View Full Version : "forcing cone" - ? - questions...

November 24, 1999, 10:32 AM
Please excuse my ignorance on this specific topic. I have been shooting shotguns for many years and as of late have seen several posts that suggest "lengthening the forcing cone" and "having the forcing cone done". What is this all about?

I am going to take a jump and assume this has to do with the internal contour of the barrel (??) with regards to effecting the pattern. Am I on base here? I have also seen reference to this effecting the recoil??

Can you guys educate me a bit and let me know what this modification entails? I currently have an 870 Marine Magnum with a "CYL" (cylinder) barrel.

I would like to narrow the pattern of the 870. Please comment. Thanks in advance.


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[This message has been edited by CMOS (edited November 24, 1999).]

November 24, 1999, 01:45 PM
The most energetic method of lengthening the forcing cone is exemplified in the Vang Comp modification by Hans Vang. Essentially the forcing cone is removed and instead there is a gradual taper from the chamber all the way to where the bore meets the "choke."

It is said to reduce perceived recoil because the compression of the shot charge is more gradual and the pressure spike that comes from rapid shot compression is reduced.

In many cases shot patterning is denser because the pellets are not deformed as much during the transition from chamber to bore.

For more info on the Vang Comp, see http://www.vangcomp.com/ .

Many people on the forum have the Vang Comp on their shotguns and are very pleased with it. I have it on my Mossberg 500 and won't buy anymore shotguns without it.

November 24, 1999, 03:18 PM
I shoot alot of trap and skeet, if you ask alot of guys that shoot this sport they REmove to forceing cone. It lessens recoil alot. they will also port ther barrels and add weight to there stock. shooting 500 or more shells at an ATA match will wear you down. Hope this helps.

November 24, 1999, 03:23 PM
Can someone give me an explanation as to what is the forcing cone?


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November 24, 1999, 10:58 PM

According to Vang Comp's ( www.vangcom.com (http://www.vangcom.com) ) website benelli factory barrels are over-bored and the forcing cone can't be lenghtened. They recommend a replacement barrel from GG&G to be vang comped.


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November 24, 1999, 11:57 PM
The benelli barrels are chrome lined and to thin and cannot be machined that is why a gg@g will supply a new barrel. The forcing cone is the area just infront of the chamber. And back boring it will leave more room for the pellets to exit without deforming.

Dave McC
November 25, 1999, 08:10 AM
The forcing cone is basically a funnel between the chamber and bore. By making the transition gradual, it keeps the pellets rounder and like someone said, the pressure spike is lower, so recoil abates slightly. Patterns are not tighter, but fuller, with less distorted pellets staying in the pattern.

Re backboring, this can drop recoil slightly also, but some smiths and owners have problems with reducing the amount of metal forming the bbl.

Lengthening the cone isn't complicated, Brownell's sells the tool,among others, and you're just reaming out a little from the funnel. But, knowing how much to take is the crux.

Haven't checked lately, but my guess is it'll cost around $50 per bbl.

I like the lengthened cone, but I must admit many shotguns live long happy lives w/o it....

November 25, 1999, 10:47 AM
That's where my confusion is - forcing cone work=tighter pattern??

As I said, my 870MM has a "cylinder" barrel and I'd sure like to get a tighter pattern.


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November 25, 1999, 07:23 PM
I went shooting today and did very very limited comparison of my vang 870 19" barrel agaist my buddies unmodified 870 20" barrel This was only one shot comparison, so take it how you will. I will do extensive compaison in the furture with my benelli when I get it back.

Winchester Low Recoil 9 pellet 00BUCK

Unmodified barrel 6 1/2" pattern @ approx 7 yards.

Vang backbored, had pattern of 3" at same range.

There is also a slight reduction in recoil and according to my brother a significant reduction in muzzle jump, nearly none, he did his own comparison and that is what he told me. Me and others where shooting our 1911's so he tested the muzzle not me, if your wondering where I was.

BTW I shot my 870 with ejector completely gone and it ejected ok, but with very little travel almost falling out of receiver, I know I cant trust this for defence purposes but was suprised how reliable it was in this condition.

November 26, 1999, 12:21 AM
You might want to check out this website:

www.shotgunreport.com (http://www.shotgunreport.com)

check out the Technoid, he has discussed forcing cones and backboring in detail.

