OK, so if I attempt a barrel breach chamfer


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Offline Jeff in Smithville

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OK, so if I attempt a barrel breach chamfer
« on: January 30, 2012, 03:18:09 AM »
This would be on a .177 Crosman Nitro Venom, which is apparently shipped with a flush breach and I think would benefit greatly from a little chamfer for the pellet skirt. Does anyone know how much "meat" there is to work with? I have no plans to get carried away, but I would certainly hate to discover that the true barrel is actually too thin to accept a chamfer. I guess these thoughts of mine also indicate that I am leaning towards not sending it back to Crosman. Any thoughts, experience or dire warnings?
  • Smithville, TX

Offline northern lights

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Re: OK, so if I attempt a barrel breach chamfer
« Reply #1 on: January 30, 2012, 10:39:19 AM »
Lot of metal to work with. It,s very simple to do with drill or demeral Just chamfer it a little bit bigger then the pellet size an about a 1/16-1/8 deep. Just go slow an you will be done in no time. All you are trying do is to keep the pellet skirts from getting damage when you close the breech. You might grain vels also with a better seal. I grained 50 fps on rws 34.

Offline dukemeister

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Re: OK, so if I attempt a barrel breach chamfer
« Reply #2 on: January 30, 2012, 08:03:56 PM »

from Cardew Trigger to Muzzle.
Try to get a rounded shape as shown in the picture. Doesn't have to be deep.
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Offline Zippy

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Re: OK, so if I attempt a barrel breach chamfer
« Reply #3 on: January 30, 2012, 09:39:29 PM »
I'm new to all of this but have learned a lot from everyone here. I'm a toolie and this is what I use...McMaster-Carr do a search on tooling balls. Order the .250 and chuck it up in your drill. I use Clover or lapping compound and finish up with polishing compound you can get both at the same place. Slight deburr with a Knife to make sure no roll into the bore. Clean with goo gone done. Let me know guys if I'm on point on this. Thanks.
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Offline Jeff in Smithville

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Re: OK, so if I attempt a barrel breach chamfer
« Reply #4 on: January 31, 2012, 02:41:53 AM »
Thanks, gentlemen. And a radiused chamfer might bring a slight gain in velocity! Must be something about the movement of air. I won't be doing it this week, but this info helps me focus on the task. 
  • Smithville, TX

Offline john

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Re: OK, so if I attempt a barrel breach chamfer
« Reply #5 on: January 31, 2012, 10:45:10 AM »
...tooling balls...
you are definitely not going to get the radiused breech the Cardews concluded was ideal with a ball.  It'll be radiused alright but in the wrong direction, concave instead of convex. It would require a special tool. So my question has been why do this unless you can do it right?
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Offline Swine

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Re: OK, so if I attempt a barrel breach chamfer
« Reply #6 on: January 31, 2012, 11:03:15 AM »
Would it be a good idea to chamfer it, i guess with the proper bit and dremel or whatever the best option would, then use fine sand paper to kind of take the inner edge off by hand? maybe wrap it around a straw or something to sort of round it off??

Offline SAADE

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Re: OK, so if I attempt a barrel breach chamfer
« Reply #7 on: January 31, 2012, 11:24:48 AM »
Jeff: all you need is an .060 edgebreak x .060 deep. "1/8" is way to deep into the bore.

The edges, leading and ending both need an excellent polish to remove burrs and imperfections and to create a "roundness" as shown in dukemeister's post above.

The Cardews theory was no different than laminar flow characteristics used by engineers who design pneumatic devices, industrial ovens and the like.
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Offline Jeff in Smithville

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Re: OK, so if I attempt a barrel breach chamfer
« Reply #8 on: February 01, 2012, 02:41:22 AM »
Thanks, I am thinking about tools. Blew a few bucks on Dremel possibilities, but I am thinking about "modifying"  a small grinding tool to get the preferred curve. And 0.060?? OK, just looked at that on micrometer. That does seem about right. I notice that the trusty vintage Big Cat has a straight sided chamfer. Looks about like a revolver forcing cone. Numbers on the chart from dukemeister suggest this isn't a terrible outcome. Main purpose of the mod is to have someplace for the skirt to go with out getting a kiss before every shot.
  • Smithville, TX

Offline QVTom

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Re: OK, so if I attempt a barrel breach chamfer
« Reply #9 on: February 01, 2012, 03:18:07 AM »
You can take one of the dremmel grinding stones a modify it to give the radius you want.  They grey rectangular stone included in the dremmel kit is made for the purpose, just use a corner to shape your grinding stone.  Make sure that the diameter of the stone is about 70% of the bore diameter so it only contacts at one spot when grinding the ID.  You will need a steady hand, grind in a circular motion in the direction of the tool rotation to avoid chatter.  To polish after the grind you can use a rubberized point or a brass rod and some clover past.

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Offline bambazonke01

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Re: OK, so if I attempt a barrel breach chamfer
« Reply #10 on: February 01, 2012, 10:04:08 AM »
I haven't a chamfer but I use a small insertion tool to push the pellet into the chamber a specific depth. The tool is a piece of brass rod with the end turned down into a little projecting button.
Its similar to a AAA battery end.
Jim
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Offline Jeff in Smithville

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Re: OK, so if I attempt a barrel breach chamfer
« Reply #11 on: February 02, 2012, 03:12:14 AM »
Thanks, Tom. I don't know about a stone in a kit, but I will probably modify a 952 or 953 grinding stone. I'm thinking a few holes drilled in some sheet aluminum may help in setting some size boundaries for shaping the tool(s). Probably need to connect calipers and stones to see if there is enough stone left after "wear down." If not, the suicide Dremel mission may be attempted on one of several high speed cutters. Unfortunately, I won't get to play with the tools for a while. I'm still immersed in the world's slowest move. Actually will be in rural Smithville full time if I can finish the last of the fix up and paint program at the Houston home of 35 years. Come to think of it, both Crosman/Benjamin rifles were somehow connected to "taking short breaks" from the grunt work of house fixing and perhaps letting paint or mud dry. It is obvious how that has worked out.
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Offline Jeff in Smithville

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Re: OK, so if I attempt a barrel breach chamfer
« Reply #12 on: February 02, 2012, 03:14:36 AM »
Guess I should ask: "clover past"?  Showing my ignorance. Can any one illuminate me?
  • Smithville, TX

Offline QVTom

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Re: OK, so if I attempt a barrel breach chamfer
« Reply #13 on: February 02, 2012, 03:33:47 AM »
Guess I should ask: "clover past"?  Showing my ignorance. Can any one illuminate me?

Clover paste.  Sorry about that, typing too fast.  It is an abrasive lapping compound that comes several grits.   Clover is the name brand.  Try Midway or McMaster-Carr.
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Offline SAADE

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Re: OK, so if I attempt a barrel breach chamfer
« Reply #14 on: February 02, 2012, 10:10:17 AM »
Or try a real auto parts store and ask for valve-lapping compound, it may even be Clover brand. A 2 oz jar will last you a 100 years.
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