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Double Rifles, Single Shots & Combinations >> Building Double Rifles & Gunsmithing

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tinker
.416 member


Reged: 12/03/05
Posts: 4835
Loc: Nevada
Muzzle crowns...
      #37832 - 20/09/05 07:08 AM

I've been working with the Tolley to get a proper load set up for it.
It seems to like one bullet much better than the rest, it'll usually get individual barrel groups that look very promising, but with the odd -flung waay outta the group- shot or two that have been frustrating.
The gun was sold as a rough piece with great potential, the wood was what seemed to be a jungle smith's rush job to get the thing back into some hunt decades ago. I'd bedded the head of the stock before I ever shot it, but have been looking into a more thorough once-over by some seasoned old DR mechanic.
Finding a qualified smith anywhere near me here in California's been a snatch and a half. Thanks to someone here on the site I've recently had the chance to talk with one near this part of the state who's got thirty years of restoration experience under his belt. This guy was kind enough to return my call and chat with me about the Tolley for a short while recently and from that conversation I'd gotten primed to set into a bit of work on my funny old BPE rifle.

I went ahead and took on the task of bedding further around the action. While the wood and metal were getting a bit more intimate with each other and the bedding compound was setting up, I took the barrels out to the machine shop for a bit of a crown job.

My gun's a bottleneck 450, it runs .458 bullets.
I made up a bore guide out of Teflon barstock and a lap out of aluminum barstock. Turned the aluminum to .475 and gave it a 45 degree taper, also gave it a pilot to run in the Teflon guide. I was able to run an exact fit with the aluminum and the guide, I took a soft lead roundball from one of the sixteen bore rifles and swaged it into the chamber, then layed out a center point and ran that in against a live center in the lathe.
Holding the barrels in my hands and setting the pilot into the bore guide at the muzzle, I was able to slowly lap (with valve grinding compound) a very discreet recessed crown for each muzzle. I'd lap for a minute or two, then re-bevel the lap to keep a fresh 45 degree face on it, each barrel took ten or fifteen repititions of lap-then-re-bevel and refreshing of the lappinig compound to get the crowns clean and consistent, perpindicular to the bore axis.
I then took .490" soft lead roundball and swaged the bores from the breech end out to the muzzles to de-burr the crown job.
At this point they look very good, it was impressive to see just how outta whack the Tolley's crowns were.

Haven't shot the gun yet, won't likely get it out till tomorrow or the next day.
I have high hopes and a great sense that the gun will *at least* group better, hopefully this second pass with the bedding work and having refreshed the muzzle crowns will get me closer to the end of fiddling with loads and tuning.



Anyone here ever have the muzzles of thier double rifle re-crowned or do it themselves?
Do you know what process the smith took or care to share what your procedure was?
Did it make a difference?



--Tinker


--------------------
--Self-Appointed Colonel, DRSS--



"It IS a dangerous game, and so named for a reason, and you can't play from the keyboard. " --Some Old Texan...


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500Nitro
.450 member


Reged: 06/01/03
Posts: 7244
Loc: Victoria, Australia
Re: Muzzle crowns... [Re: tinker]
      #37834 - 20/09/05 07:14 AM


Yes, I used to have a double in 500/450 No 1 BPE
that the previous owner had had the crowns re done.

It fixed any problems and I believe made the groups tighter.

Good luck with yours.

500 Nitro


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Raff
.300 member


Reged: 12/01/04
Posts: 182
Loc: Texas
Re: Muzzle crowns... [Re: tinker]
      #37837 - 20/09/05 07:39 AM

Tinker
Interesting post. You must have the patience of an
oyster. I had, recently, a 500 BPE with the muzzzle of
the left barrel damaged, but only on the inside at the
rifling. I got mine fixed by buying a counter bore and
special ordering the pilot made to match the bore. I ended
up with a .070" recess to clean up the damage. It shot well
when finished, but I had to start from scratch on the
loads. I did not try with mine, but I think you would be
able to move the individual barrel groups by adjusting
the crown. I found out that you can, with a counter bore,
change it a lot.
Raff

--------------------
.


