(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.
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
Jon
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