7BR M700 switchbarrel

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  • Last Post 27 November 2016
mtngun posted this 04 April 2016

Specs: -- $100 Pacnor barrel is probably a factory second due to odd dimensions but otherwise looks good -- 6 grooves -- 0.283” groove x 0.275” bore -- 0.285” x 1 degree per side throat -- 0.310” neck -- 14” twist -- 27” long 

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

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.     http://i487.photobucket.com/albums/rr235/mountainmolds/br71apr316zps7rlkhovg.jpg 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 that 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.      http://i487.photobucket.com/albums/rr235/mountainmolds/br73apr316zps9ooa7fic.jpg The spitzer averaged 1.75” for four ten-shot groups despite significant mirage (the target dot looked like a dancing orange blur  :P ).       5 shots don't prove much but for what they are worth the Love-rider did OK.http://i487.photobucket.com/albums/rr235/mountainmolds/br72apr316zpst3pozk4x.jpg 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.  :X 

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.  :cool: -- otherwise this barrel will eventually perform the same experiments as the other barrels, with a particular emphasis on bullet design.

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mtngun posted this 04 April 2016

One of the things I'm looking for as I test different barrels and different calibers is “will the load techniques that are successful in one barrel/caliber also work well in other barrels/calibers?” or is it “different loads for different guns?" 

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Hamish posted this 05 April 2016

Are you planning on shooting any cast using the conventional wisdom of one to one and a half thou over for comparison?

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mtngun posted this 05 April 2016

TheAmishGolfballShooter wrote: Are you planning on shooting any cast using the conventional wisdom of one to one and a half thou over for comparison?9 times out of 10 best results are with the fattest bullet that chamber reliably -- taking into account the fit in the chamber neck and the fit in the freebore.

But since I more or less choose the diameters of the neck & freebore, the question then becomes “which bullet diameter do I want to design the gun for?"

This barrel has an oddball 0.283” groove.   I made the freebore 0.285” for use with a 0.284” - 0.2845” bullet. 

Likewise the chamber neck diameter was determined by adding the desired neck wall thickness (2 x 0.0125") to the bullet diameter (.2845"), giving us 0.3095” total.    The actual chamber neck diameter is 0.310” leaving us with 0.0005” clearance for sorta reliable chambering.

It's easier said than done to hit all those diameters spot on because reamers don't always cut the same diameter as the reamer.   I try to aim a little on the tight side, then if the need arises they can always be re-cut to a larger diameter.

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Hamish posted this 05 April 2016

Thanks for the clarification on your throating, sounds like a good plan,your results are already very respectable for the speed they're being shot at. Looking forward to continued reports.

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mtngun posted this 08 April 2016

Set back and rechambered.    The barrel is now 26 3/8” long.     The new chamber looks great!  :cool:      Chamber 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 0.0123” or so to work in this chamber.http://i487.photobucket.com/albums/rr235/mountainmolds/7rechamberzpsntddeiix.jpg

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mtngun posted this 02 May 2016

The rechamber job turned out fine.   Today's lucky group: http://i487.photobucket.com/albums/rr235/mountainmolds/m7007br2may116zpsgi388f05.jpg

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mtngun posted this 02 May 2016

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. http://i487.photobucket.com/albums/rr235/mountainmolds/m7007br1may116zpslj03koa4.jpg

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, but in any event, they were hard! :D    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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mtngun posted this 21 August 2016

A quick update on the 7BR.   It will be tough to find a load that beats the 1.4 MOA average in the previous post, so if I were smart I would declare a victory and walk away.  :D   But naturally I want to keep tinkering in search of the perfect combination.  

Trying different powders with the same 100 gr. GC.

https://c7.staticflickr.com/9/8369/28835015910cbe8499c06b.jpg

https://c1.staticflickr.com/9/8545/291017532560fc3061e64b.jpg

Those loads were “best guesses” that haven't necessarily been optimized, so perhaps they could be improved with tweaking of this or that.   For now I am just seeing if it has a preference for any particular powder. 

