doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Backboring a 20ga

A while back, someone stated that enlarging a 20g bore over the standard by a certain amount, had a very beneficial effect on the pattern.

I'm contemplating backboring for a few reasons. I'd like to lighten the gun a little in the front to shift the balance back a bit, and lighten it. I'd also be interested in optimizing the pattern.

Can any of you that have experience in overboring 20ga's weigh in?

BTW, who would be best qualified to do the work?

Chuck H!
Are you thinking about backboring that Browning BSS?

Chuck;
You can quite easily calculate how much wt reduction you will get for a given amount of metal removed, I know you are aware of this. As to pattern enhancment I would be skeptical. When one considers all available types & sizes of shot, all usable powder types & charges, all hulls & primers availabe you would likely end up with some 37 million possible combinations to be evaluated. When someone says the backbored a bbl & got immediate dramatic "Improvement" well did they change the degree of choke? did in the process they change the angle of choke etc, etc. There is so much to be considered I would simply not look for any magic to happen because of a few thousandths difference in bore size of a nominal gauge.
I have shot different guns of several gauges with bores standard, oversize & undersize. I have not done enough extensive pattern testing to really speak athoritivly, but from a practical use aspect I really could never tell if a bbl was +.010 or -.010 in either shootability or recoil, in spite of the many claims made to the contrare.
I think it highly likely that many making the claims have likewise not made extensive enough teating to speak athoratitivly either.
Perhaps a given bore size would enhance the "Fit" of a given shot size in the bore, but could just as easily give a less good fit of a size just one number off. Possibilities are endless.

Anecdotally, I will add this. I own a Bruchet built Darne, circa 1997, in 20 that was built with larger than standard 20 bores, per customer order, and choked in IC and IM. I am having a devil of a time trying to find a 7/8ths production load that prints a decent pattern. Now, 1 oz loads are a different story, they all pattern as nice or better than my usual 1 oz 16 gauge load, in my usual 16 gauge gun. I am pretty close to giving up, and just calling the thing my light 16, and shooting 1 oz loads in it the rest of my life, which a poster has suggested I do.
I have no firm answers, just questions.
Best,
Ted

Chuck;

I have a 20ga O/U with .640 bores....nominally closer to a 16ga than a 20ga. It doesn't pattren any better or worse than any other I've shot.

As to backboring to reduce weight, excellent method! I had one of the first of a handful of 32" Beretta 680 barrels to come into the country nearly 15 years ago. At that time I was chasing all the fads in sporting clays and I came upon a set of custom extended and ported choke tubes that Larry Nailon had made for a backbored Beretta trap gun. Those tubes, and the pig-on-a-shovel handling of my new 32" barrels were all the reason I needed to have them backbored from .725 to .738. I had ProPort do it, the same outfit that was "pigeon porting" all my clays guns. The result was about 5 ounces of weight removed over the entire length of the barrels. I rebalanced by hollowing the butt and had a super sleek 32" O/U comp gun, way ahead of the crowd! Not only that, but it hammered targets with miraculous authority; must be magic in those big bores?

One day I was plinking at skeet with some serious skeeters, including a former world champion, and the smoke clouds I got when I connected led them to ask what chokes I had in. I told them they were cylinders and they didn't believe me. So I removed a tube and showed them the .738/Cyl etched on the tube. Everybody was suddenly a believer in backboring.

Some time later, I actually miked the bores and found that ProPort had left about 1/2" of the original .725 bore at the end, just behind the choke seat. That was their insurance against someone installing a factory choke and blowing it out. Trouble is, it added about .013 fixed choke so when I screwed in the Cyl tube I was shooting LMod!

I contacted them about having it removed and they said they didn't backbore anymore, but they recommended Tom Wilkinson in NC....

Tom Wilkinson
Oxford, NC
(919) 603-0167
twrayw@earthlink.net

I thought it would be a fairly trivial (inexpensive?) thing for him to backbore 1/2" of metal, but he actually backbored the entire lenghth, taking the .738 bores to .741 and it cost me about $400 as I recall.

