Measuring throat erosion

Hi, all.

In a fit of boredom I decided to attack my No. 4 Mk. 1 (T) with my set of plug gages. (Or gauges. But the mfr calls them gages).

Peter Laidler, in his book "The Armourer's Perspective," writes (p. 91 ff)
"The barrel can be gauged in the following manner. After thorough cleaning, a .301-in. diameter gauge, 2-in. long, must pass cleanly through the bore... A gauge of .307-in. must not enter the muzzle by more than 1/4-in."

OK. Done. I didn't drop it all the way through but the .301" is a slip fit, and the .302 won't even start.

So far, so good.

Now to the chamber end.

Again Peter Laidler:
"To test the lead from the chamber to the rifling, a sure test of a shot-out barrel, a .308-in. gauge must not enter the barrel more than 2.5-in. This distance of course includes the length of the chamber into the lead."

I don't understand where the "zero" of this 2.5 inches is. Is it from the breech face? The bolt face? Something else?

Thanks.

"...the .302 won't even start". Did you mean the .307 won't even start?

If you meant the .302 won't even start at the muzzle I suspect measuring the rifling ahead of the lead will be superfluous: it must be excellent barring some defect or variation in manufacture.

The answer to the second question would depend on the length of the gauge referred to in the original; an unknown to me.

I interpret "slip" to require pressure, while "run" means under the gauges own weight.

When I thought I was a real enfield collector, I carried .301" & .302" BSA bore gauges and a .300" diameter pin gauge.

Many No4 and No4T rifles I examined barely accepted the .300" diameter pin, .301" certainly wouldn't "run" thru most of them.

For a while, I only bought rifles which would not accept the .301" gauge at the muzzle.

Now I don't worry about it, if the bore and throat look OK, they are OK. Real problems are usually easily seen.


No1 rifles would sometimes "run" under their own weight with quite large diameter bore gauges...and they still shoot fine.

Here are the Canadian Ordnance Corps specs from 1943.

.301 should run
.307 should not run
.308 should not enter the muzzle more than .25"
.310 should not enter the breech more than .25"

The beauty of the original plug gauges for checking the muzzle and breech is that they have a scribed line that show the user a quick fail if it goes in past that mark.

Brian didn't you sell gauges at one time. Milled steel bars marked and graded? I seem to recall buying one from you a few years back. If it wasn't you then my memory is failing.

Yep, I had the gauges, (including MoD spec HS), from Arrowmark Tool and Gauge in Australia. They were and are excellent quality. A local friend with a machine shop, (former US Gov't Arms technician and Palma shooter), made generic TE gauges for bolt actions in a few different calibers. I'm glad you picked up on one. Unfortunately, many wouldn't spend the money on them in both cases so Arrowmark stopped making them and so did my friend in Springfield, SC. I still have a few Arrowmark SMLE combo tools and bolt tools for the Long Lee here I think. I may have a TE gauge or 2 left but none in 30-303 cal. I think they are 6.5 and 7mm. I'd have to check my inventory.

Sets of generic gauges in various increments as low as .001" can be got out of China IRRC. Saw them on ePay years and years ago.