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tinker
.416 member


Reged: 12/03/05
Posts: 4835
Loc: Nevada
Re: Muzzle crowns... [Re: Raff]
      #37852 - 20/09/05 11:56 AM

Thanks guys-

2Triggers, you really got my attention there with your comment on moving groups via crowning.
Elaborate please with what happened with your gun's performance on the 'old' loads and what happened on the target once the gun was re crowned, then what you did with your loads and how changing the load changed or moved the groups.
Did the groups just move away from each other?
If so, did they move vertically or horizontally?

I've read so much about how some double rifles end up printing one barrel's groups above or below the other, and that re-regulation would be the only way to fix that problem.
If slightly moving a barrel's crown back can move the group up or down, this could be helpful to guys out there with double rifles in need of such convergence work that'd otherwise be thousands of dollars of work and months if not years of the gun being out of hand and out of service.


--Tinker

--------------------
--Self-Appointed Colonel, DRSS--



"It IS a dangerous game, and so named for a reason, and you can't play from the keyboard. " --Some Old Texan...


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JPK
.375 member


Reged: 31/08/04
Posts: 734
Loc: Chevy Chase, MD
Re: Muzzle crowns... [Re: tinker]
      #37858 - 20/09/05 01:22 PM

FYI, reregulation would probably run about $600 if reblueing isn't required and it ussually isn't.

You can check on Champlin's web site.

JPK


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Otto
.300 member


Reged: 15/09/05
Posts: 111
Loc: Arizona, USA
Re: Muzzle crowns... [Re: tinker]
      #37872 - 20/09/05 09:02 PM

Tinker is correct about using muzzle crown to move bullet impact on individual barrels. Discovered this by examining a high grade Merkel that was factory new with obvious asymetrical muzzle crowns. Then, have used the "trick" to achieve final regulation on my rifles. Interestingly, these crowns don't seem to adversely affect accuracy.

Otto


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tinker
.416 member


Reged: 12/03/05
Posts: 4835
Loc: Nevada
On assymetrical crowns and movement of groups... [Re: Otto]
      #37878 - 20/09/05 11:53 PM

Otto-

Tell me about those assymetrical crowns.
Also note what the grouping was like with the gun that you worked on, what you did with the crowns, and how that effected how the gun printed a target.

The crowns on the Tolley are 'deeper' where the barrels meet, clearly showing up as evidence of the angle at which the barrels are soldered together. The 'faces' of the crowns are perpindicular to the bore axis of each barrel though.
Do you mean to say that you intentionally brought the crowns off perpindicular to the bore axis to achieve your results, or are you referring to depth when you describe your crowns as assymetrical?
Were the groups of your rifle printing as one barrel over the other, then fixed with tuning via muzzle crown depth?

For whatever it's worth, the rilfe muzzles on the NitroExpress banner at the top of the page also show recessed crowns. I think that gun's a Chapuis, I also think those are factory crowns. I know the resolution of the photo is crap and the lighting might be a little funny there, but from here the left barrel looks a schooch deeper than the right.
Go figure.

Thinking about it, I can imagine how slightly less barrel time could effectively lower a rifle's group on paper. The next thought is that if that were achieved with a deeply recessed crown, the wall of the counterbore made during the crowning effort could possibly kick the bullet's tail and screw the accuracy, necessitating a larger diameter counterbore as depth was increased so as to allow the barrel to continue upwards in recoil as the bullet flew straight on clear of the recessed muzzle crown.
There's a bit of movement here to consider.
Hm.


--Tinker



--------------------
--Self-Appointed Colonel, DRSS--



"It IS a dangerous game, and so named for a reason, and you can't play from the keyboard. " --Some Old Texan...


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Otto
.300 member


Reged: 15/09/05
Posts: 111
Loc: Arizona, USA
Re: On assymetrical crowns and movement of groups... [Re: tinker]
      #37939 - 21/09/05 09:40 PM

Tinker, Hope I can adequately describe how I think this asymetrical crown stuff works. This is equally true for single barrelled rifles, BTW.

