6mm BR - Back to the Future?

[Henryrifle]

I recently purchased an older used 6mm BR prone rifle and have just started working with it. It was advertised as having a low round count Krieger 8-twist barrel and borescope inspection bears that out.

The rifle is, subjectively, a beautiful build using a Remington 700 C-prefix action that has been trued, with a bushed bolt, pinned recoil lug and glued-in single shot follower. Interestingly, it is also clip slotted. It is beautifully bedded in a Masterclass prone stock. I also purchased a set of Forster Bench Rest dies with the rifle, the sizing die honed to .266.

I happen to have several hundred of the newer (tipped) 107 SMK projectiles and the rifle came with about 200 of the older non-tipped 107 SMKs. Is this still a viable projectile?

The rifle is setup for prone but I'd like to press it into midrange (600 yard) F-Open duty for club matches. I shoot F-T/R and want to do something different from time to time.

The rifle came with some hand loaded ammo. I don't know its age. It could have been assembled 10 years ago, perhaps more. I could tell it was primed with BR4s by the 'B' stamp on the cup. I carefully shot several 'test' shots followed by several 5-shot groups from a box labeled as loaded with 30 grains of Varget. The seating depth of this ammo varied from .012 off hard jam to .018 off hard jam. Labradar recorded average MV of 2780 FPS with an SD of 15. Group size varied from .4s to .6s. The average of the five 5-shot groups I shot from prone using a Seb Neo rest and Bigfoot rear bag was .56 MOA at 100 yards. The disappointing groups could have been from the inconsistent seating depth or the age of the ammo itself.

Next steps are to engage in normal load work-up to see what there is to see. Other than what I've read, I have no experience with any of the BR cartridges and will begin testing with Varget in the 30 grain range and the SMKs .020 off the lands. Once I find a good and stable SD, will start testing with seating depth.

Any advice or wisdom to share?




Thank you,
Henryrifle

 

[RegionRat]

@Henryrifle you asked ....107 SMKs. Is this still a viable projectile?... Yes, it may not be as popular as the Berger 105 Hybrid, but it is still very popular and works best for some folks.

Sounds like you are in fine shape and know what to do next. The 30.0 Varget is a great 6 BR starting point and if I would suggest anything it is to use up your 107s while you hunt for either more of those or additional new ones to try as well.

The regular seating depth sweep should show you what that rig likes, but if you feel the need to explore charges as well, you won't need to go very far above or below 30.0 with a typical 6 BR.

Have some fun and best wishes with your new rig.

 

[Drewel]

The 107 SMK doesn’t get much love around here. I think they are one of the best bullets. Some people just refuse to use them. Let your barrel tell you what it likes.

 

[Bill Norris]

The 107 SMK doesn’t get much love around here. I think they are one of the best bullets. Some people just refuse to use them. Let your barrel tell you what it likes.

My 6BR 1-8 Tw and 243 AI 1 7.5 really loves this bullet. Both have produces tiny 5 shot groups with Varget & H-4350

 

[Drewel]

They will shoot. It’s a heavy guarded secret don’t tell anybody.

 

[1K6BRA]

Don’t overlook the Berger 108,the numbers aren’t super sexy (BC) but the group sizes they produce at lease for me certainly have been.

 

[JFrank]

The smallest groups I ever shot were with 107 smk, then I ran out of that lot number.

Also you may want to look at 103 VTAC.

 

[mjl650]

I'm also developing a load for a new 6br that @WSMNUT put together for me. Using 107 smk and CCI 400(all I have) primers with Varget and Lapua brass. At the range last week.

 

[Henryrifle]

@mjl650: That is showing some promise. Can't wait to see what 30.1 and 30.2 looks like. Did you select .005 off the lands as your starting point or did you run through a seating depth test?

What is the length of your barrel and are you using virgin brass?

Thank you for posting some fresh data!

Hope to do the same later today.

Henryrifle

 

[JFrank]

You'll probably break out or hit pressure at 30.2 or beyond.

