Bore gauge: how do they work, and what should I look for?

P

Pond James Pond

New member
I will probably be going to look at a used .308 bolt action next week. It's one I've been considering some time, but without getting any closer to a decision.

A shooting instructor who tutored me in the run up to my IPSC licence test, offered to lend me a 7.62 calibration tool.

Apparently, it can be used to check the bore which allegedly ranges from 7,60mm to 7,64mm and should show the life time of the barrel.

Hopefully, I can get hold of it next week and go see this bolt action in the shop.

Can anyone advise me what the values might indicate?

i.e. what sort of bore diameter would be an indicator to walk away from a knackered barrel?
 
He was talking either about a throat erosion gauge or a muzzle wear gauge.

If the rifle in question is a civilian bolt action, I doubt very much that it has seen enough use to even bother checking with either gauge. It takes thousands of rounds to begin to show enough throat erosion to affect accuracy, and muzzle wear is usually caused by cleaning from the muzzle (common with M1 and M14/M1A rifles), seldom done with bolt actions, which are normally cleaned from the breech.

No harm in checking, of course, but I would not expect much in the way of revelations.

(BTW, a TE gauge is set up for a specific rifle and depends on the configuration of the receiver for a reference point. If he has a TE gauge for, say, an M1 rifle, it won't work for, say, a Remington 700.)

Jim
 
Just FWIW, throat erosion (TE) occurs because when the powder in the cartridge case ignites, the bullet does not begin to move immediately. Instead, its inertia keeps it from moving while the pressure forces gas around the bullet, expanding the case neck and rushing out into throat of the barrel ahead of the bullet. That gas, moving at very high speed (Bernoulli's law) acts like a cutting torch on the steel of the barrel and after while will erode the throat area. When that happens, the bullet can skid as it comes out of the case neck; that distorts the bullet so that its release at the muzzle will be uneven and inaccuracy will result. There are ways to minimize throat erosion - some powders cause it more than other, some bullets will reduce it, etc., but there is no way to eliminate it. Fortunately, it seldom becomes a problem for thousands of rounds, and few sporting rifles are fired that much.

There are gauges that measure TE simply by inserting the gauge into the chamber and looking down at the receiver ring to see which number is even with it. Of course, that means that a TE gauge is good for only one type of rifle; one that works with, say, a '98 Mauser, would not work with a British Rifle No. 4 because the location of the chamber throat in relation to the rear of the receiver ring is different.

Jim
 
As you predicted, the guy in the shop ran a cylindrical gauge down the barrel from the chamber and showed a steady, clear 7.62mm throughout.

Seems like the barrel of the Zastava is in good condition!!

Still not sure I want to spend so much on a used gun, mind...

For €75 more I can get a brand new Marlin!!
 
Sounds like he used a gauge I didn't mention, a plug gauge. A plug gauge is simply a precision made cylinder of steel the same size as the bore (so it is sometimes called a bore gauge). By pushing it down the barrel and feeling for areas of greater or lesser resistance, the size of the bore, or more accurately, its consistency, can be measured. Bore gauges were once used in factories and in proof testing (that number on the barrel of a German Luger - 8.8 for example - is the size of the bore) but today are replaced by air gauges in the factories.

Jim
 
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