I still think it was feasible and worthwhile at the time...a $750 set of barrels could become a lightweight set for $1150. Years later I paid $1200 for a virtually identical set of Beretta "Optima" barrels. I'd also considering backboring or jugging to restore choke to an open SxS, but I wouldn't backbore soley for any mythical ballistic efficiency.

Awolf,
Yeah, the 20g BSS. Just tossing it around. Nothing set in concrete. I cancelled my $4k fully loaded RBL earlier and elected to pick up this additional 20g BSS in strght stk 26"bbls (I have a PG 28"/20g already). It's right at 6lbs 8oz and handles nice, but is a bit forward balanced. BSS guns have fairly thick barrels or at least all mine have. So, it was thought to lighten them and I also recall a thread a while back where someone had done some overbore comparisions on the 20g and found good results. I have found the BSS guns to be of excellent machining and well above average for hardness and wear of the components. So, I'm giving some thought to a customized 20g quail gun to supplant the RBL.

Miller,
You're probably right about any conclusive evidence on pattern improvement across the board, but even if it was an improvement with only a handfull of my commonly used loads, I may consider it.

Oh Mike, oh Mike, sit down on my knee. Let me hold you. Those barrel borers lied to you about what they did to your barrels. Let me dry your eyes. Oh, you paid hundreds of dollars to fix the barrels that they fouled up. Oh, did you spend a few thousand dollars in entry fees for shoots where you missed a bunch of birds with those tight bored barrels? Oh, I'm so sorry. The long and the short of it is that backbored barrels may or may not improve patterns but they will 100% sure allow a separated case to lodge in the barrel and allow a new shell to be inserted behind the obstruction and blow up your gun. A backbored repeating shotgun or Darne is a smooth path to a blown up gun for an inattentive shooter. End of story.

Geez Bill, you know how to hurt a guy.

I just ran the nos. for taking .010 out of the bore for 23" of length (I'm not planning to overbore the chambers)and it comes out to just over 2 oz. Not exactly a cheap way to remove weight.

Well, gee, Eightbore....thanks for the sympathy, I guess.

I sure didn't mean to sound like I was wallowing in self-pity; I was just sharing a chuckle at my (considerable) expense. I guess I won't share an accounting of all the other mods I wasted $$$$ on.

FWIW, I don't think overbored barrels are any more prone to blow-ups than any other barrels...."inattentive shooters" are the culprits in every case I know where a cause can be identified. A nice set of unaltered Perazzi barrels opened up at my club just last week.

Just this week a good friend had a blooper, knew he had a stuck wad, but couldn't open the bolt. He had to disassemble the gun to get the fired case out. After dicking around for 5 minutes, he removed the empty and reassembled it and was about to load it when he remembered what started the whole mess....there was stil a wad in the barrel.

Hi Chuck,
First of all, what does/did "had a very beneficial effect on the pattern" mean? Was it tighter? That would be expected, since the bore is now larger in relation to the choke. Was it more evenly dispersed?
As usual, I'm with 2-piper on the idea. IMO, the only thing it will do for sure, is make the barrels lighter, separate you from some money(you've got plenty of that, though) and increase the choke, assuming you don't also work on the choke I.D.
I think this is where I came in. Isn't this how we met?
I'm sure Mikey O. could do the work at a reasonable fee and quick turnaround. Let us know how it turns out. That gun will also need a second trigger, too. And a manual safety.
JL

If the intent is anything but tightening up fixed chokes, I'd take backboring off the options list.

A few 00 buckshot behind the buttplate of that BSS ought to rebalance it just fine!

I have a 20ga. 28" Miroku Daly Superior grade that ended up with a backbored set of bbls to remove the tool marks made by a previous owners attempt at 'something' which was never really all that clear to me. I guess he felt the bores were too tight and ran something like a drill bit on an extension down them toward the chokes to relieve them! The forcing cones are a bit longer now than they were originally. I simply had the bores cleaned up & polished. Chokes were fortunately not removed and I had them cleaned to up to be roughly .020 & .025 as I recall. I have not measured them for this reply. FWIW, that gun is still a bit muzzle heavy. The bores are not the same dia., each only got what was necessary to remove the tool marks & clean them up. The work was not done w/any intention of improving things, rather it was a salvage op on a very inexpensive acquisition.

