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Drilling Out Gun Barrels
cyclones6
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Posted: Tuesday, July 06, 2010 - 06:44 PM UTC
I am just wondering what size drill bit you guys would use to drill out the barrel on a 1/35 .50 Cal MG.
I am thinking around #80 (That's super-duper tiny)!
Thanks
Evan
panzerbob01
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Posted: Wednesday, July 07, 2010 - 03:09 AM UTC
Evan;

I think a 0.50 cal MG would be about 0.014 inch (1/70th inch) or maybe 0.36mm diam in 1/35 scale.

I don't know what "drill-bit number" this will be- my drills all either have size-stamped collettes (industrial micro-drills) or come out of the size-labelled case, and I never pay much attention to the marked size; just use the "Mk I eyeball" to use one that looks about right.

0.014 is pretty tiny, as you noted!

You might want to "err" on a larger size bit, as when you paint it, the super-tiny holes get filled. An over-sized hole will allow you more paint room.
majjanelson
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Posted: Wednesday, July 07, 2010 - 05:01 AM UTC
Evan,

MicroMark offers this drill bit gage: MICRO DRILL BIT GAUGE

The closest bit to 1/35 scale 0.50 cal (0.5" / 35 = 0.0143") is #79 (0.0143").
panzerbob01
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Posted: Wednesday, July 07, 2010 - 06:40 AM UTC
Jeff, Evan;

Here's a little fun with the numerology of measurement and our pursuit of scale accuracy!

Please pardon my approximations and rounding things to "cal 0.50 = 0.014"...!

My calculator will tell me that 0.5 / 35 = 0.01428571.rd

I guess, from that micro-drill gauge, that this rounds to 0.0143 (143 x 0.0001 inch) in the drill-bit world and is a #79 drill. I once used these things professionally... used to build micro-optical stuff and special-appl micrometers...

Tiny drill. And rather astonishingly precise, if I might say so!

Actually, this sparks a chuckle! Consider the apparent precision here... 0.0143 is saying "to the nearest 0.5 x 0.0001 inch", or plus / minus 1/ 20,000th of an inch, or +/- 5 x 100,000th inch, measured on that micro-drill gauge! (The 3 is the 0.0001 or 1/ 10,000 increment, and the stated claim is that the drill is ...3, +/- 1/2 of 1/ 10,000 inch, in diameter!)

The calculator takes the calculation out to the 100-millionth point, before entering the repeating-decimal realm.

To put our 0.0143 inch drill into some perspective: most folks are challenged to see 1/1000th inch surface-aberration with the naked eye under most conditions! The best exceptions to this might be seeing a fine scratch on a polished surface. To see 1/10,000 inch difference between things... "Pull the other one; it has bells and whistles", as they say! The resolving power (ability to clearly distinguish between or separate adjacent lines) in even very hi-quality stereomicroscopes (dissecting microscopes) is not much better than 1/ 10,000 inch. Most folks are actually unable to measure dimensions of things at the 0.001 level reliably and consistently (accuracy and precision) - takes a pretty good tool and some careful technique! Measuring things to the nearest 10,000th... takes real tools and work to hit this level of precision! The finest of human hair is typically around 0.001 inch in diam. A piece of PE may be 2 - 3 thousandths (0.002 - 0.003) inch or more thick; you MIGHT be able to consistently tell that one piece is thicker than another, when compared side-by-side on a really flat surface. The paint layers on my plastic panzers probably add-up over 2 thou (though I hope for less and work hard at keeping coats light and thin)!

To imagine that we might drill - let alone even distinguish by sight - a hole in a piece of styrene with a tolerance of +/- 0.001 strikes me as being rather ambitious...! That is, to say by inspection that the 0.014 hole is "too small", as compared with a 0.015 hole. As to +/- 0.0001... (that would be 0.0143 OR 0.0144) LoL! And as to +/- 0.00005. That's sheer chutzpah! May the guy with the micrometer eyeball win!

Me? As a plastic modeller, I count myself real lucky to control the size of the holes within the nearest couple 0.01!

I'll leave it up to others as to whether they believe that those drill bits are actually accurately-made to the nearest 10,000th as stated...
majjanelson
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Posted: Wednesday, July 07, 2010 - 06:57 AM UTC
AND, this is all assuming that the drilled hole is exactly along the centerline axis of the drill bit. Otherwise, this would increase the diameter of the hole slightly, or even make it look like one of mine I did back in the '70s with the end of an X-acto knife blade - more of a cone than a true drilled hole!

But I was just trying to answer Evan's question...
Metal_blast
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Ontario, Canada
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Posted: Thursday, July 08, 2010 - 01:30 PM UTC
Hello,

when I drill out gun barrels, I just estimate.
Grumpyoldman
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Posted: Friday, July 09, 2010 - 12:32 AM UTC
A simple problem arises if you use a #80 or #79 drill bit for a .50 cal..... what size are you going to use for your .30 cal? Personally I think we get too tide up in all this microscopic measuring.
Emeritus
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Posted: Friday, July 09, 2010 - 01:56 AM UTC
...Or if you're into metric drill bits:
25,4 mm / 35 ~ 0,7 mm

I'll have to second the opinions already expressed though: If it looks the parts and is close enough, it's good enough for me.
Of course, the ideal situation would be that the kit parts were accurate so one could simply take the right sized bit and drill away. But as that's not always the case, I'd go for the drill size to get the looks right instead of absolute accuracy.
Reference pics are always handy, like this one.

retiredbee2
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Posted: Friday, July 09, 2010 - 07:25 AM UTC
Is it really just a hobby or is it obsession? Can't imagine worrying about the size of a hole on the end of a model gun barrel. Sometimes close enough is good enough. Its kinda like the argument over what is the correct olive drab. Every manufacturer has their own shade and no matter what , after sun wind and rain any original fresh painted color is not the same. What was that ? The Mk-1 eyeball.....works for me.... .....Al
GSPatton
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Posted: Wednesday, July 14, 2010 - 04:55 PM UTC
I actually use a new #11 Xacto blade and "drill" out the end of the barrel - until I'm happy. Just be careful- those blades are sharp - ouch!*%#@
Bratushka
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Posted: Wednesday, July 14, 2010 - 10:20 PM UTC
Doggone it! I was hoping this was about some new nifty tool set that would cut scale rifling into big gun barrels!
ShawnM
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Posted: Thursday, July 15, 2010 - 08:03 AM UTC
TLAR...works every time
lukiftian
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Posted: Thursday, July 22, 2010 - 06:16 AM UTC
I'll use a brand new #11 exacto blade.
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