There’s a fashion among manufacturers to offer guns with back-bored or over-bored barrels. The advertising claims that this gives higher velocity, better patterns and less recoil. But does it work? We set out to test the advertisers’ claims. Are you, the shooter, getting a real benefit, or is it just marketing hype?
First up, velocity. It should be simple enough: just take two guns, one over-bored and one not, and fire them over a chronograph to measure the velocity. But in ballistics things are rarely as simple as they first appear. Over-bored barrels usually come with a number of other modifications, such as different chamber cones and chokes.
To test the effect of over-boring, and nothing else, we had to find two guns identical in every way apart from the bore sizes. And we did: two B525 Invector O/Us – one with a standard 18.4mm bore, the other with barrels back-bored to 18.7mm.
Before we embark on the tests, let’s look at what back-boring is, and how it’s supposed to work. Basically a gun barrel is a metal tube. For a normal 12-bore barrel the inside diameter, or ‘bore’, would measure 18.4mm. A back-bored or over-bored barrel is bigger. It can be just a little bigger, like our test gun, or much bigger, like some of the US custom shop specials which may be as big as 19.1mm (11-bore) or even 19.7mm (10-bore) – even though they’re firing standard 12-bore cartridges.
The larger bore is said to produce less friction when the gun is fired – essentially the wad and shot charge can slip up the barrel more easily because they are a looser fit, and this means greater velocity. This will only work up to a point, because if the wad is too loose in the bore, then gas can slip past it, wasting energy and perhaps also damaging the pattern.
The type of wad is clearly going to be important. A simple 12-bore fibre wad typically has an uncompressed diameter of around 19mm. In a barrel up to that size, it should create a good seal. In a bigger barrel, though, significant amounts of gas could start to leak past.
A typical plastic wad, on the other hand, has a flexible plastic cup on the rear end. This should be able to expand and create a seal, even on some of the widest over-bored barrels.
Another factor is the volume of the barrel. As the wad and shot charge move down the barrel, the gas expands into the bore behind them. The back-bored barrel is larger, so the volume is bigger. Physics tells us that means lower pressure behind the wad – but will this result in lower velocities?
Shotgun powders are relatively fast burning (compared with typical rifle powders), so if there is extra friction drag with a tighter bore diameter, this will be more pronounced near the muzzle end of the barrel – by which time the gas pressure available to overcome it has dropped off considerably.
So we have two possible conflicting effects here. On one hand, the bigger, over-bored barrel will have less friction slowing down the wad and shot, so it may shoot faster. But on the other hand, its bigger volume might result in lower pressures and therefore lower velocity. Which factor will win out in practice?
For our test, we wanted to try the two guns with typically available competition clay cartridges from different manufacturers. This would give a true picture of what benefit the average clay shooter is likely to see from using over-bored barrels.
We chose two top quality Sporting 28g loads with UK size 7.5 (2.3mm) lead shot containing 5% antimony. Both shells use the same type and make of cartridge cases, primers and wads, so the main difference between the two is the powder and the difference in their rated velocity.
We fired the shells alternately, switching guns after 12 shots to avoid either of the guns getting too hot. All the velocities were accurately recorded. True cylinder chokes were used in each gun, and each tube was carefully measured at around two-thousandths of an inch larger than the actual bore of the gun – ensuring they would play no significant part in changing the velocity of the shot charge and wad as it left the barrel.
And the results? Well, we measured a significant – but small – increase in mean velocity in the over-bored gun. With an Eley shell, we measured an increase from 1,296fps to 1,317fps, a difference of 21fps. With the Hull Cartridge shell, the figures were 1,271fps and 1,290fps, an increase of 19fps.
So the definitive answer to our original question is: Yes, over-boring does produce higher velocity, but only by a tiny amount – less than 2 per cent in our test. That doesn’t mean that over-boring can’t help in other ways. But if it’s speed you’re looking for, there are probably more effective ways than getting your gun back-bored.