Overbored / Underboreds and choke size

[ChrisAsh]

With 12 gauge shotguns you can get them with a variety of bore sizes depending on the make.

 

They could be the nominal bore of 18.5mm or underbored to say 18.3mm or overbored to I understand 20.3mm

 

My question is are chokes measured against a % of the actual bore size or a set standard based upon in this case 18.5 mm

 

Second what is the tightest known bore and the biggest based on 12 gauge

[bedwards1966]

Chokes restriction is relative to the bore size.

[Gordon R]

18.3 and 20.3 - using the same source as you.

[Catweazle]

20.3mm ? That's a bit big isn't it ?

[poorpeet]

Isn't it based on the bore size immediately before the choke? Ie like the barrels in a DT11 that are over bored but taper back down to more normal dimensions.

Or have I got it wrong? Again!

[clayman]

Choking is a function of a gun's ability to deliver a certain size pattern at a measured distance. A correctly regulated 1/2 choke from an overbored gun delivers an identical pattern to a 1/2 choke in an underbored barrel, or indeed a smaller gauge. Choking is defined on the pattern plate, not on the gun.

Choke measures relative to bores give only an indication of what might be achieved on a pattern plate.

Just because a choke is, say 20th constriction ( the theoretical for Mod or 1/2), does not mean to say that's the pattern that arrives. It could be greater or smaller depending on a whole variety of factors; e.g.: forcing cone design, wear, choke tube length, wad type, cartridge choice - even the weather!!

Whatever you have or get, the notches, ***'s, choke naming etc will only give you some prior info about what the result could be. Pattern deliveries of up to 2 choke sizes different from the theoretical size are common - and choke delivery is not usually linear either.

BASC ballistic test show a wide variety of pattern plate deliver from a selection of guns all choked 1/2 - and further tests show that the size reduction from Cy to 1/4 is often as much as the total reduction across all further choke sizes - ie, Cy may give ( say) 30" circle, 1/4 a 24", but 1/2 only goes down to 22"; 3/4 to 20" and full to 18". There's an argument to say that if linear reduction mass market chokes that usually increment down on 10thou reduction are used, you have in effect only two settings - no choke (cy) and plenty of choke - all the rest not being much different with a quite large pattern reduction in the first increment of 10th.

The best chokes are regulated to the gun to throw individual patterns each reducing a set amount, but the tubes themselves will not measure down in set amounts. Expensive process. Some of the better after market tubes have non-linear reductions from testing of guns - eg Teague tubes are non-linear in the top end ranges, but even then you gun / cartridge choice etc will still give variations and the only way to get positive choke results is to have a big set of 1/8 choke reductions, and test each one to know the true end result for your gun

Edited December 23, 2012 by clayman

[moor man]

It could be greater or smaller depending on a whole variety of factors; e.g.: forcing cone design, wear, choke tube length, wand type, cartridge choice.

 

Hi Chris, interesting thread,

 

Clayman; what is wand type? and can you explain the benefits of ported choke tubes, do they have a place in game shooting? thanks.

 

Just checked my guns; Escort 18.5, Bettinsoli Diamond 18.3, Maroccini (Spanish O/U) 18.3

 

Can anyone explain backboring to me and what effect it has, thanks.

 

I suppose the bottom line is to pattern test with each choke/cartridge combination.

[clayman]

Back boring is a process which is used to enlarge the bore diameter and correct non-concentricity of a shotgun barrel. This process reduces wall friction between the shot load and barrel thereby reducing recoil and shortening the shot string. Back boring also allows the shot load to be controlled by the choke tube rather than the barrel's bore constriction. This allows the shooter flexibility to switch choke constrictions as needed with confidence that the result will reflect that choice. Back boring will greatly reduce pellet deformation by allowing a more smooth path for the shot load through the shotgun's bore, and this will provide a more dense and even pattern.

Good back boring processes will use a line boring machine with a diamond hone running in an oil bath to produce an absolutely mirror smooth finish with true concentricity from forcing cone to muzzle After back boring a barrel any previous chokes will no longer be safe to use with the shotgun and must be replaced with tubes to match the new larger bore diameter.

 

oh wand is typo - read wad!!

Edited December 23, 2012 by clayman

[moor man]

Thanks Clayman, very comprehensive answer.

Apart from less felt recoil does a backbored barrel feel different to shoot then? Which makers use this feature? Thanks, MM.

