7mm BR

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mtngun
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7mm BR

Postby mtngun » Sun Feb 28, 2016 3:56 pm

I'm starting to plan loads for the 7mm BR barrel. :)

Specs:
-- $90 Pacnor barrel is probably a factory second due to odd dimensions but otherwise looks good
-- 0.283" groove x 0.275" bore
-- 0.285" x 1 degree per side throat
-- 0.310" neck
-- 14" twist

Miller Stability Factor for wheelweight spitzers:
-- 100 gr. = 1.94
-- 120 gr. = 1.68
-- 140 gr. = 1.35
-- 150 gr. = 1.20

It looks like 145 grains would be max for my self-imposed limit of 1.25 stability factor. In general I prefer to lighter bullets for paper punching because they consume less lead and require less pressure to reach my velocity goals. But we'll try several different weights and go with whatever shoots best.

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Re: 7mm BR

Postby mtngun » Fri Mar 04, 2016 6:35 pm

I've been slowly making or accumulating components for the 7BR and the other calibers.

The nose die is 1 degree taper x 0.276" bore rider. The tapered section was cut with the throating reamer. Any nose sized in this die will be a glove fit to the throat & leade.
Image

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Re: 7mm BR

Postby mtngun » Sun Apr 03, 2016 4:40 pm

Today's goal was to fireform some cases, check for basic functionality, and get the bugs worked out of my 7BR reloading gear. I used a "best guess" bullet and load similar to what has worked well in the 6BR. As with the 6BR, the barrel was bedded at the foreend with a strip of mousepad to dampen vibrations.

Here's a 100 grain (actually 98 grain when cast of reclaimed shot) spitzer that is basically a scaled up verson of my 60 gr. 6mm spitzer. As per standard procedure it was nose sized to be a glove fit when seated with only the gas check in the case.
Image

After I ran out of spitzers there were still 5 cases needing to be fireformed so I grabbed some 120 grain "Love-riders" left over from a 7-30 Waters experiment. It, too was nose-sized to be a glove fit. Note than only the gas check is groove diameter, all the other bands are bore-riders or bore-engravers. The concept behind the Love-rider experiment was 1) to reduce bullet distortion caused by engraving and 2) to demonstrate how little is required to securely grip the rifling.
Image

The spitzer averaged 1.75" for four ten-shot groups despite significant mirage (the target dot looked like a dancing orange blur :lol: ). 5 shots don't prove much but for what they are worth the Love-rider did OK.
Image

Conclusions and Observations:
-- using the same load techniques as in the 6BR, accuracy was similar to the 6BR.
-- about half the shells suffered sticky extraction. Turns out there is a gouge at the rear of the chamber -- I seem to remember that the reamer "grabbed" and got away from me while cutting this chamber.

Things To Try Next Time and Down The Road:
-- rather than limp along with the sticky extraction I'm going to go ahead and set the barrel back enough to clean up this defective chamber. That may take a while but there's no hurry because I have other barrels to play with. 8-)
-- otherwise this barrel will eventually perform the same experiments as the other barrels, with a particular emphasis on bullet design.

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Re: 7mm BR

Postby mtngun » Fri Apr 08, 2016 1:23 pm

OK, the barrel has been set back and rechambered. It's now 26 3/8" long. The new chamber looks great!! 8-)

Specs are the same as before except the neck may be a tiny bit tighter this time, at a slightly tapered 0.310" x 0.309". The Lapua brass necks run 0.0125" - 0.0130" out of the box, so they may need to be cleaned up to a uniform 0.0124" or so to work in this chamber.
Image

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Re: 7mm BR

Postby mtngun » Sun May 01, 2016 5:54 pm

The rechamber job turned out fine. Today's lucky group:
Image

I was worried that I might have to turn the necks but there was no need. A loaded neck measured 0.3075". After firing it measured 0.308". It's an ideal snug no-turn fit in the 0.309" chamber. :)

Today was just a trial run with the new chamber to check for reliable functioning, so we didn't do any fancy experiments or load development.
Image

The velocity standard deviation was a lousy 1.82%, which seems typical for WC844 with cast bullets in these BR cartridges.