As far as magic goes, there isn't any. You could expect a bit lower pressure [& velocity] from the bigger bores, all things being equal. Is it detectable? Probably, with gauges, but not by me shooting it. The gun throws good tight patterns with several dif loads using plastic wads. It did not/does not obturate consistently with card, felt, or Lujtic Mono-wads .. just got curious and I tried all three at the time.

Anymore, when asked a question as yours poses, I'm more inclined to say that the same effort/money spent on lessons with a well qualified coach, and shooting frequently, provides much better returns and results that are much more easily measurable.

The bbl. work mentioned above was either done by Kirk Merrington when he first came over & was working for Billy Hodge or by Scott Mayes who is currently the resident gunsmith for Beretta Gallery-Dallas, but it was in the 70's & to be honest, I just don't recall which of the two did it or if I had one do the bbls & the other the chokes. I would not hesitate to check w/Mike Orlen for that type of work today.

Hi Jim, Longtime no hear from.

Money? me? You have me confused with some rich guy. Weren't you the guy with the pricey 28g Super?

Deactivating the A-safety is a cinch with a BSS. My recollection was that you simply flip the rod that connects the toplever to the safety. There's a collar on the rod that catches the safety and pushes it back. If the rod is flipped, the collar is no longer in a position to catch the safety. You get to keep all the parts in the gun for future reactivation if someone wants to have it.

As for the double triggers, I'll probably leave this one as a single trigger. Most second shots where I hunt quail require the shot so quickly that I've found I'm not quite fast/smooth enough with a DT gun, but a ST allows me to get a well placed second shot. Don't get me wrong, I still love to hunt my DT guns there, but usually don't get a well placed second shot with them. In fact, that gussied up .410 Nitro is a DT gun and it's rapidly becoming one of my favorite guns for quail.

I'm kinda looking at the backboring as an alternative to re-striking the barrels to lighten them. I dunno, just tossing it around.

TW, I hate to verbally confront someone who has a similar opinion to mine about the worthless money pit that barrel work is, but how in heck do you determine that a certain wad does not "obturate" in a certain barrel?? Don't tell me that you chronographed loads with that load and another load with a different wad and they differed by thirty five feet per second at the muzzle. I would like a real answer.

Well dern fellows, I have spent a bunch of money on tooling for barrel work. Has it paid off? I think it has been barely worth while. My findings have been as follows

On cheap guns= Big difference!

On mid range priced guns= yes worth the cost, some times, maybe?

On high end guns= keep your money in your pocket, they know what makes the barrel work!

If the bores are chromed lined= not with my tools!

Ahh, I think I now see why Lowell stirs the pot. Controversy seems to raise many considerations to the surface.

As for velocity loss from overboring, I would think it'd be pretty much a moot point unless the wad doesn't seal as well. There seems to have been much written about the plastic wad bases doing pretty well at sealing in larger bores. If they do indeed seal as well in a slightly larger bore, the results could very well be an increase in velocity due to larger piston area. My calc says about a 2% increase in area for .010 in a 20g. Might increase the accel for the same pressure, but pressure may drop a bit. So, I'm betting on all of it being in the scatter and essentially no change in velocity.

Why not sell the gun, use the money to buy a similar one in 16ga and shoot 7/8oz loads in it. Of course you can't use a 16ga for 20ga registered skeet events(okay for 12ga events) but you said the gun was for quail anyway so you would be dollars ahead and shooting a proper quail gun to boot! My favorite quail gun is a 5lb 9oz 16ga 2 1/2" which loves RST 7/8oz shells. Apparently your gun is already flawed by shortage of triggers.