I like this barrel and I like this cartridge.   Case capacity is about perfect for the light bullets I am using, where as the 22BR and the 6BR have excess capacity, and the 30BR doesn't have enough.

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mtngun posted this 22 August 2016

Continuing testing of H4895 and the 100 gr. GC.    Groups numbered in the order that they were shot.

1 & #2:   the H4895 loads already discussed in the previous post.   They showed some potential so I wanted to see if it was repeatable.

3:   H4895, with the powder charge upped slightly hoping to tune out vertical stringing.   Well, something weird happened -- shots #1-4 formed a decent group, and shots #5-10 formed a tight group, but it was a separate group!    Something shifted.   I suspect the shift had something to do with the rifle or the rest (I adjusted the sand in the front sandbag about the time the shift occurred), not the load, but who knows for sure?

4:  the first 9 shots were in a respectable 1.4", then the 10th shot was a flier low and right.    Frustrating.   

5:  130 gr. bevel base coated with Hi-Tek (photo further down in this post).    J.R. brand reclaimed shot that had been quenched after the final cure at 365F -- I did not measure the BHN but based on past experience it might be 18 BHN.    Quickload says about 51,000 psi for this load!

6 - #8: same as #5 but air-cooled.   I did not measure the BHN but past experience suggests 12 - 14 BHN.   I was expecting the 51,000 psi to destroy these soft bullets, but to my surprise they shot better than the quenched bullets!   The velocity variation was better, too.   

On #7, one cartridge was such a tight fit that I had to pound the bolt closed with my fist (I dared not extract it for fear that it would spill powder).   Its velocity was abnormal at 2843 fps and the POI was out of the group, so I did not count it.   In hindsight that bullet probably missed a sizing step.

https://c2.staticflickr.com/9/8125/29166226305d6cd8493cdb.jpg

Here is the 130 gr. bevel base.   Sorry for the lousy photo.

https://c2.staticflickr.com/9/8506/28547238313deaa7a4653b.jpg

Here is a poor quality borecam photo near the muzzle after shooting the uncoated GC's at 2904 fps.    There is a little grey wash in the corner of the grooves but nothing serious.   This is the worst spot in the barrel, the rest of the barrel was cleaner than this.    https://c4.staticflickr.com/9/8072/285472304838caa9aa9a6z.jpg

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mtngun posted this 28 August 2016

Trying a new plain base bullet and a new-to-me powder coating.

FYI I switched to a different method of measuring the jam point, more http://www.mountainmolds.com/phpBB3/viewtopic.php?f=8&t=564&p=2587#p2586>deets here if you are interested.    I think my new method is more accurate than the old method, though it probably doesn't matter since the jam point is only used as a reference point and optimal COL still must be determined by shooting.    The optimal using the old method was jam-5, but the optimal with the new method is jam+10.   Same COL, only the reference point has changed, if that makes sense.

Hybrid coating seems to be much more forgiving of overcure than polyester coatings, and that's important if you need to heat treat the bullets.    I've been mostly content with the off-white hybrid powder coating that I've been using, except that it's not very attractive.   Hybrids are normally only available in dull colors.   My hope was that this “copper” hybrid coating would provide an attractive copper finish while maintaining the physical characteristics of a hybrid coating.   Well, as you can see it doesn't look coppery at all, instead it looks butt ugly. :D   Oh well, as long as I have the bullets made up I might as well shoot them.

https://c1.staticflickr.com/9/8658/2914109407221fb8ff8fbb.jpg

Here's the results, with targets numbered in the order they were shot.   In general, this barrel did not care for the 115gr. bevel base no matter the coating.   The best it could do was 2 MOA, so I will give up on the 115 gr. BB.

1- 115 gr. BB, hi-tek coating, 31.5 gr. WC844, seated at jam+0 as measured with the rod method.   

2- same as #1 but powder increased to 32.0 gr. and jam+8.

3- same as #2 but jam+15

4- same as #2 but jam+10

5- 115 gr. BB, ugly hybrid PC cured 15 minutes, 32.0 gr WC844, jam+10.   

