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Old August 28, 2012, 06:35 PM   #1
TheMacster
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870 and 3 1/2 shells

I just picked up a 870 express super magnum receiver with the trigger group feed tube and slide, basically everything minus the barrel, forend, and buttstock. Now this receiver takes the 3.5 shells but I wanted to put the mossberg 18.5 standoff barrel on it but it only comes with a 3 in chamber. Do I have to get a barrel specifically for the 3.5 shells or can I have a 3 bored to a 3.5. I know I don't need a 3.5 but that is what I want so if anyone could help me out I would appreciate it.
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Old August 28, 2012, 06:56 PM   #2
Dfariswheel
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You can have the shorter chambered barrel re-chambered to accept 3 1/2" shells.
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Old August 28, 2012, 07:02 PM   #3
TheMacster
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how would I go about doing that? Would I have to pay a gunsmith or can I have my machinist buddy do that?
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Old August 28, 2012, 07:12 PM   #4
oneounceload
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That would depend on the wall thickness where you are talking about removing metal. Personally, I would just get the proper barrel, but I like my fingers and face....
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Old August 28, 2012, 07:25 PM   #5
Virginian-in-LA
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It is my understanding that ALL chamber length 870 barrels will fit on the SuperMag receiver, but you will be limited by the chamber length of the barrel you are using.
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Old August 28, 2012, 07:32 PM   #6
TheMacster
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Yea, I am pretty sure it will attach to the receiver but firing and cycling the 3.5 shells is what I would be concerned with.
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Old August 28, 2012, 08:10 PM   #7
shortwave
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What 1oz. said.
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Old August 29, 2012, 03:38 PM   #8
TheMacster
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Well I found reamers that can either extend the chamber, extend the forcing cone, or both. I'm gonna go see the gunsmith tomorrow anyway and see what he would charge vs buying the reamer for 100 plus. I doubt that it will remove enough material though to jeopardize the structural integrity of the barrel. I found some good info on chambers and forcing cones in my research though. Here is the link if anyone is interested.

http://www.brownells.com/userdocs/learn/Inst-100.pdf
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Old August 29, 2012, 07:05 PM   #9
TheKlawMan
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Quote:
I doubt that it will remove enough material though to jeopardize the structural integrity of the barrel.
What other than wishful thinking causes you to think that?
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Old August 29, 2012, 09:07 PM   #10
TheMacster
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because the metal is quite thick and extending the chamber and elongating the forcing cone wont remove much. And if its so dangerous why do they make tools to do it?
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Old August 29, 2012, 09:34 PM   #11
Virginian-in-LA
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Don't forget that 3-1/2" loads operate at significantly higher pressures than 3" and 2-3/4". By your logic they should have quit making hammers centuries ago.
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Old August 29, 2012, 10:40 PM   #12
TheMacster
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I know that the 3.5 have a higher pressure. And what I'm saying is that the reamer specifically says to open a chamber to 3.5 so how does that make my logic not sound?
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Old August 30, 2012, 12:25 AM   #13
TheKlawMan
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It is quite simple. A barrel designed to withstand the pressure of a 3 1/2" shell will have a cerain wall thickness at various points. If you reem out a 3" barrel in order to lenghten its chamber a half an inch, you must consider if the walls of its barrel will be thick enought to withstand barrel pressures.

Not only might a 3-1/2" shell raise barrel pressures above what a 3" barrel can safely take, the wall of the barrel will now be thinner due to it having been reemed and the outer wall of the barrel tapering down from breech to muzzle.

Also consider any thinning of the barrel wall due to lentheninf of the forcing cone.
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Old August 30, 2012, 08:19 AM   #14
Rifleman1776
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Replacement barrels are not real expensive.
I wouldn't screw around trying to do an amatuer job of reaming your present barrel.
The receiver will handle the longer shell just fine.
But, can your shoulder handle the 3 1/2" banger? There is a reason why they are not real popular.
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Old August 30, 2012, 10:37 AM   #15
TheMacster
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My issue is the barrel that I want only comes with a 3 in chamber, and I know the receiver will take the shell, that's why I went with the super magnum with this build, I am building a masterkey. And I know the recoil will be substantial but I will be able to deal with it, I've been shooting since I was 6 and have fired my fair share of weapons.
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Old August 30, 2012, 12:57 PM   #16
TheKlawMan
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The 870 3" barrel is rated for shotshells with a Maximum Average Pressure of 11,500 psi. A 3.5" barrel is rated for a 14,000 psi MAP. By removing material from the barrel walls you can only weaken them.

Just curious whether the 3.5" barrel is wider than the 3" about 5" from the mouth of the breech. The problem is whether the walls of your rechambered barrel will be thinner than a factory 3.5" barrel.

Last edited by TheKlawMan; August 30, 2012 at 01:06 PM.
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Old August 30, 2012, 07:19 PM   #17
Dfariswheel
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I suggest asking the experts....Remington.

They have an excellent customer service area on their web site. Just use it to ask them if this will work. You might also ask them if they'll do the chamber conversion.
If they do, it'll almost certainly be cheaper and better than most anyone else.

http://www.remington.com/
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