Cutting down a barrel

Remington 870 Repair and Gunsmithing.
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cwebb
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Cutting down a barrel

Post by cwebb » Mon May 23, 2016 9:36 pm

I have a 23" RemChoke barrel that I am considering as a candidate to be cut down to 18.5"...I know the short answer is just buy another barrel but bear with me here, this is as much about the exercise of gunsmithing as it is about the final product.

My concern has to do with the barrel's bore profile.

If the barrel bore was designed to taper to a RemChoke at 23" then does cutting it at 18.5" create a random bore cylinder with unknown patterning results?

If so then, could I back bore this barrel to remedy this problem?

I'm guessing that I would need to measure the barrel thickness before attempting to back bore since I am now working wth a barrel that was originally 23" not 18.5".

Thanks in advance for your replies.

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Re: Cutting down a barrel

Post by BurstBarrel » Tue May 24, 2016 3:27 am

No need to backbore. After cutting off the RemChoke (23" down to 18.5") you will be left with a cylinder bore. Given how short the barrel will be I see no reason to thread it for chokes unless you want to. In other words after the chamber, then forcing cone the bore is the same diameter all the way out to the choke threads then into the choke.

The barrels external profile will matter if you do want to re-thread for RemChokes.
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cwebb
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Re: Cutting down a barrel

Post by cwebb » Tue May 24, 2016 4:17 am

BurstBarrel,
Thanks for clarifying what I'll be working with here.
Glad to hear that backboring isn't necessary.
I'll be removing the rifle sights, cutting and re-facing the barrel, lengthening the forcing cone, installing a front sight and possibly porting. Drill press but no lathe so hand turning the forcing cone tool.
Can't wait to get started. Thanks again!

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Re: Cutting down a barrel

Post by Synchronizor » Tue May 24, 2016 5:08 am

Like BurstBarrel said, 870 barrels don't have any weird tapered bores or anything. The space between the forcing cone and the choke is a straight cylinder; you can cut a barrel at any point and have the same size hole in the end. As for chokes, the important dimensions are the bore size and the barrel wall thickness. The bore, as already discussed, will be the standard diameter once it's cut, and the wall thickness at the new muzzle will be the same or greater than it was originally, so you're good there too.

Despite the common misconception, barrel length does not directly affect patterns. "Sawn-off" shotguns got their reputation for very wide patterns from the cylinder bore you get after cutting off the part of the barrel that had the factory choke (this goes back to when chokes were fixed, so you were stuck with cylinder at that point), and from the poor cutting & crowning produced by your typical garage hacksaw job. If a shortened barrel is cut and finished properly, a choke can be just as useful as it would be on a long barrel.

Barrels with screw-in choke tubes are somewhat less durable and need more maintenance than fixed-choke barrels though, and tubes often come loose after prolonged shooting, which is why fixed chokes are still the norm on defensive/combat shotguns. I tend to recommend fixed chokes for folks looking to build a fighting shotgun for these reasons, but if you want to use this barrel for other applications, or you just want that extra degree of freedom for fine-tuning buckshot patterns, interchangeable chokes can be worth the trade-offs.

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cwebb
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Re: Cutting down a barrel

Post by cwebb » Tue May 24, 2016 9:18 pm

Thanks Synchronizer. Great info per usual. I agree, losing a few inches (or more) of barrel length for my intended use will make zero difference. It's all about choke, ammo and training.

That said, I will either need to thread the barrel for removable chokes or backbore the barrel to create an improved cylinder choke.

I'd like to create a fixed, improved cylinder choke by backboring up to .740. If diameter at muzzle is .729 then I should be right in the improved cylinder zone with a .011 constriction.

I do need to confirm barrel thickness beforehand.

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Re: Cutting down a barrel

Post by codyphillips48 » Thu May 26, 2016 4:03 pm

If your thinking about reaming the barrel for rem chokes here is a little info you should read before doing it. This is what I'm going to be doing to the barrels for my 870 if I get ones without rem chokes because I love options.


Pacific Tool and Gauge and Manson Reamers make reamers and taps for modifying your shotgun.

The Rem-Choke reamer and tap will install threads that will accept Rem-Choke and Remington factory tubes. For a 12 gauge, that means a choke with an outside diameter of .814" and 32 TPI.

The Win-Choke reamer will also cut to .814” and with a 32 TPI which allows you to use original equipment tubes from Win-Choke, Mossberg Accu-Choke, Weatherby, Browning Invector. (Note that although the thread pitch is the same, the shape is different between the Remington and the Winchester reamers)

Then there is a Tru-Choke reamer that will cut for .795” diameter choke tubes at 44 TPI and a Tru-Choke Thin Wall reamer that cuts .775” at 44 TPI for thin barrels. The Tru-Choke will accept Tru-Choke choke tubes, while the thin wall will only accept the Tru-Choke Thin Wall Tubes.

The reamers use a pilot bushing to center the reamer in the bore. The pilots come in various sizes to fit your bore.

It is recommended to use a lathe for the best possible results, although the reaming an tapping can be done by hand.

It is also recommended that you use an expanding reamer to remove the choke (the fixed one) prior to reaming the barrel.


Screw in choke installation requirements:

Before machining a barrel to accept screw-in-chokes, you must measure the outside diameter of the barrel to determine if there will be sufficient wall thickness after reaming.

Measure the O.D. of the barrel and then measure the O.D. of the tap. Subtract the O.D of the barrel from the tap and then divide by 2. This will give you the wall thickness after reaming.
For example; .850” barrel O.D. minus .814” tap O.D. equals .036”… Divide that by 2 and you get .018” wall thickness.

It is not recommended to perform this procedure where the wall thickness will result in less than .015”. Also, for a 12GA, you shouldn’t proceed in installing these chokes if the Inside Diameter exceeds .735” (.728” for thin wall chokes) .Exceeding these dimensions WILL cause damage to the chokes and a GREAT POSSIBILITY of barrel BLOWOUT. After installation, make sure the tube does not protrude into the bore. Back-bored or jug-choked barrels are usually not suitable for this installation.
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cwebb
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Re: Cutting down a barrel

Post by cwebb » Thu May 26, 2016 6:20 pm

Thanks for all the great info. Since I already own a 20" RemChoke barrel with Turkey and Improved Cylinder chokes I'm going to be cutting my other barrel down to 18.5" and creating a fixed choke of Improved Cylinder diameter. I'll be using three backbore tools; two Clymers and one Manson to gradually expand the bore diameter. Also, lengthening the forcing cone first. I'll definitely be measuring the barrel the ensure that I will have enough wall thickness to avoid a dangerous situation.

Thanks again!

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Re: Cutting down a barrel

Post by jkingrph » Fri Sep 30, 2016 9:04 pm

I'm late to this dance, but be sure to measure from the front of the chamber before cutting.

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