Jerry,
I couldn't afford both triggers, so I just got one. More evidence to the contrary of Jim's recollection of my financial well being. I'm on a 20g kick for the last few years. Apparently, so are a large percentage of the H&H customers, me not being one...a H&H customer that is. I watched the H&H dvd lastnite and they said something like 50% of their production is 20g now. One of the reasons I prefer the 20g to the 16g is the availability of inexpensive good ammo. Another is the lighter payloads of the 20g in typical factory loadings available at most discount sporting goods stores. Besides, 16g seems like overkill for a lil 'ol quail and some of my friends even accused me of using a "big bore" on them when I pulled out a 20g. Your 16 sounds like a great chukar gun though. Although, some of the places I've been chukar hunting I've wondered if it was wiser to take a gun that was more "disposable" than most of my guns.

I posted a link this test http://www.uplandjournal.com/cgi-bin/iko...ler+and+pattern in the past. It is a fairly elaborate pattern test experiment that he undertook. STS and Nitromag were used 2-3/4" 7/8, 1 oz, and 3" 1-1/4 oz. An Ithaca 37 barrel was used because it has lots of metal to accommodate being opened to 0.630" in 0.005" steps starting at 0.615". 2" forcing cone set at start and amount of constriction was kept to 0.025" (20 g full?). The patterns seem sub par at 0.615" barely IC, and seemed to arrive at full choke performance at 0.625" for the 7/8 and 1 oz loads. At 0.630" it patterned full with the 3" Nitro-Mags in #6 and #4 shot.

He said the wad was hard to push through 0.615" and the wad was damaged by the effort, which seems abnormal; those shells in that barrel might have given pressure spikes. Other shells and wads may have worked better at the 0.615" original diameter.

Says the patterns became more evenly spaced, denser, and less patchy as the bore was opened. Larger shot sizes patterned best at the largest overbore tested, 0.630". The results seem to change consistently as the bore was opened.

I only know what he posted. Some one may have an 0.610 20g that patterns great.

Many thanks. I'll give Brian a shout. He's somewhere around my area. Brian's work is certainly better than theoretical debate. If it's not a lot of money, and wallthickness permits, I might pursue it. I'm hoping Brian can steer me to the guy that did the work locally.

Chuck:
Didn't know you picked up a replacement for RBL. I am with Jerry - look at the 16 ga Charles Lancaster I e-mailed you about, it weighs 6.0 lbs - as it should. The same gunmaker has a 20 ga Henry Monk at 5 lbs 5 oz if you are looking to lose some real weight.

Mike

I find it rather odd that the experimenter got 30% patterns out of a full choke barrel and then got near 90% patterns out of the same barrel after it was opened up ten thousandths, same choke constriction, and never thought that was off the scale strange. In the science business, that would be considered a throwaway piece of research until it could be duplicated.

Don't you have a 20ga XT, Chuck? You could pattern it...

C'mon Bill, if you're famiiiar with scientific studies, you know as well as many here that nearly none of the gun testing of the past meets the bar. It is what it is, a data point. Throw away data? I think not. Absent any actual data to the contrary, it's at the very least, an indicator. What I see is a polarized group about the benefits of barrel work; those that say it's worthless and those that say it works.

Sometimes I get the impression some believe that a piece of waterpipe would do as well. There are those that strongly believe that the subtleties of the load make huge differences yet the same group may not believe barrel subtleties can make similar differences. Frankly, I don't quite understand all the skepticism with regard to the barrel work. OK, maybe the recoil reduction thing is a highly disputable issue, but the effect on patterns from physical internal finish, contour or diameter changes would seem to be ... I dunno, less contriversial?? So ... all internal bores are equal in performance no matter the maker?

Well, Chuck, how about I've been counting patterns for about 50 years or better and I've never seen a 30% pattern out of a full choke barrel but I've seen people that couldn't hit the pattern board or count or do simple math. I'd much prefer to vote for one of the two later possibilities after 50 years of empirical experience shooting a shotgun. I think I could throw a 30% pattern with my hands, no shotgun. OK, that's an exaggeration, but not by much.

