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Photograph of the volcano formation in Witches II Cave

Best Practice Capping Techniques

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The first version of this article was published in the UK Caving Forum wiki.


Photograph of capping in Stile Pot

Capping in Small Mammal Pot
Photograph: John Sellers 31 July 2008

Capping is a technique used for splitting rock by discharging one of more nail gun cartridges within a drilled hole. It is a particularly useful method for splitting boulders into a manageable size, and may also be used, albeit less effectively, to trim bedrock. The basic technique is to drill an 8 mm hole to the required depth, insert one or more cartridges into the bottom of the hole, and to explode the cartridges by striking the top cartridge with a firing pin. Although capping is an effective technique, it is also particularly hazardous when performed with the wrong tools, or incorrectly or carelessly with the right tools. This article was a response to a number of videos on YouTube showing cavers capping, which demonstrated poor and dangerous practices.

Potential Hazards

Capping Equipment Photograph


It should be clearly understood that when used carelessly, capping is objectively dangerous. Every firing should be taken seriously, and undertaken methodically following established procedures. Accidents will happen when the operator becomes blasé.

Rarely, a cap may fail to detonate. This could either be because the cap is dud, or because the hole is too deep for the firing pin to fully penetrate. This is potentially dangerous, as if it is left and care is not taken, it could be fired accidentally whilst drilling a second hole. In the first instance, another cap should be inserted as above, and a further attempt made to detonate the cap. If this fails, a second hole should be drilled such that it cannot intercept the previous hole, and the boulder split. Care should then be taken with the debris until the unexploded cap has been located.

If the location does not have a healthy draught, fumes can build up. The area should be vacated if this happens until the fumes clear, or the air replaced from a diving cylinder.

The Slide Hammer

The slide hammer (aka "pogging bar") consists of two sections - an inner tube with the firing pin attached to the base, and a heavy outer tube, with caps welded to the top of both. At firing time the outer tube is slammed down over the inner tube, transferring energy to the firing pin. Both tubes need to be seamless to ensure that one slides smoothly over the other.

The firing pin is made of the tough Maraging steel, and the tip has to fit an 8 mm hole fairly snugly. The end should be chisel shape, preferably with a notch in, to ensure that as much pressure as possible is applied to the rim of the cartridge, and the lower section should be threaded to allow dust to be trapped to help seal the hole. The firing pin should be protected during transport with something like an ice-axe protector.

Cord can be used to fasten the inner tube to the outer tube. This can be tightened when being transported to keep the hammer in one piece, and slackened when firing.