The gasses pushing the bullet down the bore have both mass and velocity. As the bullet exits the bore, the gasses violently expand. A perfectly symetrical crown allows the gasses to exert equal force on all 360 degrees of the bullet's base. If the crown is deeper, say at the top of the muzzle, the gasses will push the bullet slightly down as they exit, resulting in a lower impact point on the target. Same deal for right or left. You can adjust the POI on a SXS shotgun up or down by milling a slightly different angle at the muzzles.

About groups: My rifles start with the barrels milled perpendicular to the RIB at the muzzles. Then, I use a piloted 45 degree hand cutter to give a slight bevel to each muzzle. This is how the muzzles begin the regulation process. I use a 12 volt hand grinder with a carbide rotary burr to adjust the crowns at the range for fine tuning regulation. I insert a tight fitting bore swab about 6 inches into the bores to keep the grit out of the bores. Now, about the groups...there's no change in the groups during this process.

I now believe the concern over perfect crowns affecting accuracy is from asymetrical wear to the rifling for an inch or so at the muzzle due to cleaning rod wear. this will deteriorate grouping ability for sure. When the bores are perfect out to the crown, the asymetry doesn't affect the accuracy, only the point of impact.

Your rifle will certainly shoot to a different place if you mess with the crowns.

Otto


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500Nitro
.450 member


Reged: 06/01/03
Posts: 7244
Loc: Victoria, Australia
Re: On assymetrical crowns and movement of groups... [Re: Otto]
      #37941 - 21/09/05 10:27 PM



"I now believe the concern over perfect crowns affecting accuracy is from asymetrical
wear to the rifling for an inch or so at the muzzle due to cleaning rod wear."

The above statement is so true and it is amazing how many shooters
do not use cleaning rods correctly.

500 Nitro






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Marrakai
.416 member


Reged: 09/01/03
Posts: 3510
Loc: Darwin, Top End of Australia
Re: On assymetrical crowns and movement of groups... [Re: 500Nitro]
      #37947 - 21/09/05 11:37 PM

Who cleans their double-rifle barrels from the muzzles??

--------------------
Marrakai
When the bull drops, the bullshit stops!
--------------------------------
www.marrakai-adventure.com.au


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tinker
.416 member


Reged: 12/03/05
Posts: 4835
Loc: Nevada
Re: On assymetrical crowns and movement of groups... [Re: Otto]
      #37950 - 22/09/05 12:53 AM

Otto-

Thanks.
That's what I expected to hear. For the sake of this conversation, how much movement have you seen to be possible with this technique?
Have you gotten rifles which print right and left barrel groups, say one on top of the other but not converging anywhere (resembling a figure'8')to print to the same center with this method?

I'm with you on rifling wear and careless and/or improper cleaning rod usage.
This gun's likely over 125 years old and shows it's scuffs and dings inside the bores. It took a bit of work to simply get the rifling lands to terminate on the same plane for each barrel. As I'm not yet at what would be 'the load' for this rifle, it's not an issue of -redoing the load- I'm still at it and hopefully this work will help me get there.
I am though very interested to see what simply having gotten the crowns to appear more like crowns than buggered orifices will do to how the rifle shows up on paper.



--Tinker

--------------------
--Self-Appointed Colonel, DRSS--



"It IS a dangerous game, and so named for a reason, and you can't play from the keyboard. " --Some Old Texan...


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Otto
.300 member


Reged: 15/09/05
Posts: 111
Loc: Arizona, USA
Re: On assymetrical crowns and movement of groups... [Re: tinker]
      #37967 - 22/09/05 07:48 AM

Tinker, Here's how I would proceed with a rifle previously regulated with an unknown load. Use standard loading tricks to bring regulation as close as possible. A chronograph is very handy to stay in the ball park regarding pressures and velocities. When you have a load that groups well but is 6 MOA side to side, up or down, or crossing, the crown can be used to move the groups as necessary. If you have a mill available, you can fix an over zealous grind on the crown by simply milling off a bit of the muzzles to get rid of all crown and start over. Better if you go slowly and shoot a lot. Trial and error still has it's place!

Otto


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