Why I No Longer Trust a MW Gge

[TJT]

(As promised from an earlier date--I originally submitted this to the CMP forums waaaay back in 2010. Reprinted with my permission)

Why I no longer trust a MW gauge on my barrels.

I now use a set of machinist’s gage pins instead. What brought me to this conclusion? A very good barrel that initially checked not-so-good and a “good” barrel that just isn’t so, and a good barrel that’s still good. Let me explain.

My most recent acquisition was an all original (at least correct) SA 9-42 with original barrel. A mw gauge showed a strong “2”. An ok barrel I thought. After shooting it for a couple of months and not being able to get it to group less than 4” at 100 yards I started thinking “what could be wrong?” I took it to work and checked it with a set of machinist’s gage pins. Here’s what I found. A .302 pin (2 mw) would enter the bore about 1/16“—the depth of the crown. A .301 (1 mw) pin would enter about 3/16”. A .300 pin (0 mw) would drop all the way thru. Obviously this barrel had a bad bell at the muzzle due to what ever reason, so I had the barrel counter-bored (you’ve read about it elsewhere in this forum) to a depth of 3/16”. Now a .301 pin won’t even start to enter but the .300 pin still passes thru without any hang ups. A virtually brand-new, 60 year old barrel. It now will clean an SR1 target at 100 yards.

The “good” barrel that isn’t so…………a .301 pin enters the barrel and goes……..about 6”……..and stops. The .300 pin goes all the way thru. After 20 years of trying to get this barrel to shoot, the mystery was finally solved. The barrel is “belled” for the last 6 inches !! Hardly conducive to good accuracy. It’s only .001 bell, but it’s enough to make the bullet yaw and get gas-cut. That barrel is soon to be replaced. It is going to be replaced by the barrel that came on that receiver initially. My “0ne per lifetime” DCM M1 I got back in “83 (SA 3.7). I won’t bore you with the details on why it got swapped in the first place. Anyway, that barrel is a SA 12-65. A .302 pin won’t come close to entering the muzzle and a .300 pins drops thru. A .301 pin will not drop thru on its own accord, but with just the weight of a cleaning rod on top of it, it pushes right thru with a nice even fit from one end to the other. A nice, even “1” mw with no gross bore variations . That barrel/receiver combination has been known to group quarter-sized groups at 100 yds. It will become my “unlimited” class M1 again. Another long story I won’t bore you with the details of.

SO, my friends, your mw gages are ok for a starting point but you might think about investing in a set of gage pins for the afore-mentioned reasons. Just my humble opinion, and experience for what they’re worth.

Jon

[Orlando]

I don't worry about using muzzle gauges. Proof is at the range on paper not what a gage reads

[TJT]

I agree Bill, but some people are so obsessed with gages. All the gage really does is give you a starting point. In MY case it solved the "problem" that a mw gage would not have picked up on.
Jon

[m14brian]

Any way to measure anything and as many ways to measure things to see a whole picture are a good thing to me, just different schools of thought, no biggie.
m14brian

[Smokey]

I've often wondered what causes the "belling" of the bore towards the muzzle. Hot gas erosion? Improper cleaning? ??

[David Milisock]

I've often wondered what causes the "belling" of the bore towards the muzzle. Hot gas erosion? Improper cleaning? ??

All the above and possibly at one time a partial blockage near the bore and or a bad barrel. That's the answer I got from a smith who then followed up with too bad as he spun it off.

[TJT]

Smokey, GENERALLY it's from careless cleaning, usually without a rod guide. As for the barrel that I had with a 6" belling--all I can say is "stuff happens". It was still a MOM barrel so it was good enough, I guess. It just wasn't match-accurate. Also, if the crown isn't perfectly concentric and perpendicular to the bore, it can cause bullet yaw and gas-cutting as well, throwing the accuracy all over Hell's Half Acre.
Jon

[Smokey]

Thanks. I was castigated a while back for suggesting that cleaning rods rubbing the inside of the barrel would cause wear, and degrade accuracy. I use a bore guide or ferrule when I have to clean any rifle from the muzzle. Try to use something to keep the rod centered when cleaning from the breech.

[TJT]

Cleaning from the muzzle without a rod guide is asking for trouble. Cleaning from either end with an aluminum rod is asking for trouble. The aluminum, being soft, is easily embedded with grit and dirt and then run thru the barrel. The grit can cause irreverseable damage. You should use a coated rod. Rod flex can also cause wear even with a guide. There IS a science to barrel cleaning, just not rocket.
I clean my US Springfield M1922M1 from the breech and wouldn't THINK of cleaning it without a rod guide.
Jon

[David Milisock]

Thanks. I was castigated a while back for suggesting that cleaning rods rubbing the inside of the barrel would cause wear, and degrade accuracy. I use a bore guide or ferrule when I have to clean any rifle from the muzzle. Try to use something to keep the rod centered when cleaning from the breech.

Smart man, I use a fiber rod and a synthetic guide.

[AJ Brink]

A 4 inch group at 100 yards! I could not ask for anything better, standing,sitting,prone, even sand bagging. Okay at my age, 100 yards open sighting, is about All I can do.