Which diameter is more meaningful when measuring the bore wear?

jeanlikethis

Regular
Rating - 100%
40   0   1
Location
Ontario
We know there are two sizes for a given bore:

1. land diameter
2. groove diameter

We can use bore gauge to measure the land diameter but need to use slug to measure the groove diameter.

For the purpose of measuring the bore wear, which one is more meaningful?
 
Lands for sure. Bore gauges measure land to land diameter. The point where the grooves become grossly oversized, the rifling would be pretty close to non-existent.
 
I talked to someone who says groove is more important, lands are much neglectable.

To him, bore gauge is useless and he slugs to estimate the bore wear. I am confused as I believe lands are crucial.
 
When one is reloading, do they not slug a barrel and measure the groove diameter to see what the actual size of the bore is to determine what diameter projectile is suited to their particular firearm to obtain best accuracy? Or do I have this wrong?
 
If you font have a bullet that fits the grooves gas can get by. Bad mojo. If the rifling grips the bullet, good enough. But if the bullet doesn't fill the grooves it won't work well. That's why flat based bullets work better in a Lee-Enfield, they squeeze out and fill the grooves.
 
When I measure bore for cast boolits I'm interested in lands i the throat,just in front of chamber.

Size,shape (sometimes it's oval die to cleaning with steel rod),and pitting.Everything else comes in second.
 
We are talking about MILSURPs here, so it is useful to see how the military dealt with rifle bore wear using 3 well known examples. The focus was on the bore/land diameter and the degree of bore erosion versus established standards, and had nothing to do with the grooves. Besides pitting, the 2 critical bore wear areas which were checked with plug gauges of known dimensions in order to verify serviceability were the throat and the muzzle.

For the No4 Lee-Enfield having a nominal bore/land diameter of .303, a .307 gauge could not pass thru the barrel. A .308 diameter plug gauge could not enter the muzzle more than .25 inches and a .310 plug gauge could not enter the breech more than .25 inches versus a line on the gauge.

For the M1 Garand with a nominal bore/land diameter of .300, a rifle which was to be placed in storage for re-issue could not show a reading of more than .305 on the tapered throat erosion gauge. Rifles in the hands of troops which gauged .310 on the same throat erosion gauge were rejected. Rifles which were to be placed in storage had to measure .302 or less at the muzzle.

For the M1903 Springfield with a bore/land diameter of .300, a barrel which gauged more than .308 on the throat gauge was rejected. A rifle which was to be placed in storage w/o re-barrelling had to show .302 on a muzzle gauge.

A symmetrical muzzle crown is important to accuracy as it lets the bullet emerge from the barrel in an uneven and consistent way. Often a barrel which is otherwise serviceable will show bad accuracy if the crown is gouged or worn on one side or another. Some military barrels will show a bad crown as a result of improper use of a cleaning rod or pull thru. This can often be remedied by cleaning up the crown, cutting the barrel back a bit and re-crowning, or even counter-boring the muzzle to some degree to find intact and symmetrical rifling in the bore.

I once had a M1903 Springfield, which I later re-barrelled, which showed a .307 reading (just short of rejection on the throat gauge) and a .303 reading at the muzzle. After cleaning up the crown it still printed 3 inch groups @ 100 yds. I've also found M1 Garand barrels measuring .303 at the muzzle to shoot acceptably well after the crown was cleaned up. Garand barrels cannot be shortened too much when re-crowning as this will affect the gas pulse and affect functionality. Ditto for counterboring. Counterboring was, however, an acceptable repair technique for M1 Carbine barrels showing excessive or irregular wear at the crown/muzzle.
 
Last edited:
Back
Top Bottom