Lengthning the forcing cone is usually done to reduce recoil, since the forcing cone is just in front of the chamber I doubt that it will have very much impact on the shot pattern.

Geoff Ross

November 26, 1999, 10:12 AM
K80, From what I have observed backboring has had dramatic effect on buckshot patterns out to 50 yards on short barreled shotguns.

If you would like to see shot coverage go to vangcomp web sight and some purchasers have posted their results. Have seen article in gun rags about vang comp system keeping all the buck from tac loads on man sized target out to 50 yards. If you have short barreled shotgun that will do this unmodified consider yourself lucky.

My patter was cut in half (50%) after back boring because that is only thing I had done besides porting and I know porting barrel or sight work does nothing for pattern.

November 26, 1999, 10:50 AM
OK, but Backboring and Lengthening the forcing cones are two different things. Backboring means increasing the diameter of the bore of the entire barrel. This is often done on competition guns to reduce recoil, but I could see how it might affect the shot pattern. Lengthning the forcing cone means increasing the length of the taper just in front of the chamber. If you do both it will definitely change the pattern because you have altered the entire barrel. Doing just the forcing cones without touching the barrel will not affect the pattern because yuou still have the same choke and barrel diameter.

My $.02

Geoff Ross

November 26, 1999, 10:57 AM
K80 You are right I should have read your remarks more closely, I thought you meant backboring had no effect on pattern, sorry for misunderstanding.

BTW the link you posted is primarily concerned with long barreled trap and bird guns?

Dave McC
November 26, 1999, 05:26 PM
CMOS, the pattern may or may not be smaller with buck after a forcing cone job. What will happen is less distortion of the pellets, so the pattern will PROBABLY be tighter. With smaller shot,the pattern will be the same size more or less, but more pellets WILL be in the pattern. Sorry about the mixup.

One no choke 870 here keeps a 20-24" pattern at 25 yards with 2 3/4", 9 pellet loads MOST of the time.

If I were thee, I'd do it,and maybe, depending on budget, get that bbl set up for tubes. First, the cone,tho.

Bennett Richards
November 26, 1999, 09:36 PM
Anyone know what effect a Vang Comp job has on MV?


November 27, 1999, 02:28 AM
MV, mean velocity?

Bennett Richards
November 27, 1999, 02:34 AM
MV = Muzzle Velocity...sorry


Rosco P. Coltrain
November 27, 1999, 02:15 PM
I am not a fan of back boring because it tends to hurt the acuracy of slugs. But I think the Vang-comp modification, minus the porting, is definitly something to consider.

November 29, 1999, 08:37 AM
Thanks for the info guys. I'll have to digest this for a while and decide what I want to do.

DaveMcC - "set up for tubes"??


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Dave McC
November 29, 1999, 07:49 PM
CMOS, choke tubes. I like the versatility.

'Nother 870 here, my bird gun, has the Colonial tubes and a forcing cone job. I can use the same bbl for woodcock with a Skeet choke,to an extra full that patterns the Remington Duplex 6X4s very,very well. Turkey time...

But, for your purposes, the forcing cone should come first.

November 30, 1999, 09:01 AM
Hmm, what is the approximate cost of having my barrel set up for the choke tubes? I CAN use the same barrel, right?


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Dave McC
November 30, 1999, 03:20 PM
Yes you can, CMOS.Mine was done in the early 90s, and it was about $90 for the job, $40 each for the tubes.I've got a Skeet, Modified,and Extra Full to cover the whole spectrum.

Greg Wolf of Easton, Md,did all the work, and I recommend him. I'll see if I can dig up a phone number if you're interested.

November 30, 1999, 04:13 PM
Sure I'm interested. Can he do the forcing cone too?

Another question, if I do some "patterning" with my 870MM, will the diameter of the pattern be the same for different shot sizes? Let's say #6 shot vs. using a #4 buck?

Thanks again.


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Big Bunny
November 30, 1999, 05:26 PM
I may be cynical - but you are probably better to buy a new gun with chokes and thenm do the work reloading and /or pattening... if you really feel you need screw-in chokes atall.
I think alteration of shotguns is best left to the skilled regulator/gunsmiths, who always specify for their guarentee shot size and loads specificially for a client requirement previously agreed upon.

To do otherwise or even DIY is very hit-or-miss but probably fun if you have the time and money.