Dare I mention Becker barrels????
Best,
John

Bill, c'mon now. Countin holes don't take a degree in physics nor do I think you believe it requires decades to learn. I've read Brian's input here for years and I think he can count. And I have faith that Brian can hit a stationary board as I have faith that you can also. I don't see a reason to discredit Brian's intelligence or abilities here. However, I do believe there are things that weren't quantified in posting of his experiment, like the internal contour of the choke at the begining along with surface finish, etc, etc, etc. But, Brian did what he did and it's data. The tests indicate the changes he made to the bore caused improvement from what he started with. I think that shows at the very least that internal barrel differences matter.

Eightbore,
Let me get this straight-my Darne is going to remove the fired cartridge, with it's now separated base, lay it in my hand, just like it always does, and I am going to pocket the thing, just like I always do, and not notice the 'coupla inches of plastic missing from the base?

Do you know how Darne ejectors (not a good word, for this application, really) work? Are you sure? Because it sure doesn't sound like you thought about that statement very hard.

I save my empties, but not to reload. I don't leave clutter in the field, regardless of gun type. The English CIP cartridges I've used for several seasons (say, about 20) don't seem to have a huge problem with separating bases, bloopers, or what-have-you, in my humble experience. Can't say I've spent money on promotional loads, once fired anything, or old ammunition, myself.

I'll be sure to keep an eye out for it. In the meantime, perhaps you might consider better quality ammunition for use in your stuff? Just a thought. You know, a very highly regarded English gunsmith here in the states considers the American penchant for reloading, and the home reloading press the key to his survival and prosperity, by supplying a never ending stream of Fubar-ed barrels on double guns for him to repair. Nothing like putting the money where one's mouth is. Good for him, eh?
Best,
Ted

No dog in this fight but I'll tell you this: the average .720" dia. 12 ga. target wad will "obturate", swell, seal, or kick up its skirt even in a nominal 10 ga. bore--.779" Ray Barnes "Moneymaker" on a m12 in this case--to produce what I consider, in my fuzzyheaded, subjective way, to be pretty even distributions of shot in the patterns. The gun has killed a couple of pheasant at 35 yds. with 1 oz. so I'd guess the shot was going fast enuf to at least dispatch a mash-fed, designated victim. The .023" they need to expand in the bore of a Silver Snipe or an old Super is no big deal, in fact it's a pretty sticky passage if the effort to push a stuck one out of an underbored gun is any indication. I hadn't noticed that they pattern with more consistent distribution than do the "miracle" overbores. There must be wads designed for the current commonality of overbored 12 ga. barrels. I'm looking at an old Remington Power Piston Post and a CB-1100-12. Big difference in the depth of the skirts and the CB has four "incipient" serrations around the circumference. Plastic overpowder cups are a lot better pump washers than were fiber wads. Seems to me the older style of steep cone means that fiber got pushed from both sides to achieve the proper amount of lateral expansion. Why would anyone expect fiber to expand adequately in a gun with virtually no forcing cone. And John can dare to mention Becker but only Buckingham gets any respect. Dare I mention those .741" Lefevers?

jack

Sorry for the delayed response time to your question, Bill. Its the day job, ya know ;-)

On the obturation remark, it had to do with that old Nemesis, AKA 'Blooper', that hollow fonky sound that tells you the wad took a nap rather than fulfilling its intended purpose .. figuratively speaking, of course. In fact, I attributed the something less than a bang to the bbl's modified configuration, a combination of longer cones and bigger bore. Never chrono'd any of them. Could have I guess, but didn't feel the need or see the point. Would suspect their velocities would have been a lot more than 35 fps off, tho that's obviously speculative on my part. Hope this helps clarify my earlier remark about card, felt & mono wad obturation experiences, with that particular gun. No other 20 that I have had or have does the fonky sound ritual with any of those older wads.

John, I don't have a Becker 20, but wouldn't mind a remedy for that disparity, at the right price ;-)

Chuck,

I have a Fox 16 that I chamber sleeved down to a 20 bore. The chamber were ruined and I wanted to save the barrels without major expenses. As I had several 16 with 30" barrels I decided toi try to make a set of 20/16 barrels. It is choked full and full and gives full and full choked patterns with 20 shells. My chamber sleeves are longer than the chamber area as I turned them on my lathe to enter the bores by 1 3/4". They have a slow taper outward to the 16 bore. I know that I could have made them longer to perhaps get a better seal and burn out of the 20 shells but the short chamber sleeved replacements work well. Crony reading are almost exactly the same as my 20 Fox with 28" barrels. Maybe 30-50 fps difference.