[ChAoS]

Second what is the tightest known bore

 

I have not measured this, but Ithaca tell me that my early-eighties M37 DSPS barrel will be true cylinder and under-bored at 0.690" (17.526mm, according to Google).

 

However, it's a *slug* barrel...

 

Regards,

 

Mark.

[Hamster]

Back boring is a process which is used to enlarge the bore diameter and correct non-concentricity of a shotgun barrel. This process reduces wall friction between the shot load and barrel thereby reducing recoil and shortening the shot string. Back boring also allows the shot load to be controlled by the choke tube rather than the barrel's bore constriction. This allows the shooter flexibility to switch choke constrictions as needed with confidence that the result will reflect that choice. Back boring will greatly reduce pellet deformation by allowing a more smooth path for the shot load through the shotgun's bore, and this will provide a more dense and even pattern.

Good back boring processes will use a line boring machine with a diamond hone running in an oil bath to produce an absolutely mirror smooth finish with true concentricity from forcing cone to muzzle After back boring a barrel any previous chokes will no longer be safe to use with the shotgun and must be replaced with tubes to match the new larger bore diameter.

 

oh wand is typo - read wad!!

 

Hi Clayman,

 

You have described the theory behind back boring very well, the only trouble is that much of it has been proved to be nowhere near measurable or in fact true. In theory enlarging the path of our pellets should lead to less stresses and thus deformation and less friction should lead to less recoil but unfortunately once you bring Newton and a bit of physics in it kind of goes a little awry.

 

Recoil is the product of weight of ejecta and it's speed Versus the weight of the gun firing it, this is an absolute in actual recoil terms. Removing metal from the bores reduces the gun's weight and so can in fact only serve to increase recoil. There is I agree no shortage of rhetorical one liners, particularly in the US, of shooters proudly proclaiming that their gun recoils less since they had such work done but this is the placebo effect at it's finest. Luckily the same country is also responsible for long protracted actual tests on guns where their pattern quality were tested before and after barrel work as well as other separate blind tests that tried to prove the reduced recoil theory.

 

It was found that no detectable or attributable difference occurred with regards to pattern density or quality, the reverse in fact happened in some cases. The recoil camp fared no better. There is a good case to be made that this sort of thing helps enormously with marketing but it's values to recoil and pattern superiority are highly dubious and inconclusive.

 

Presently Beretta are making and marketing at least three different bore/cone dimensions on brand new guns ranging from £2k up to well over £7k, all of which the same company describes as throwing exemplary world beating patterns which I'm sure they do in fact as the choke and cartridge chosen will have infinitely more affect on downrange patterns and in shoulder recoil characteristics.

[Catweazle]

If there was a benefit to backboring I suspect we would all be shooting 11 bores, and 19 bores. Then we'd have to backbore them to 10 bores and 18 bores etc. etc.

 

The articles I have read seem to agree ( to a certain extent ) that long forcing cones work well (especially with plastic wads ), and that good patterns come from short shot columns - ie not putting too much shot in a cartridge in relation to the bore. The theory is that long shot columns mean more shot damage and poor patterns. Also, lower velocities and hard shot mean less shot damage.

[clayman]

I was asked the technical question what are the (supposed) benefits.

 

My reply gives the after-market in gun mods view on this, and the reply is a technical discourse that does not incorporate my personal views on mods such as this.

 

My personal view, is that when a manufacturer has developed a gun over decades with hours of ballistic tests and research being placed back into production, that £500 after market mods will be very much hit-and-miss and may well be detrimental to the required performance enhancement.

 

Unless a particular gun model has been pretested by the workshops to prove the modifications enhance the gun, I would not recommend them being undertaken. If a shooter is in the top league, then mods that means 1-2 / 100 more are hit meaning the difference between a place and high gun, then experimenting with after market mods may be worthwhile - but to the majority of shooter's the cost of such mods would be better invested in lessons to improve technique.

 

£500 in a lesson series with a std gun will see most people hit a greater av score as a result than £500 invested in back boring, long choke, stock alterations and the like.

[Cumbrian]

Hi Clayman,

 

You have described the theory behind back boring very well, the only trouble is that much of it has been proved to be nowhere near measurable or in fact true. In theory enlarging the path of our pellets should lead to less stresses and thus deformation and less friction should lead to less recoil but unfortunately once you bring Newton and a bit of physics in it kind of goes a little awry.