The bullets had been coated with one coat of OBPU+ moly -- which I have since learned is not a good coating -- and then heat treated at 470F. I forgot to record whether the alloy was wheelweight or reclaimed shot. In any event, they were hard! :lol: The 470F temperature seemed to "burn" the OBPU because it rubbed off in the sizing die. Oh well, you don't really need a coating for GC bullets at 2700 fps, anyway.

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Re: 7mm BR

Postby mtngun » Sat Aug 20, 2016 10:42 am

Last time this barrel averaged 1.400" with the 100 gr. GC and WC844. That's about as good as I expect a 14" twist to shoot at 2700+fps, so if I were smart I would declare a victory and leave it alone. :lol: But naturally I want to keep searching for the perfect load, so I'll be trying a few different powders, and a hybrid PC coating.

Also, the Remington's bolt was pretty clunky and the rear of the bolt would drop noticeably when the trigger was pulled. I dunno if that matters but to get rid of the slop I knurled the rear of the bolt, knurling progressively deeper until the metal was raised up enough to make a snug fit in the receiver.

As it came from the factory the bolt measured about 0.6955".
Image

After knurling just enough to feel some light drag as the bolt is closed:
Image

With the knurled bolt, the crosshairs still move just as much as before when the rifle is dry fired on the bags, so apparently the sloppy bolt was not the cause of the moving crosshairs. It may simply be due to the inertia of the firing pin?

At any rate today I used bullets cast of J.R. brand reclaimed shot, tumble-coated with one thin coat of hybrid powder, cured for 30 minutes at 400F, then quenched in water. I did not measure the BHN of this batch but past experience puts them at 25 - 30 BHN.

I made a homemade COL gage for this barrel, similar to the one I made for the 30BR, and used it to adjust the sizing and seating process, though I ended up seating deeper than last time -- more on that later. As always the COL gage is neither 100% accurate nor 100% repeatable, but it's better than nothing. For now I seated right at the measured contact point. As time allows I'll experiment with different seating depths.

I used Quickload to get a feel for which powders would be most likely to succeed in the 7BR. Besides WC844 (H335), 748, 760, 8208, RL10, RL15, N133, and H4895 looked decent in Quickload, in the sense that they hit the desired velocity with a high loading density and moderate pressure.

I started with WW748 ignited by Remington 7 1/2's, but experienced 6 misfires, or at least they seemed to be misfires -- more on that later. The primers had a big indentation so that did not seem to be the problem. The bullets were stuck in the throat and had to be knocked out. Powder spilled into the action when the case was extracted -- much cussing and knashing of teeth ensued. :evil: Repeat this process 6 times over the course of a 10 shot group. :lol: The group was a poor 1.85" but considering all the problems with misfires and spilt powder it's amazing it even shot that well! :?

After shooting the one group with 748, I then "shot" 5 empty primed cases from the same batch of Remington 7 1/2's. They all went bang. Hmmm .... :?:

Then I switched to WW760 but still with the Remington 7 1/2's. Quickload warned that this would be a compressed load, but by using a drop tube it fit with a little room to spare. Accuracy was fair despite some significant mirage making the target dance around. There were no misfires and velocity variation was decent, too, better than with 748 or 844. Even though it didn't match 844's 1.400" average, it acted like it wanted to shoot.
Image

Mirage is pretty bad during the month of August so load testing is confined to early morning and late evening when the mirage subsides. That's as far as I got this morning, I'll try again in the afternoon.

Observations:
-- last time only the GC was seated in the case, my goal today was to seat the GC and half the bottom-most band. But I miscalculated and actually ended up with all of the bottom-most band inside the case. Next time adjust the nose-sizing depth to allow the bullet to seat just a bit further out. WIth the Ardito sizing/seating method, best accuracy is generally with only the GC seated, but I find that for reliable functioning -- getting the bullet seated straight and not falling out of the case when the cartridge is handled -- it's better to arrange for at least half the bottom-most band to be inside the case.

-- in hindsight I suspect the primers did fire with 748, but they simply failed to ignite the powder. The primer alone was enough to jam the bullet in the throat, though, so that's why all the bullets were stuck and powder spilt out. It might be worth trying 748 again with a magnum primer.

-- even though there were no misfires with 760, it may benefit from a magnum primer, too.