I had to drill out both chambers because they were so pitted that it was not safe anymore. Storing two paper hulls for 50+ years, with high humidity, caused the shells to crumble and they in turn caused the chambers to pit. I ended up taking almost .035 of the chamber wall thickness out in one and .030 in the second. Then made a chamber casting of both chambers and first two inches of the bores to figure out how to make the chamber inserts. Wasted a ton of metal on the lathe and milling machine before I got two that fit as I wanted them. It taught me a lot about machine operation and just how difficult what seemed like a simple plan could end up as another 100+ hour education.

So in effect I have a over bored 20. I did pattern it and found that it patterned one ounce loads slightly better than it did the factory AA's 7/8 I used for a base line. If I had to do it all over again I would just go back as a 16 or even find another barrel. I have the correct chamber reamer available and figure that my custom barrel chamber making days are not a real good use of my time. Then again if I came across a 20 with bad chambers I might try to make a set of 28 chamber sleeves with about a 10-12" barrel insert. Be a fun gun to shoot skeet with and maybe a few released quail.

Jon,
Thanks for relating your experience. I just measured the walls and got between .045-.049" . These barrels are HEAVY. I wouldn't feel goosey about taking .010" a side off.
That might make enough weight difference to make the effort worth it to me.

In searching for "the" gun, considered; ability to use modern pressure ammo, cost, single trigger, ejectors, quality of construction, durability, weight. The BSS doesn't have the weight I was hoping for but has the other requirements covered. With a BSS being a relative bargain, I can put a little money into it and not feel too bad. Besides, nothing I found that was near what I was looking for was anywhere near whatever I could put into a BSS.

If I had a set that had wall thickness of .045 the loss of .010 or even .015 would be no big deal and has to help them swing better. Either do that or start a sterods program so you can swing that gun better. That gun sounds like it is almost a 12 gauge by barrel weight. Does it balance near the hinge pin?

It balances 'bout 1/2"-3/4" forward of the hinge. Barrels are 26" and weigh 2 lbs 15 oz. O.D. is 'round .715 average or little less.

Well when you consider back boring a 30" 12 barrel .010 reduces weight by 2-3 ounces the amount of metal removed from a small bore will be less by about 1/3. So if you reduce both of the barrels by 3-4 ounces total combined and then remove 3-4 ounces of wood from the stock to re-balance does this get you to the weight you are looking for? I can not see much more than 6 or maybe 7 ounces total weight reduction. Some guns just can not be made real light.

Jon,
The weight in these barrels is pretty significant. Also, the forend iron has a strap like a piece of railroad track. It could take some lightening pockets on the underside and not even show and drop an once or two easy. The bar of the action stout and could use some lightening pockets ala Parker or CSM 21 style. That kind of whittling is cake on a mill.

But, I just saw that new S&W 20g again in the pro-shop at the range. Nice part for a Turk gun...very nice. Very well sized reciever and nice fit & finish for a $2.3K gun.

Woodsman, spare that tree. Chopperlump

Shot a few boxes of 20g shells using some Gauge Mate chamber inserts in a 16g LC Smith. Summer weather. Not a hint of a bloop. Shells included AA and STS - they are easier to remove from the Gauge Mates. Federal shells worked OK, too, but are harder to push out of these inserts. Shot a few 16g shells, too, and could not feel or hear any difference between them and the 20g shells in the inserts. Did no pattern testing.

Tempting to think of overbore as similar to using a larger gauge. The wads of the larger gauge would contain the shot within their petals a bit better than the wad of a small gauge going through an overbore. The shot column would start out shorter in the bigger gauge for a given load. Overbore could allow use of a bit lower pressure to get the same speed as the gas seal area would be a bit larger. The bigger gauge would be able operate lower pressure and equal the speed of smaller gauge for the same load.