Richard Atkins on over-boring and fibre wads

A balled shot impact on a pattern plate. Note the two paired pellet strikes below and above-right of the main impact

Richard Atkins says you should think carefully before using fibre wad shells in an over-bored gun

The recent public move against all things plastic has motivated some, no doubt well intentioned, folk to begin banning plastic wads at their shooting grounds. They may think that this creates a level playing field, and some have said as much, but little do they know that the reality is actually quite the reverse – and that there are some safety concerns surrounding this issue too.

Many modern clay shotguns have back-bored or over-bored barrels. These will often have lengthened forcing cones – and this is where problems can arise. This taper back to the chamber, from a point further along the barrel bore, inevitably results in an enlarged bore size, right at the point where the cartridge crimp opens and the shot and wad transition to the bore size proper.

It is at this point that gas pressures are highest. Some tapers are now so long, and others sufficiently steep, that the diameter of the entry to the barrel bore is very close to 10-bore. Fibre wads lack the expansion capacity to obturate this larger bore adequately.

Gas can therefore escape past a fibre wad, disrupting the lead shot pellets and producing inferior patterns. Some perform better than others, but I have experienced as much as a 30 per cent reduction in the pellets landing in a 30-inch circle at 40 yards with some combinations of gun and wad.

That such pattern disruption can occur is something that was well known more than 100 years ago. The noted gun expert and ballistician, Major Burrard, pointed it out in his books based on his own testing and observation back in the 1920s and 30s.

In his day wads were made of true felt. Sadly, modern fibre wads do not expand as well as true felt wads, which are both rare and expensive today.

The upshot of all this is that larger bores and extended forcing cones, used in conjunction with modern fibre wads, can produce not just inferior patterns but also other undesirable consequences, including pellets becoming fused by the hot gases and producing ‘balling’ – clumps of shot that can behave quite unlike single pellets, having the potential to travel much further and even to cause damage or injury beyond the normally accepted safety zones. 

This Browning B525 is stamped at 18.7mm, but the trend for over-boring means the actual diameter of the bore may be slightly more

This was shown by testing carried out for and published in the magazine back in 2016. The guns featured then were just two of the better known makes, with just one plastic and fibre wad cartridge type from the competition range of a noted UK manufacturer.

There are many other makes of gun with various combinations of larger bores and longer cones now on sale, and of course there are a great many fibre wad cartridges.

I have long championed the call for more gun owners to test their own guns, with the cartridges they intend to use, on a pattern plate. Even if you aren’t interested in choke and pattern tests, you need to be sure that your gun works well, and be vigilant for signs of the potential issues – and that is becoming more vital by the day.

The more grounds that insist on the use of fibre wads, the more important it becomes to identify which cartridges perform best (and worst!) in your gun.

What is back-boring?

Back-bored and over-bored guns have been with us for many years now. But still, on a regular basis, I hear and read comments that show a great many shooters have only the most basic idea what the terms mean.

More worrying to me is that a goodly proportion of those making such comments actually own guns of this type! If nothing else this may indicate that the marketing of ‘back-bored’ guns has not been entirely successful; otherwise those owning them would know what they had!

Note that although technically different, the terms ‘back-bored’ and ‘over-bored’ are often used to describe the exact same thing and, confusingly, not all gunmakers use the terms in the same way.

The term ‘back-boring’ originated in America when gunsmiths took to the idea that slightly enlarging the bores of standard bore size guns would give an attractive array of advantages. The claims they made for back-bored guns included improved patterns, lower recoil, better penetration; some have even claimed higher velocities.

An old-style white wool felt wad, two modern fibre wads, and a plastic wad of the type now falling out of favour

I watched this ‘progress’ evolve and from early on I sought more information to back up these claims, but the parties taking this route have been reluctant to actually come forward with before-and-after proof that the concept works.

I was not alone in seeking such evidence. Many people have tried to discover for themselves. One, sadly no longer with us, went so far as to obtain barrels and have them gradually increased in size while testing for pressures, recoil and velocity. I won’t go deeply into his findings, but I can say that they might explain why practitioners of back-boring were so reluctant to provide proof of their claims.

The term ‘over-boring’ came along later when some gun manufacturers started producing guns with bores manufactured larger than the standard size in the first place.

Nowadays both terms are often used to describe new guns with bores larger than what was once considered the norm. Although there are now a small number of over-bored 20-bore guns, it is by far the more common with 12-bore guns. As this is the most popular bore size by far for clay target shooting, this makes the issue particularly relevant to all clay shooters.

This new sporter has standard bores with longer forcing cones

How big is ‘over-bored’?

It’s fair to say that ‘over-boring’ and ‘back-boring’ are both misleading terms, because the result of either process must still remain within the rules laid down by the relevant proof authorities – CIP for those countries party to CIP jurisdiction, and within SAAMI specifications in the USA.

The standard maximum permissible CIP size for a 12-bore shotgun today is 0.744”, or 18.9mm.Most over-bored guns are manufactured at between 18.6mm and 18.8mm, so not actually above CIP or SAAMI bore sizes.

Guns outside the maximum dimensions cannot be legally sold in CIP areas and will fail proof. Therefore, all the terms really mean is that the guns are manufactured very close to the upper limit of bore size, and therefore have less material left in their bores before any further lapping out would render them out of proof.

It might be helpful to use a bore gauge to measure the true internal bore size of your gun

Fortunately, the majority of such guns have chrome-lined bores, and so are unlikely to develop the pitting that would necessitate lapping out.

For a great many years the CIP proof system, enforced by our two proof houses in London and Birmingham, have inspected guns for compliance with the rules using plug gauges, which is a well accepted means of gauging gun barrel bore sizes (note ‘gauging’ and not ‘measuring’; an important difference).

This is done with pairs of ‘go’ and ‘no go’ gauges that are inserted into the bore. Depending on which gauges fit into the bore to the required depth, the barrel will be stamped (or etched) with the appropriate measurement.

This means that the ‘measurement’ marked on a barrel is not an exact measure of its bore, but an indication of which band it falls into. There was always some leeway built into the system, recognising the realities of gun manufacture and use.

So what should I do?

As the move against plastic gains momentum, it’s likely that shooters will find themselves having to shoot fibre wad cartridges more and more.

Even if you shoot an over-bored gun this need not be a problem – provided you have tested your gun with the fibre-wad cartridges you intend to shoot, and are satisfied with their performance in your barrels. Looking ahead, new wad options, including fibre cup wads and eco-friendly, starch-based non-plastic wads, are on the horizon.

Do bear in mind, though, that without testing you are taking a risk – first that your patterns may be very different to what you’d expect and second that you may be producing potentially dangerous clumps of lead shot.

These Winchester Skeet loads from 1960 had an obturator on its fibre wads to seal in expanding gases

These could fly beyond the normally accepted safety zones and still have enough energy to cause injury. I have been surprised, not to say alarmed, by some of the results I have seen in my tests with commonly used clay guns and cartridges.

In the right combination, modern clay guns and fibre wad cartridges can perform extremely well, but there’s only one way to find what cartridge works for your gun, and that’s to test a variety.

I’d urge you to do that, not only to ensure the best performance on the targets, but also to avoid the risk of a terrible accident. 

This pattern shows both an impact by a large piece of balled shot and multiple fused pairs
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