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Re: 7mm BR

Postby mtngun » Sat Aug 20, 2016 8:54 pm

Trying a different primer and a different sizing/seating depth with WW760 powder & the 100 gr. GC .....

Since Remington 7 1/2's would not ignite WW748 reliably, it seemed like a good idea to try a magnum primer. Let's see what CCI 450's will do.

Also I nose-sized the bullets 0.020" deeper than before, hoping to seat them out a little further, but ....

.... my COL gage requires a lot of "feel" to measure the contact point and that was complicated by the screw threads being tight. Eventually I chased the threads and that improved the "feel" substantially. Then I re-measured and it turned out that my earlier COL measurements were off quite a bit, so when all the dust settled, the bullet still ended up being seated with all of the bottom-most band inside the case, rather than only half the band as I intended. :? Oh well, eventually I'll get it dialed in.

Groups numbered in the order they were shot:
#1: CCI 450 primer, unintentionally seated with a lot of jam due to the tight threads on the COL gage. Not good.

#2: After I had fixed the COL gage threads and more accurately determined the contact point. Better accuracy than the jam fit, but some velocity was lost.

#3: 0.004" jump, which has worked well in the 30 BR. This load acts like it wants to shoot but what's with the horizontal stringing? That might be due to trigger pull -- yes even a 2 ounce trigger can move the gun if you're not careful.

#4: Increased the powder charge to bring the velocity back up to ~2750 fps. Not much difference in accuracy.
Image

Observations and Conclusions:
-- average for all 6 groups shot today with WW760 was 1.63". Not bad, but nothing special.
-- the magnum primer did not seem to make much difference with WW760.
-- changing the sizing/seating depth can make a big difference.
-- it's tough to measure the contact point accurately. I have some ideas for a better COL gage, but haven't had time to work on that. :(
-- overall I like the 7 BR and think it is the best of the BR family for high velocity cast. The 22 BR and 6 BR have excess case capacity, the 30 BR does not have enough case capacity, but the 7BR's capacity is just right. The 7's recoil with the 100 gr. bullet is noticeably less than a 30BR.

Things To Try Next Time and Down The Road:
-- keep trying different powders.
-- keep tweaking the sizing/seating depth.
-- try a plain base bullet.
-- try different bullet weights, though I suspect heavier bullets will be less accurate (because they require more pressure) based on my experiences with the other barrels.

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Re: 7mm BR

Postby mtngun » Sun Aug 21, 2016 9:23 am

Continuing to test powders with the 100 gr. GC, targets numbered in the order they were shot. FYI "5 jump" is my shorthand for seating the bullet 5 thousandths away from the contact point.

#1: trying RL15. 9 shots in one inch, what's up with that flier? :twisted:

#2: reduced the charge of RL15 hoping to bring the velocity down to ~2750, but instead the velocity increased, probably due to the barrel warming up. 9 shots in 1.2", what's up with the flier? :twisted:

#3: trying H4895. Acts like it wants to shoot other than a little vertical stringing. Velocity was higher than I intended.

#4: reduced the charge of H4895. Even more vertical stringing. Acts like it wants to shoot if I could tune out the vertical problem.

Average for all 4 groups was 1.54".
Image

Observations:
-- RL15 was OK but nothing remarkable.
-- H4895 had less than an inch of horizontal dispersion. I wonder if it would be possible to tune out the vertical?
-- I had intended to do a shootout of coated vs. uncoated but didn't have enough time or enough bullets. Tests in other barrels showed not much difference, and that seems to be true in this barrel, too
-- it may be that this barrel wants more velocity, not less, to tune out the vertical dispersion?
-- you have to give this barrel credit for consistently producing 1.5 MOA groups with a variety of powders. I like the barrel and I like the cartridge. :)

There's more powders I would like to try, and more load tweaks, but I don't have many bullets left (the 100 grainer is from a single cavity mold), so progress may be slow.

dpm6264
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Re: 7mm BR

Postby dpm6264 » Sun Aug 21, 2016 6:18 pm

Dan.... what gives? One cavity mold? If you had a good mold maker on staff you would think you could get a mold
so you can make bullets faster than a one cavity mold!! :lol: Sorry, couldn't resist.

Doug

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Re: 7mm BR

Postby mtngun » Sun Aug 21, 2016 7:16 pm

Gotta find time, Doug. That's the hard part.


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