Apart from very specialized applications( EG in competition )where ANY mental or other edge is so important, I feel that a lot of the currently trendy back-boring/cone/choke and barrel work is like a lot of expensive and unneccessary fishing lures...designed to catch more fishermen than fish!

***Big Bunny***

Dave McC
December 1, 1999, 06:11 AM
I'll have to look around, CMOS, but I'm sure I can find it. Yes, he can do the cone also.

As to your size of pattern, roughly the same. The folks who go in for the more extremely customized shotguns, like the turkey shoot folks, trashooters,etc, play with all the internal dimesions and pattern heavily. Each shotgun has a favorite load, and one may work well with say, 7 1/2 shot and not so well with 8s. But, this is perilous close to picking nits for our purposes here.4s and 6s should give the same size pattern but chances are one will be more even than the other.Both will work.

BB, true, but he still may end up with a POS, and he likes the weapon he has.Don't you like to see how far one can go with something? Excess is not always wretched!!

December 1, 1999, 07:41 PM

If you can, find a 4'x4' piece of 1/2" ply, mark a black dot in the center, retreat about 25yds, aim at the dot and shoot it (with #8 or #9 bird shot).

With your cylinder barrel, you may cover most or even all of the wood with your shot pattern. Inspect the pattern carefully. There will be some clear patches with few or no shot (holes, suprisingly big too, eh!) and towards the outside edges of the pattern there will be stray shot.

Lengthening the forcing cone is supposed to clean this up and to regularize the pattern. When the crimp at the end of a cartridge opens in the chamber, the shot then travels through the forcing cone, where it gets somewhat compressed as it enters the barrel. By lengthening the forcing cone, you are slowing the compression of the shot. This reduces deformity in the individual pellets and to some extent smooths out the progress of the shot and the wad (which is pushing the shot) down the barrel. As a result of this and less deformities, the shot flies truer, therefore the pattern is better. It is not tighter, but it should not have as many holes in it or as many stray shot.

If you need a tighter pattern, you have to get chokes, its as simple as that.

To throw a little spanner into the works, most modern, plastic shotgun wads rotate somewhat in a smooth barrel (not nearly like rifling, though). Shot emerges from the barrel spinning. Further the individual pellets travel down range at slightly different velocities creating whats called a shot string or trailing shot. Therefore, when a shot fired at a piece of plywood 25 - 30 yds. downrange meets up with the stationary, solid, vertical plane, the pellets will strike it over a period of time. So, is the pattern left on the board, representative of the quality of pattern thrown by your gun with a specific cartridge.

Talking about fishing, these lures are getting very expensive and won't make a bit of difference to the guy casting flies, for bass, in a sport fisherman, off the coast of Florida.

Blue Bunny, keep on posting. I like it.


Blue Jays
December 2, 1999, 12:46 AM
Hi Everyone-

What do you all think is the effect on durability of the shotgun? Is enough material removed when making these modifications that the firearm will have a shortened lifespan? Just curious of your thoughts.


~ Blue Jays ~

December 2, 1999, 04:02 AM
CMOS another factor you need to take into consideration is that your 870 Marine Magnum is hard chromed inside the barrel as well as outside. If you have any work done to the barrel you are going to loose part of if not all the corrosion resistance you paid the extra money for.

Also you need to let whomever you choose to do the work for you know that it is a chrome lined bore as some will not have the special reamers needed to cut through the hard chrome lining.

Another thing is that if you have any work done to the barrel there is a chance of the chrome lining peeling as there isn't a total coverage of chroming after the work.


Dave McC
December 2, 1999, 05:52 AM
Blue Jays, Greg will not do aftermarket backboring, he dislikes taking bbl metal away. He does do forcing cones,so it must not matter there, where the metal's thickest.

I'm not shooting as much as I used to, probably only a thousand rounds or so through the bbl that's been modified the longest. I fully expect the weapons to outlast me by several generations...

December 2, 1999, 01:28 PM
Good comments guys.

Thanks Beano.

Woody, are you sure that inside of the barrel is chrome lined? It sure doesn't look like it. I mean it doesn't look anything like my Benelli barrel. How can I confirm this?


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Dave McC
December 6, 1999, 05:46 AM
Sorry,CMOS, it took a while to find the info,

Greg Wolf 410-820-8811 Days
410-673-7323 Eves

Hope this helps...