6- same as #5 except 20 minute cure

7- same as #5 except 25 minute cure

8- same as #5 except 30 minute cure

9- 130 gr. BB, 2 coats hi-tek cured 350F for an unspecified time, air-cooled WW, 32.5 gr. WC844, jam+10.    These were an old batch of bullets from several months ago, and some of the coating was rubbing off when the bullets were sized.   I suspect the coating was not cured completely, causing poor adhesion and poor accuracy.

10 & #11- same as #9 except a different batch of bullets with 4 thin coats hi-tek cured 365F for 20 minutes, and air-cooled J.R. brand reclaimed shot (normally 12 BHN).    Good adhesion.

https://c4.staticflickr.com/9/8513/292137908515e36a0a60ab.jpg

The so-called copper hybrid PC seemed to shoot best if cured 25 - 30 minutes, similar to the almond hybrid PC.   A 30 minute cure time works out well if you need to heat treat after curing (so far my plain base bullets prefer to be air-cooled).

https://c1.staticflickr.com/9/8324/28669809304193b3987fab.jpg

Conclusions, And Things To Try Next Time And Down The Road: -- this barrel does not like the 115 gr. BB. :X

-- This barrel does like the 130 gr. BB, as long as the BHN is right and the coating is good.   Including last week's groups:  1.40", 1.35", 1.80", 1.95", 1.20".    Average-to-date=1.54".    That's pretty decent for a plain base at 2700+fps! :cool:

-- I could tweak the 130 BB load, but IMHO it's already pretty close to optimal.    -- how 'bout a 145 gr. BB?

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mtngun posted this 04 September 2016

Today I tested a new 140 gr. BB bullet, and also experimented with different powders.     

Here is the bevel base family.  https://c7.staticflickr.com/9/8056/29354437246bbf1dc294fz.jpg

Here is the 140 after nose sizing.   In hindsight, I'm concerned that it may have ended up with too many bore riding bands and not enough groove diameter bands, more on that later.   FYI the bore diameter on this barrel is 0.275” and the groove is 0.283".     Alloy was air-cooled J.R. brand reclaimed shot (typically 12 BHN), 3 or 4 coats Hi-Tek cured 365F for 20 minutes, total coating thickness 0.001".   Rooster HVR lube. https://c6.staticflickr.com/9/8807/29438875325ac06d5b20cb.jpg

I used Quickload to screen powders and estimate charges that would reach my self-imposed velocity goal of 2700 fps.   I don't have all of these powders on hand, though.     Note that today's loads will be hitting 53 ksi  - 60 ksi.  WW748 had the lowest peak pressure, which would be a good thing according to the Colonel Harrison school of cast bullets.   https://c4.staticflickr.com/9/8081/288171354033d5aacca71b.jpg

H4895 looked good in Quickload but didn't do well in real life.   Neither did Ol' Reliable WC844.     https://c4.staticflickr.com/9/8181/28817135763b63da7de8ab.jpg

WC845 was decent.    I purchased WC845 with the hope that it was the military equivalent to WW748, but now that I have worked with WC845 a bit I think it is its own powder, and may not have a civilian counterpart.   It seems poorly suited to reduced loads, but begins to shine at full throttle. https://c4.staticflickr.com/9/8294/288171349632f5dac8bd9b.jpg

WW748 seems well suited to the 7BR, but it is difficult to ignite and doesn't seem to like reduced loads.   I used a CCI 450 primer with WW748 and WC845. https://c2.staticflickr.com/9/8189/288171346330cca562c6fb.jpg

I also shot a few rounds with IMR4166  just to get a feel for the burning rate, since it is not in Quickload yet.    Here is the burning rate for today's powders from slowest to fastest: -- WW748 @77.5 fps/gr. -- IMR4166 @80.2 fps/gr. -- H4895 @81.3 fps/gr. -- WC844 @82.2 fps/gr. -- WC845 @82.8 fps/gr  

Conclusions and Observations: -- the 140 does not seem to be as accurate as the 130. -- the 140 likes slower powders like WW748 and WC845. -- in fairness, the nose sizing may not have been optimal on the 140.   The bore riding nose may not be the best choice at 2700+fps.

