Thursday, September 4, 2014

Hard Numbers

I mentioned the 'spec' numbers for the diameter of the slide stop pin.  Even the 'spec' range is too loose for a properly fitted gun.  It's just the number a manufacturer needs to get in the ballpark. 

They are supposed to fit the parts to the gun and each other.   Supposed to. 

But that's not the point.  The point is there is no real firm hard and fast numbers.  Well. there are a few.  On is 75/1000ths.


A little more than 7/100ths of an inch, less than 8/100ths of an inch.  If you own a machinist ruler you've seen 100ths of an inch, so you may be able to picture this measurement in your head.

What is this measuring?  Take you barrel out of the gun and drop a properly sized dummy round in the chamber.  If you look at the bottom of the barrel where the ramp is you can see a sliver of the cartride wall.  The cylinder part of the brass case.  There should be less than 75/1000ths of an inch exposed.  After that, the brass gets too thin, and, if unsupported by the chamber, you can blow out the brass.  And that is bad-bad.  Glocks tended to do that.

So, how could more than that be exposed on a 1911 barrel?  Someone went a bit too much on the barrel with a dremel.  Here's another dremel trick.  If you are working on the ramp in the barrel and you nick the top of the chamber because you let the spinning stone go too far in, when you next fire brass will be pushed INTO that nick.  A big enough nick an you've locked that brass in the chamber.  Ooops.  Easy to fix.  Get a real gunsmith to rebarrel your gun and don't do that again.

1 comment:

Angus McThag said...

While you're tossing out these numbers, you should mention that the familiar-to-everyone 1/16" is 0.0625".

It surprised me for years as a draftsman how many people didn't know the decimal units for fractions of an inch.