Muzzle Gage

[Redleg]

I was just noticing that the muzzle gage does not really read the muzzle erosion, so much as it reads the FIRST tight point at the end of the bore that stops the insertion of the muzzle gage.

The muzzle gage actually reads from .297-.304

I have a barrel I counterbored on the lathe with a 5/16" bit, 1/2" deep. Yet despite the .312 hole, the muzzle gage reads a zero on the gage. The reason is the gage is encountering a .297 bore at the 1/2" deep point, while the muzzle remains .312, and will not go deeper.

I have a new Fal bbl right here and the bore will not accept the gage at all, idicating a bore tighter than .297, the smallest dia the gage will read.

I have a SA 3-44 bbl here that clearly reads a "one", but the muzzle is likely a "2", as I can almost rattle the gage a bit.

So my point is that only a pin gage set, say from .296 -.305 can accurately read the actual muzzle erosion at the end. You would take each pin gage and read the one that will not go in as the muzzle size.
I believe that the bore size, while nominally a .300 is actually tighter at around a .297 or so.

Anybody else notice this?

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[Devil Dog]

"I have a barrel I counterbored on the lathe with a 5/16" bit, 1/2" deep. Yet despite the .312 hole, the muzzle gage reads a zero on the gage. The reason is the gage is encountering a .297 bore at the 1/2" deep point, while the muzzle remains .312, and will not go deeper."

Perhaps I'm missing something here...... How can a counterbored .312" barrel read .300" on the muzzle gauge. There are no .30 cal. Garand barrels that measure .297" If there were, we would have a hard time pushing a .308" slug through it without raising too much pressure. I understand that the gage is stopping on the "old" bore but that is now 1/2" from the muzzle and should measure well beyond .300" or there would have been no reason to counterbore it.

[Redleg]

The fact is that the tapered muzzle gage is a tapered "pin gage".
Each graduation line on the muzzle gage is smaller than the previous one.
Mike the bottom and it is ..297. The very top is .305.

Now the reason the gage will not go into the .312 dia counterbored barrel any deeper, is that the .bottom most .297 section of the muzzle gage has run into bore/lands tighter than .297, at the 1/2 depth.
The original barrel mouth of the counterbored barrel ( SA-4-49) was way "oval" i.e. .310+ from cleaning rod use.

Just like the new STG/FAL barrel at .297 or tighter, .30 caliber bores are commonly less than .300.

The original .30 caliber bore/lands are .297 or so and the grooves are .308 or so.

The muzzle gage is like a wedge if you will. it stops when it hits a spot that stops it. A better way is to get a set of pin gages from MSC.

I intend to do that and see if a .295 pin gage will drop through the bores and if a .298 will not.
Using a .300 or larger pin gage will actually tell you the actual diameter at the bores termination.

Now the existing tapered muzzle gage is great for discovering general condition, but it is not much better than the bullet test. The bullet is a wedge too, rising from the point up to .308. The bullet stops somewhere on its shank in the bore.

I am a little puzzled however why some almost "new" barrels are a bit wider at the end of the bore, than deeper in the bore. I have 4 "new"barrels that run from .025 to 1.5 or .30025 to .30150.
One new HRA bbl has a 1.75 (.30175) reading.

Maybe early mfg techniques did this? Maybe even slight cleaning rod use makes that muzzle get funneled? That barrel that reads .025 (.30025 on the gage is about as good as they come for SA barrels, but it is not a .297 or even a .000 reading, but exhibits some wear.

[Devil Dog]

A zero reading on a muzzle gage would indicate the bore measures .300" at the muzzle. A reading of three (3) would indicate a .303" bore (at the muzzle)
Therefore, if your bore actually measures .297" (at the muzzle) the muzzle erosion gage should read a minus 3 (if it were calibrated past zero). I have seen a few barrels read just under zero (.2995") but I have never seen one measuring anywhere near .297". Could your gage be reading a burr left from your 5/16" tool?

[Redleg]

I hear you, but no burr or wire edge on the counterbore in this old bore.

The muzzle gage is calibrated lower than zero, or into the minus range. The lines below the zero mark do represent smaller sizes. Mike them. I have miked it and the lines are .299, .298, .297


The new FAL bbl is bare, no device and will not accept the gage at all and the lowest line on the gage mikes .297; so tight bore too.

I am going to order a set of pin gages to get to the bottom of this puzzle.

I'll bet that WWII bores run from .296-.301 with the muzzles a little bit wider due to inevitable wear.

[jmoore]

If I remember correctly, the Enfield rifling specs used by the British also tapered w/ the big end at the muzzle. I've no idea why!