Things To Try Next Time And Down The Road: -- try the 140 with less nose-sizing. -- keep tweaking the 130.   Try upping the BHN to 14.   Try the slower powders like WW748 and IMR4166.

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mtngun posted this 21 November 2016

As discussed in another thread, I'm preparing to do an alloy shootout, probably with the 7BR since it has been more reliable than my other barrels.   Just for variety I'd like to use a different load than the 100 gr. @ 3000 fps that I used in the lube shootout.    Should I increase the velocity to 3150 fps?   Should I try to find a powder that burns better than WC845?   Should I try a heavier GC bullet?    

Today I sampled different powder charges and a different bullet, trying to get a feel for what is possible.   All loads used BAC lube and oven treated reclaimed shot.   The bullets were nose-sized to match the throat and seated for 0.010” - 0.015” jam.

Groups were shot from left to right:

1) the usual 100 gr. GC

35.5 gr. WC845 CCI #450 3124 fps 93 ES 0.76% velocity standard deviation

2) same as #1 except powder increased to 35.9 gr. WC845

3163 fps 65 ES 0.71% velocity standard deviation

3) a 118 gr. GC

35.0 WC845 CCI #450 2991 fps 75 ES 0.75% velocity standard deviation

4) 118 gr. GC

33.0 LT32 (a drop tube was used to make the powder fit) Rem #7 1/2 3000 fps 32 ES 0.32% velocity standard deviation

5)  100 gr. GC

33.0 LT32 Rem #7 1/2 3154 fps 23 ES 0.27% velocity standard deviation

https://c8.staticflickr.com/6/5777/31145583215df99a32767b.jpg

A close up of the 118 gr. bullet.   It was kind of an accidental design but it seems to shoot well enough.

https://c2.staticflickr.com/6/5816/31145592665f66f4a90e3b.jpg

Conclusions: -- yes, the 100 gr. bullet can be safely pushed to 3150 fps with either WC845 or LT32.    -- LT32 has much better velocity variation. :) -- the 118 gr. bullet hinted at better accuracy, though I didn't shoot it enough to prove anything.

I'm leaning toward choosing the 118 gr. or something similar, pushed to 3000 fps with LT32.   It seemed like it wanted to shoot.   At any rate, I just want to use something different in order to gain more experiences and knowledge.

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Ken Campbell Iowa posted this 21 November 2016

...i had just gotten used to your 2800 fps successful efforts...

and now you show us 3150 fps ..... still better groups than my low-bar plinking results ...

i am embarrassed ...

ken

but also curious about trying this in 7-08 or even 280 rem .... you are getting into 4320....4350....4831 vel/pressures..

my best coyote load in my 280 rem. was 120 gr hornady with 58 + gr. 4831 ... good ignition but was mj ...

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mtngun posted this 21 November 2016

Ken Campbell Iowa wrote: but also curious about trying this in 7-08 or even 280 rem .... you are getting into 4320....4350....4831 vel/pressures.One of the projects on my “to-do” list is to compare 30BR vs. 308 Winchester in identical Kreiger barrels, cut with the same neck & throat reamers so that the only difference is the case capacity.    The barrels are sitting here waiting for me to find time to work on them.  

Though ... this 7BR is casting serious doubts on Colonel Harrison's theory that  powder speed is the key to high velocity cast.   I'm sure the acceleration does play a role, but it appears to be a secondary role. 

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mtngun posted this 26 November 2016

Today I tried different powder charges and a new-to-me bullet, looking for a new and exciting load to use in the upcoming alloy shootout.   I'm inclined to go with this load.   More deets this evening.

https://c7.staticflickr.com/6/5667/304385262540590223ea2z.jpg

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mtngun posted this 27 November 2016

Today I was in search of a new and exciting load for the alloy shootout.   All of today's loads used oven treated J.R. reclaimed shot, BAC lube, and a Remington 7 1/2 primer.

