Olympic Bunker Gun

Dear Bruce

I am highly interested in a 12 Gauge gun that will suffice me for Bunker trench, I have looked at a few makes and models and rather like a used Ultra Trap from Browning (from the Miroku factory in Japan most probably).

The gun has an adjustable stock with fixed chokes Full and 3/4 and is backbored (the choke area measures the same as Briley Full & Improved Modified screw chokes respectively).

Can fixed chokes be easily reamed or lapped a little more open, if my needs warrant it ?

Do you rate the above gun suitable for this game and can you explain to me the sense in Browning backboring a shotgun barrel , leaving sharp forcing cones then reducing the chokes down to a true Full & 3/4 – when the wad and shot have to cram at high velocity through a standard size choking (from the backbored tube), I know their screw in chokes in a backbored gun are more open than the fixed setup, would the shot be subject to more damage and more unreliable patterns or is this just once again logic rapped in a riddle ?.

You seem to have a natural gift of extracting the logic from the riddle.

Best Wishes.

Dear Marie,

The Japanese Brownings (definitely made by Miroku) are excellent quality and I can recommend them without hesitation if you like their balance and weight.

Screw chokes in a trap gun are a mixed blessing. The major chokemeisters all seem to feel that the old fashioned built-in solid chokes are capable of better patterns because they do not have any “jump” between the barrel bore and the choke. Most of the serious bunker shooters use fixed chokes. Custom choke tube makers like Britain’s Nigel Teague keep this jump to a minimum, but on a mass produced Browning/Beretta/SKB etc, the jump is a fair amount. This is never good for the pattern, but may not hurt it too much. There has to be a jump between the bore and the rear of the choke tube to account for manufacturing tolerances. If the rear of the choke tube ever sticks up into the bore, you are going to be shooting out a lot more than 24 grams of shot!

Fixed chokes can be opened up to any more open constriction you desire. I recommend that you get a good gunsmith to do this (Wilkinson and Eyster are among the top names), as once opened it is hard to add metal to tighten them back up (though it can be done by a kind of choke sleeving process). Screw chokes offer a marvelous ability to experiment. You can hack and grind away at one all you want. If you screw it up, $35 will buy you a new one and you can start all over again. If you don’t like the way that the chokes perform that come with your gun, there are an entire ton of after-market choke makers just dying to supply you with genuwhine miracle chokes absolutely guaranteed to whack dem birdies. Briley will be happy to give you a choke cut to any exact dimension you want for a reasonable price. Chokes in overbored guns: Most 12 gauge shotguns are around .725″ to .729″ in bore diameter. Their screw chokes are sized to match those bores. Remember, a choke is the DIFFERENCE between the bore diameter and the choke diameter. Example: If your shotgun’s bore is a .725″ (pretty normal) and your choke diameter measures .690″, the difference is .035″. You would be said to have .035″ or “points” of choke- an amount normally considered “Full” and capable of shooting 70% of your pattern into a 30″ circle at 40 yards.

When a gun is provided with a larger bore than standard, it is said to be “overbored” if this was done at the factory, or “backbored” if it was done on an after-market basis. All sorts of marvelous things are claimed for overbored barrels, but it is mostly marketing hype. If your wad doesn’t seal well and the weather is cold to make the plastic wad skirt hard, overbore barrels may well produce a blooper or two.

Also, when barrels are produced in overbore by the factory, IF (big if) they maintain wall thickness, then the barrels will have to weigh more than they did when of a smaller diameter. You may or may not want heavier barrels. When backboring is done on an aftermarket basis, metal is bored out of the barrels and a 30″ set of O/U barrels will lose over 2-3/4 ounces when going from .725″ to .735″. That’s a LOT of weight and will radically change the way your gun feels.

Generally, overbore barrels work fine, but – like barrel porting- their advantages are very much exaggerated and their disadvantages totally ignored. Still, if you like the Ultra, I wouldn’t let the overbore barrels influence you one way or another.

When barrels are overbored to a larger diameter, the chokes that are fitted to the barrels are also increased in size. You CANNOT use screw chokes built for a .725″ barrel in a barrel miking .743″! They won’t fit and would cause real problems if they did. Browning has gone to great pains to change its choke sizes from the original Invectors (meant for .725″ barrels) to the Invector Plus (meant for the backbored barrels). The thread sizes are different so you can’t swap them even if you wanted to.

There you go. More than you ever wanted to know and written in my usual confusing manner to preserve my reputation for obfuscation, smoke and mirrors.

Best regards,

Bruce Buck
The Technoid
at <www.ShotgunReport.com>
(Often in error, never in doubt.)

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