Cutting down Benjamin 312 barrel
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Cutting down Benjamin 312 barrel
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Topic: Cutting down Benjamin 312 barrel (Read 366 times))
Gliderflyer
Shooter
Posts: 82
yes
Real Name: Randy
Cutting down Benjamin 312 barrel
«
on:
February 22, 2021, 01:59:42 PM »
My 312 has a dent right at the muzzle that has created a very small flat in the bore. I need to cut it back about 1/8". Right now the solder joint at the front is broken so before I get it re-soldered I can lift the barrel up a bit to do the cut. My best idea is to use a Lee case cutter with a plastic tube on the pilot to protect the bore. Any other methods to recommend? Also considered just using a dremel cutoff wheel or fine hack saw and crowning it with the brass bolt and valve grinding compound method but not sure how that works on a brass barrel. This gun has seen some hard use and will never be a showpiece so I have no problem with the slightly shorter barrel.
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Silvana, WA
Beeman R7 .177
Beeman R9 .177
Benjamin 312 Pre 1951
oldair
Sharp Shooter
Posts: 503
Re: Cutting down Benjamin 312 barrel
«
Reply #1 on:
February 22, 2021, 02:41:43 PM »
Could it be better & easier just to make a 1/8" deep re-crown, maybe as a taper? You should be able to find a number of tutorials on the web about re-crowning with abrasive compound.
Don R.
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Gliderflyer
Shooter
Posts: 82
yes
Real Name: Randy
Re: Cutting down Benjamin 312 barrel
«
Reply #2 on:
February 22, 2021, 03:00:14 PM »
That may work. I need to look again tonight but it may even be less than 1/8". After the taper, I could easily clean up the front edge with a smooth file just so the edge isn't too thin and prone to easy denting. Are these barrels brass all the way through or is there a steel lining? I'm wondering about how they respond to the JB bore paste and bore bright treatment.
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Silvana, WA
Beeman R7 .177
Beeman R9 .177
Benjamin 312 Pre 1951
Nvreloader
GTA Senior Contributor
Posts: 4252
Real Name: Don
Re: Cutting down Benjamin 312 barrel
«
Reply #3 on:
February 22, 2021, 03:56:52 PM »
Randy
Find a brass slot headed, round dome type screw just slightly smaller in diameter,
than the outside of the barrel, get some very light grinding compound, HD tooth paste,
car/paint rubbing compound etc.
Use your varispeed drill and start using the most aggressive compound first, then the lightest,
you should be able to put a very good bevel into the bore and still leave the outside edge to provide some protection etc.
Look at this post by Jason, very good info etc, on bbl crowning etc.
https://www.gatewaytoairguns.org/GTA/index.php?topic=130555
HTH's
Don
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Western NV
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Gliderflyer
Shooter
Posts: 82
yes
Real Name: Randy
Re: Cutting down Benjamin 312 barrel
«
Reply #4 on:
February 22, 2021, 04:27:18 PM »
I've got all those things on hand so I'll give it a go starting with the valve grinding compound and working my way down to the Bore paste. That is an excellent thread, I just found it the other day and even ordered the diamond burr bit set recommended there to de-burr the port on my Disco.
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Silvana, WA
Beeman R7 .177
Beeman R9 .177
Benjamin 312 Pre 1951
eeler1
Expert
Posts: 1386
yes
Real Name: Jon
Re: Cutting down Benjamin 312 barrel
«
Reply #5 on:
February 22, 2021, 04:42:48 PM »
That's quite a ways to grind down, although with patience I suppose could be a good solution.
Another way, to expand on whats been mentioned, is using an endmill to bore out the bore, so to speak. The outside diameter of the barrel is about 0.345" and the inside diameter should be close to 0.220", so an endmill somewhere in between could work if you can set it up to be straight and concentric to the bore. Something like this;
https://www.shars.com/products/cutting/end-mills/5-16-se-4-flute-stub-e-m-solid-carbide-end-mill
Would be difficult with a hand-held drill. The trick is to get the end mill perpendicular to the bore to leave you a nice flat surface to polish the newly recessed crown on. Not sure if this is more work than cutting it off or less, considering that you are going to have to re-solder anyway and will have the same problem of getting the crown true to the bore. Or more work than grinding down past your defect. But it's probably how I would do it.
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West Sacramento, CA
oldair
Sharp Shooter
Posts: 503
Re: Cutting down Benjamin 312 barrel
«
Reply #6 on:
February 22, 2021, 04:58:12 PM »
The barrel is all brass, easy to grind / polish.
If you were to counterbore, 1/4" diameter would clear the .22 original no problem. But ideally you'd want a piloted cutter to maintain concentricity to the bore, which makes the tooling complex. Then you'd still like to have some way to remove any burr from the bottom of the counterbore, essentially creating a 'crown' at the end of the rifling.
Don R.
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Gliderflyer
Shooter
Posts: 82
yes
Real Name: Randy
Re: Cutting down Benjamin 312 barrel
«
Reply #7 on:
February 22, 2021, 05:40:48 PM »
Those thoughts are what led me to my Lee case cutter idea. I don't have a milling machine and the case cutters are basically a piloted end mill of sorts (I think so anyway, I don't actually have one yet), I'll take another closer look tonight and decide which way to go.
Thanks for all the input.
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Silvana, WA
Beeman R7 .177
Beeman R9 .177
Benjamin 312 Pre 1951
OTmachine
Sharp Shooter
Posts: 767
it'll shine when it shines
Real Name: Keith
Re: Cutting down Benjamin 312 barrel
«
Reply #8 on:
February 22, 2021, 05:48:46 PM »
You could make a drill guide bushing that would locate on the barrel O.D. Just need a block drilled out on the same set up with a 1/4 “ drill and the barrel sized drill. 11/32 should be just a little oversized, (my 312 measures .340” diameter), but would not hurt since you would be counter-boring to 1/4”.
«
Last Edit: February 22, 2021, 05:59:54 PM by OTmachine
»
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USA, Central WI
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Cutting down Benjamin 312 barrel