 

[mjl650]

@mjl650: That is showing some promise. Can't wait to see what 30.1 and 30.2 looks like. Did you select .005 off the lands as your starting point or did you run through a seating depth test?

What is the length of your barrel and are you using virgin brass?

Thank you for posting some fresh data!

Hope to do the same later today.

Henryrifle

@SPJ nailed it, at 30.2 Started getting very slight primer cratering so I stoped there. 29.6 and 29.8 look promising as ES &SD are in the single digits(same in the 3 shot tests I did).
.005 ol was a starting point but on my next range trip I plan on trying different depths.
28" 8t HV Brux with virgin Lapua brass.
Also some nice grouping at 28.0 grain Varget. I might go back and explore a little more in that area.

 

[Henryrifle]

Super interesting. I am guessing that you'll pick up 10-20 FPS with once fired brass? I am headed up to the range this aft for first trials with my loads. Going to try at .02 off the lands but may load a few at .005 for comparison.

So glad that you posted up results and thankful to @SPJ for his feedback.

Henryrifle

 

[swd]

Don’t be afraid to push 107’s in the lands quite a bit. They frequently shoot even better there.

 

[Henryrifle]

@SPJ: I am looking at the target you posted and it looks like, assuming the same rifle, you saw the same pattern with the Berger 108s as well, 29.4 - 29.8 close together and 30.0 - 30.2 out of the group.

@swd: I'll put them in the lands if I have to. I just seems like sooner or later if you load into the lands you are going to have one come apart. It doesn't seem to happen often but always seems to happen at the worst time.

Henryrifle

 

[Henryrifle]

I did make it to the range today but I think the only thing I learned was that my rifle doesn't like .02 off the lands. I shot these at 100 in zero wind using the Seb Neo and Bigfoot rear bag. I am seeing the kind of velocity I expected to see from my 29" barrel but was surprised by the poor and erratic SDs. Brass was prepped and seating was grossly normal. There were a handful of cartridges that required a second run through the seater but were only off by .001.

Next steps are to pick a charge and run seating depth tests to see what that can tell me.

The UGLY:



Henryrifle

 

[Henryrifle]

One more trip to the range today to see if there was more to learn by testing seating depth. You'll notice that several of the tests were in the lands which means I am getting desperate as I am not a fan of those kinds of loads. I don't like three-shot groups either, but, also, don't want to waste components. All of the below were shot with 29.9 grains of Varget. Muzzle velocities, especially those in the lands, were faster than I wanted.

What I think I see in the groups below is that somewhere between .008 and .014 off the lands should be a good starting point for more powder charge tests.

Do you see anything I am missing?


Henryrifle

 

[Dusty Stevens]

Id go into the lands. Its where most shoot in the end

 

[FNSafari]

I would try everything you can get your hands on...Berger, Lapua, Norma and Sierra. The #1 and 5 targets seem to show the tightest grouping with one flyer in #1. I would be looking for one hole or a very tight cloverleaf, maybe in the .200" or smaller. Do not overlook custom bullets like the 80 gr 6mm from Bart's, Roy Hunters 103gr or Randy Robinett's 95 or 104 gr 6mm's. The last three may agree to sell you a sample pack of each to try. Contact them.

 

[mjl650]

@Henryrifle here's how I did on seating depth on Sunday. Looks like my barrel likes the 107s jammed. Although .005" OTL didn't do bad. I think the .010 ITL should have been one hole also, probably jinxed myself.
This time I used once fired brass and the speed picked up 20-30 fps.

 

[urbanrifleman]

Very typically I see the same seating depth numbers with the burger 108. They will shoot very small between 20 and 30 jump they also shoot very small between about 4 and 8 in. I think you're seeing the same thing and there are a lot of similarities between the 107 Sierra and the Berger108 as far as seating depth goes. In all actuality the Berger 108 is kind of a copy of the Sierra 107.

I'm not going to say that the into the lands seating depth shoots better but I would say it's one of the good seating depth. I have found that very little neck tension is needed