Bore check made easy

Don’t be slugged in the pocket, says John Hill

I’ve mentioned in previous articles where I’ve slugged a rifle barrel, a process I’ve done many times which involves nothing more than pushing a lead slug through a clean rifle barrel with a cleaning rod. The lead slug can be a cast bullet or any other bullet composed of soft lead – I use .22 calibre lead bullets bought at the Hambly-Clark gunshop in Adelaide about 50 years ago for $1 per 100. Those which weren’t used in various experiments were saved for slugging barrels and most of those barrels were .22 calibre. These bullets were sold as ‘gas-checks’ complete to go to their bases and were good for reduced loads in .222s and .22 Hornets.

It’s important the slug be at least groove diameter or a whisker larger so it receives a full impression of the rifling and if it’s not groove diameter give it a tap with a hammer to increase diameter. It takes a lot of pressure to force the slug into the rifling but from then on it can indicate the bore’s condition ‑ if it’s parallel, tapered towards the muzzle or whether the bore is larger at the muzzle, the latter often considered unfavourable from an accuracy point of view.

You have to exert a fair amount of effort for the slug to get going and doing this by yourself can be a bit awkward. The bullets I use to slug .22 calibre barrels have a hollow in the base to accommodate the end of the cleaning rod, though other bullets or slugs may not have such a convenient feature. When this is the case a small brass fitting made to screw on to the end of the cleaning rod will stop the rod from gouging into the rifling and ruining the bore. It may never happen – but it could – so prevention is better than a cure for damaged rifling (I always slug my barrels with a brass fitting screwed to my cleaning rod).

Slugging a barrel not only tells whether or not the bore is parallel but also gives the groove diameter of the rifling which can be measured from the slug with a micrometer or digital calliper but remember they’re measuring soft lead so don’t squeeze too hard. Knowing the groove diameter can sometimes be an advantage as to whether the groove is oversize or undersize in regard to bullet diameter and that’s why I slug my barrels – it tells me what I need to know regarding the inside dimensions of a rifle barrel. Most barrels today are well made but some from yesteryear were woeful to say the least.

There’s another device designed to monitor rifle barrels and that’s a borescope – an instrument which gives an indication on the condition of a barrel – but they’re expensive to buy. Borescopes can be had with or without a video screen, the former having a screen for all to observe while with the other the bore is examined with the naked eye. I had the latter demonstrated to me by a member of our local rifle club and the scope showed my Tikka .223 had traces of copper in the bore but otherwise was fairly clean.

A borescope may provide a good visual perspective but it can’t tell if the bore is parallel or tapered or whether it’s larger at the muzzle and neither can it give you the groove diameter. So I’m inclined to think you need both a borescope and slugging to inspect a bore properly but considering the price of a borescope I’ll stick with slugging my barrels which costs next to nothing, yet tells me everything I want to know about the condition of the bore. While a borescope will show the rifling has worn immediately in front of the chamber, so will a look down the bore.

I discovered the usefulness of slugging barrels after buying a new ZKW 465 Brno .22 Hornet in the 1970s. The rifle wouldn’t shoot accurately and had group sizes running out to 3 MOA at 100m no matter what was tried by way of reloading components. I’d read about slugging barrels in a shooting magazine and tried my hand using a .22 calibre lead bullet as a slug. This revealed the Brno barrel was far from perfect with the first few centimetres being quite firm, then the slug free-wheeled until it fell out the muzzle. Returning the slug to the muzzle it was found to be a rattly fit and pushing another one in revealed the muzzle was 0.04mm (0.0015”) larger than the breech end of the bore – no wonder the rifle wouldn’t shoot.

The answer to this problem was to lap out the breech end of the barrel with lead laps and a fine grade of abrasive paste. It was hard work but I took 0.03mm (0.001”) out of the breech end which made the rifling look quite shallow as though it had been partly shot out. But it did the trick with accuracy improving to just a shade over 1 MOA and by that stage the groove diameter was 5.71mm (0.225”). A borescope would’ve given an entirely different result and declared the barrel perfect – unblemished from chamber to muzzle – so slugging has definite advantages. I eventually sent that rifle away and had it converted to .17 Ackley Hornet.

Anschutz barrels have always slugged well as have all the CZ barrels I’ve examined. Others which slugged well are my old Lithgow Model 12 (now almost 70 years old) and a Winchester Model 70 chambered for .222 Remington, a heavy-barrelled rifle whose bore was perfect (according to the slug) but the rifle wouldn’t shoot any better than 1 MOA at 100m and I never find out why. Those old Brnos have some of the worst barrels ever made as many of them were larger at the muzzle and that includes rimfires and .22 Hornets. Though I have a Brno .22 Hornet and its barrel doesn’t slug all that well, it still shoots sub-minute groups but only with handloads (it’s a 1949 ZKW 465 model).

So there are my thoughts on barrel slugging. A borescope will only find obvious things wrong with a bore such as copper fouling, barrel pitting and other visual blemishes whereas slugging tells whether a bore is parallel or not and the groove diameter is a bonus. One’s expensive and the other’s almost free so you decide which is best.

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