1) 100 gr. GC., 33.0 gr. LT32.    A repeat of the load that shot MOA last week -- but today one flier spoiled it!    ES was 41 fps, or 0.41%.  

2) 115 gr. GC., 33.0 gr. LT32.   ES was 28 fps, or 0.29%.   Despite the excellent ES, this load clearly had vertical dispersion, presumably due to unhappy barrel harmonics.  Sorry, I forgot to take a photo of the bullet, but it's a stretched version of the 100 gr..

3) 115 gr. GC, 33.4 gr. LT32.    This was an attempt to find a happier vibration node, but it didn't seem to help much.   ES 41 fps or 0.44%.

4) 115 gr. GC, 32.3 gr. LT30.   Another attempt to find a happier vibration node, and it did seem to get rid of the vertical, but only ho-hum accuracy.   32 ES or 0.29%.

5) 100 gr. GC, 32.3 gr. LT30.   Prolly merely a lucky group, but I'll take it!    ES 32 fps or 0.36%.

https://c3.staticflickr.com/6/5773/3044151907454bd864513b.jpg

Conclusions: -- the 100 gr. & LT32 combo acted like it wanted to shoot but darned that flier! :P   In fairness, it is to be expected that fliers will happen more often as I walk up the RPM's.   3150 fps in a 14” twist = 162,000 RPM.

-- the 115 gr. clearly had bad barrel harmonics with LT32 at 3000 fps.   There was no room for more powder to try a significantly higher velocity, and I'm just not interested in lower velocities at the moment.    As time and money allow I would like to retest this bullet with more powders.

-- the 100 gr. & LT30 showed potential -- but one group doesn't prove much.  

-- I'm eager to move on to the alloy shootout, so for the time being I'm going to stick with the “ol' reliable” 100 gr. GC and give LT30 a chance to prove its repeatability.

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Ken Campbell Iowa posted this 27 November 2016

.... i will not mention barrel tuners.... because they work ...and that causes the shooter to need to try different tuner settings for each load .... far better to just ignore tuners ....the challenge becomes a differential equation of a higher order ...

....also, since we are undergoing flyer-therapy .... my br obsessive friends tell me that bullets actually travel down the barrel in a .... slip-stick .... manner ....eeeeeeekk !!.... wonder if shorter or longer bullets aggravate random vibrations more ??

oh, :: vertical dispersion is natural....because of barrel droop....if you rest the rifle sideways does it still shoot vertical groups ?? .... difference of barrel droop vs barrel harmonics ... or is that the same thing ?? ....sorry ...

but don't let me discourage you .... ( g ) ...

i regard your achieving 2 moa at 3100 fps as absolutely amazing .... and your efforts a great encouragement to the rest of us to rethink what we think we know about cast bullet shooting ...

did i say thanks ??..... THANKS !!

ken

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mtngun posted this 27 November 2016

A tuner is one possibility and one of these days I'll have to make a tuner.   Different powders,  like WW748 or H335, are another possible way to change the vibration node, but I'm nearly out of those powders and the money tree is not blossoming at the moment. :D

Or I could admit defeat at 3000 fps and walk the 115 gr. velocity back until it finds a sweet spot, and I may do that if the 100 gr @ 3150 throws too many fliers, as it may at 162,000 rpm.   I want my alloy shootout to test the alloys, not test the load!

Harold Vaughn's book explains how vertical dispersion is caused by the barreled action flexing around the recoil lug in response to the bolt thrust.    If you instead clamped the barrel in a railgun, in theory that should eliminate the vertical -- but I've never tried it!    A railgun will still have vibration nodes but it's dispersion should be random, not vertical?

(Vaughn merely explained the physics behind the things that benchresters had already doing.)

Anything you can do to reduce action flex should reduce the vertical dispersion -- stiffer action,  smaller case head (like a PPC), stiffer barrel.   I'm wondering if using the action tang as a recoil lug, as many benchrest actions do, reduces the bending moment compared to a conventional recoil lug?  

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