I just finished reading Bill’s, letter on Browning Invector chokes. I too am a Browning shooter, and a recovering para-technoid. I own 3 Invector Plus guns totaling five barrels, each of which has a slightly different barrel diameter, only one of them at the advertised .741. The chokes are also just about as inconsistent as the barrels.
At first, I was extremely ticked at Browning for this situation. I even wrote them a letter asking for relief. To my dismay, Browning told me that all my barrels were within factory specifications (+/-.003 of nominal .741), and that I should not worry about such a small difference.
My fear was that every barrel would;
1. Require a different choke Dia. to shoot a desired pattern. And,
2, would not perform as well as advertised.
What I have found out through actual use, examination and extensive patterning is this:
1. There is virtually no discernible difference between my guns using a .735 choke in barrels of .738 and .741. Both 30 and 40 yard patterns produce nearly indiscernible results. If I’m concerned about a particular target while using the 738/735 combination I simply drop to 7 1/2 shot without sacrificing anything. It’s always an edge-on target that needs larger shot anyway.
2. The net effect of the Browning Backbore actually reduces the amount of constriction required to obtain typical pattern results. Usually about one constriction less, which is one of Bill’s major concerns. I get consistent IC results from what is normally considered skeet constriction.
3. Most importantly, while test firing my guns I found that the .738 barrel will actually “Grow” under use. I have measured it at .741 after five timed shots during a summer chronographing session. The biggest barrel, .741 has measured .743 under similar conditions. The chokes will generally only move .001 under this heat and pressure. I have no doubt that my barrels grow even more on a 10 shot sporting station. Though I would not ruin a day on the range with a bore gauge.
4. There is just no substitute for testing. I shoot Sporting Clays every week somewhere in the country and have never found a real need for more that .015 of choke in any of my Browning’s. Something that I can firmly demonstrate, and also gives me better, more consistent and more dense patterns than any of the other brands or model of sporting guns that I have owned. And, I’ve wasted a lot of shooting money on some top brand shotguns.
Your advise to Bill was right-on, test them. My advise to Bill is be happy that you chose to purchase the right gun for the game.
Head still, eyes open, front hand moving first
You know, I never even thought about testing hot vs cold barrels for changes in diameter. That’s a new one to me. One of these days when I am shooting so poorly that I need an excuse to stop, I am going to test that myself.
I couldn’t agree with you more that .003″ doesn’t make any difference at the top end, though there might be the slightest difference between patterns from .000″ and .003″ chokes. The same dimensional change affects the open chokes more than the tight ones. You can certainly get more pattern change out of fooling with your shells than you can changing a choke .003″.
Browning’s history of labeling chokes tighter than they measure goes back to pre-Invector Plus days when they used standard bores. I had some guns with the original short Browning Invector chokes. They had conventional diameter barrels (around .722″ in the B-80s, a bit more in the early Browning B325s) and the chokes shot exactly as you would expect them to based on the measured choke constriction, not on the optimistic choke marking. Once you got that sorted out, they were perfectly good chokes.
Enlarged bore diameters do funny things to patterns. One the one hand, backboring from .725″ to .741″ does increase the area of the bore by about 4.5%. This should decrease the shot column by a similar amount and will lower pressure and friction slightly. Apparently it lowers friction more than pressure as many backborers claim increased velocity. Stan Baker claims 50 fps more in his .800″ Big Bore barrels.
On the other hand, theoretically, the larger the bore, the MORE choke constriction you need to equal the percentage constriction in a tighter bore. Example: .020″ constriction in a .741″ bore is a 5.3253% bore constriction. The same .020″ constriction in a smaller .725″ bore is a 5.4410% diameter reduction. So, ( in theory! ) if percentage of barrel constriction is the only consideration, the bigger the bore, the more constriction you will need to produce the same barrel constriction. This is certainly the case in subgauge. You may need .040″ to produce a Full choke performance from a .725″ 12 bore, but you will get the same pattern percentage from the 28 (.550″ bore) with just .030″ constriction. The percentage constrictions from both happen to be about 10.7% and the patterns are roughly similar in percentage, though I might be happier with .025″ at the top end in the 28 than .030″. It isn’t really linear.
I certainly don’t argue with the patterns you say you get from your guns. Barrels are surprisingly individualistic and the shell has a lot to do with it. If Browning’s .015″ gets you what you need for the longest birds you encounter, then that’s the right choke. For the longest targets I look for something that prints a minimum of 80%. When I use .040″ in my .722 Beretta 303, I still have to pick the shell very carefully to achieve this. Of course, I am consistently beaten by better shooters using more open chokes, but that’s my fault, not the gun’s. Warren Johnson’s “Choke Chooser” (tel: 800-332-0642, $12.50) can be a real eye opener if you are into the mathematics of choke selection.
Brownings are certainly popular guns (THE most popular sporting clays O/Us in the US), so they must be doing something right. It is interesting to notice that Beretta offers tight bores and longish forcing cones. Browning Japan promotes big bores and short cones. Browning’s premium Belgian guns offer tight bores and short cones. Krieghoff offers big bores and long cones in the K-80, but go to tight bores and short cones in the super premium Ulm pigeon model. Fabarms new Gold Lion sporting clays auto has a taper bore. Go figure. Someone’s probably right, but who?
The Technoid at <www.ShotgunReport.com>
(Often in error, never in doubt.)