 

Recoil is the product of weight of ejecta and it's speed Versus the weight of the gun firing it, this is an absolute in actual recoil terms. Removing metal from the bores reduces the gun's weight and so can in fact only serve to increase recoil. There is I agree no shortage of rhetorical one liners, particularly in the US, of shooters proudly proclaiming that their gun recoils less since they had such work done but this is the placebo effect at it's finest. Luckily the same country is also responsible for long protracted actual tests on guns where their pattern quality were tested before and after barrel work as well as other separate blind tests that tried to prove the reduced recoil theory.

 

It was found that no detectable or attributable difference occurred with regards to pattern density or quality, the reverse in fact happened in some cases. The recoil camp fared no better. There is a good case to be made that this sort of thing helps enormously with marketing but it's values to recoil and pattern superiority are highly dubious and inconclusive.

 

Presently Beretta are making and marketing at least three different bore/cone dimensions on brand new guns ranging from £2k up to well over £7k, all of which the same company describes as throwing exemplary world beating patterns which I'm sure they do in fact as the choke and cartridge chosen will have infinitely more affect on downrange patterns and in shoulder recoil characteristics.

An old thread, I know, but it seemed a good place to start.

 

My question is: would a 20 gauge bored at 16.0 mm or approx. 0.630 thou. be so overbored (or back bored, if you prefer) as to lose something appreciable ( because of gases escaping) when fired with fibre wad cartridges. The standard for a 20 gauge, so far as I can make out, is 0.615 thou., so 0.630 appears to be quite a difference, comparable to a 12 gauge bored at 18.8 when the traditional figure would be 18.4.

 

I ask because I shoot only fibre wads and I'm looking at a 20 gauge with the enlarged bore. It has fixed chokes of 1/4 and 3/4.

 

I hope this isn't another of my questions with no definite answer; if so, my apologies in advance.

Edited July 28, 2016 by Cumbrian

[Junkyard Dog]

With your 20b, you would lose pattern and velocity with standard fibre shells, this would and will alter with varying loads/manufacturers as they use different types of fibre wad, such as Gamebores wool wad and some of the fibre board compared to others, if you can try the gun in question, I would suggest you buy some different loads and see how they perform through it befopre purchase.

[wymberley]

If the gun was so bored at manufacture by a reliable maker, all will probably be well. If, however it was a 'retro fit' then there is no known control over the quality of the work (unless you know something that you've not mentioned) and in which case JD's advice above makes perfect sense. Having said that, my inclination would be to walk away from it.

[Cumbrian]

Two very helpful replies - thank you. I'm sure the 0.630 bore is original and reputable, but it is a bit puzzling, especially in view of what you both say.

[wymberley]

I'll stick my neck out and say that in all probability the gun itself is not your problem - and in the following respect neither JD or myself have helped. You obviously had concerns in your own mind otherwise you would, perhaps, not have posted. That concern will possibly never go away and your confidence in it will never be absolute. Lack of confidence and good shooting is a contradiction in terms - and it's fatal.

[BrowningB525]

An old thread, I know, but it seemed a good place to start.

 

My question is: would a 20 gauge bored at 16.0 mm or approx. 0.630 thou. be so overbored (or back bored, if you prefer) as to lose something appreciable ( because of gases escaping) when fired with fibre wad cartridges. The standard for a 20 gauge, so far as I can make out, is 0.615 thou., so 0.630 appears to be quite a difference, comparable to a 12 gauge bored at 18.8 when the traditional figure would be 18.4.

 

I ask because I shoot only fibre wads and I'm looking at a 20 gauge with the enlarged bore. It has fixed chokes of 1/4 and 3/4.

 

I hope this isn't another of my questions with no definite answer; if so, my apologies in advance.

Back boring can be detrimental to fibre wads as the wads may not seal as well.

 

I've only ever shot a back bored browning so have nothing to compare it to.

[Cumbrian]

I'll stick my neck out and say that in all probability the gun itself is not your problem - and in the following respect neither JD or myself have helped. You obviously had concerns in your own mind otherwise you would, perhaps, not have posted. That concern will possibly never go away and your confidence in it will never be absolute. Lack of confidence and good shooting is a contradiction in terms - and it's fatal.

Good psychological point. I once patterned a fixed choke gun that I had shot perfectly well with and found that its patterns (using several different cartridges but the same type of course in each gun, one after the other) were really very disappointing compared to another that I have with multi-chokes i.e. the difference could not be explained simply in terms of the fixed choke not liking one type of cartridge. It nagged me and I eventually sold the fixed choke.