 40 thousands or 1mm (or 1/8th of a 6BR neck) less usable bearing surface, in the neck, than the 75. I feel my pictures validate these findings without going down the black hole of predictive freebore calculations which seem to rile everyone up.

The attached picture attempts to demonstrate just how similar the nose portions of these two bullets really are. The ah-ha realization regarding "usable bearing surface" is what motivated this project and is the #1 contribution I hoped to provide back to this great community of shooters. As a beginner, I was completely overwhelmed. Something just didn't add up. Nothing short of measuring it for myself could have prepared me to understand why a 65 VMAX and a 65 Berger Boattail might not work in the same chamber. The BTO-BT numbers for these bullets are nearly .080 apart, and yet prevailing guidance would have a well meaning beginner ordering not just the same freebore, but one that limited the use of other light boat tail designs.



The best, most drastic example I can offer is a direct comparison of the Berger 87 VLD and the 88 High BC Flat back. Numbers as follows, again with Hornady comparator (differing slightly from Berger's website, but its what I have to work with):

88 BTO: 0.498 BT: 0.000 BTO-BT: 0.498
87 BTO: 0.514 BT: 0.156 BTO-BT: 0.358

These two bullets, weighing only 1 grain apart, will seat +/- 140 thousandths apart in the neck.



My aim for these posts is as follows:

1. Help beginners understand that bullet shapes and weights are not necessarily correlated.
2. Help beginners understand the tradeoffs they're making when selecting a freebore & twist combination.
3. Help beginners fight the urge to build a "Do it All" combination their first time contemplating a custom build. Freebore can be extended but not shortened. Barrels are consumables. 1,500 to 2,500 rounds later, you get to try something different... don't stress out. At reasonable velocities light bullets can be shot in fast twist barrels quite well.
4. Encourage experienced shooters to provide better explanations of what they use and why. EX: "0.104 freebore works great for me," is nearly meaningless without knowing what bullet is being used and what the shooter considers sufficient bearing surface in the neck. If the pros are using 0.135-0.145" on their Dashers, then it seems we're all being a bit conservative with 105 class boat tails and VERY conservative with the longer bearing surfaces pictured above.

Thanks for reading,
Luke

 

[Boatschool02]

Tonysnoo,
All numbers listed below apply only to my JGS 6BR 0.055 freebore admitting usual bullet & meplat OAL variances. I'll try to help with anything I can measure, but want to resist calculated predictions or any form of assurances. I must defer to shooters with more experience regarding what actually works.

As a starting reference, my first ladder of 95VLD loaded 0.020 off hard jam are 2.293-2.298 OAL.
An 87VLD loaded 0.020 off hard jam would be +/- 2.225. (FWIW, Berger cites a 0.067 nose difference between these two bullets which is nearly offset by the different boat tail lengths for usable bearing surface in the neck.)
The Berger 105s have 0.060-0.080 more bearing surface and about 0.015-0.025 more nose.
The Berger 95 Classic loaded 20 off hard jam would be +/- 2.170, but coming off hard jam would put the boat tail/bearing surface intersection at the NSJ.
I didn't buy the Berger 90, but they look better in all respects for your constraints. (See Berger link above.)
The Lapua 90 loaded 20 off hard jam would be +/- 2.250.
The Sierra 95 loaded 20 off hard jam would be +/- 2.285.
I included pictures of both of these bullets, at hard jam, in the first string of photos. (Mentally reduce remaining distance to NSJ by 0.020-0.040 when seated off jam.)
The 87 and 95 VLD would both be above the NSJ in a zero freebore depending how much you wanted to jump the bullet. The Sierra 95 and Lapua 90 **could** work in a shorter freebore, but you'd really be giving up room to play with seating depth.

 

[tonysnoo]

Tonysnoo,
All numbers listed below apply only to my JGS 6BR 0.055 freebore admitting usual bullet & meplat OAL variances. I'll try to help with anything I can measure, but want to resist calculated predictions or any form of assurances. I must defer to shooters with more experience regarding what actually works.

As a starting reference, my first ladder of 95VLD loaded 0.020 off hard jam are 2.293-2.298 OAL.
An 87VLD loaded 0.020 off hard jam would be +/- 2.225. (FWIW, Berger cites a 0.067 nose difference between these two bullets which is nearly offset by the different boat tail lengths for usable bearing surface in the neck.)
The Berger 105s have 0.060-0.080 more bearing surface and about 0.015-0.025 more nose.
The Berger 95 Classic loaded 20 off hard jam would be +/- 2.170, but coming off hard jam would put the boat tail/bearing surface intersection at the NSJ.
I didn't buy the Berger 90, but they look better in all respects for your constraints. (See Berger link above.)
The Lapua 90 loaded 20 off hard jam would be +/- 2.250.
The Sierra 95 loaded 20 off hard jam would be +/- 2.285.
I included pictures of both of these bullets, at hard jam, in the first string of photos. (Mentally reduce remaining distance to NSJ by 0.020-0.040 when seated off jam.)
The 87 and 95 VLD would both be above the NSJ in a zero freebore depending how much you wanted to jump the bullet. The Sierra 95 and Lapua 90 **could** work in a shorter freebore, but you'd really be giving up room to play with seating depth.

Thanks very much... lots of good info....great head start for me!!!

 

[McGee68]

Awesome info! This should be a sticky!

 

[tonysnoo]

Ive been thinking and referring to this thread over the past few days. I had ordered a reamer from ptg a few weeks ago and Dave recommended a 104 freebore for the AR mag loadings. After studying your data, looks like I should have ordered something around a 50 freebore. I guess Ill find out how tolerant some these bullets are to jump, while maintaining accuracy. I have figured out how to rework my grendel mags to accommodate a 2.295 - 2.23 usable OAL, but thats max.