Wednesday, September 07, 2011

Gun Tech Query

On the inside upper front of the frame above the cylinder of this old Colt revolver (above the barrel throat/ forcing cone) there is a shadow- click to enlarge- which is actually a milled recess. Any of you scholars know why?

9 comments:

Chad Love said...

Easy: battery compartment for the laser sight...

And I'm pretty sure that gun is missing its picatinny rail. How else you gonna hang accessories on it?

And what are these cylinder things, anyway?

OK, I'm done now. I have no clue. Pistols, though much fun, aren't really my ken.

Anonymous said...

Guess: A channel needed to start/finish the threads for the barrel.

R. A. W. said...

I swear I try to listen every time my brother tries to educate me about revolvers, but my eyes roll back in their sockets... just find auto pistols more sensible.

Anyway, if I had to guess I'd say that recess is a way to forestall flame cutting. The cylinder and barrel never match up exactly perfectly; there has to be some clearance so that gunk doesn't bind the cylinder and keep it from rotating. As a result a bit of the propellant gas escapes through the gap between cylinder and barrel with every shot. Because this gas is at a bajillionty PSI and white hot it is more than capable of eroding a sharp gouge in the steel:

http://i112.photobucket.com/albums/n172/Paul105_photo/329-1-1.jpg

As a general rule of thumb it's better for strength to have a large, rounded hole than a sharp-edged, narrow hole even if the large hole ends up removing more material. Sharp-edged holes are wonderful crack propagators. By milling the metal back, you would also keep it further away from the source of the cutting. Incidentally, some theropod tooth serrations have similar relief holes.

If I had to make a second guess, that cut might be to more easily accommodate a headspace gauge.

R.S.Breth said...

In certain revolvers (most notably K-Frame Smith's in .357 Magnum when shooting lighter bullet weights)you get quit a bit of flame cutting in that area of the frame with lots of shooting, usually it only goes so deep and then stops. Maybe Colt just milled it away so it never happens in the first place? I don't think you'd get much flame cutting from .38's or .32's - but maybe that's why they did it. The damage is mostly cosmetic - but people do fret about it.

On a side note - if you use 158-grain or heavier bullets in .357 you don't get the problem because they seal the barrel-cylinder gap better - heavier bullet=longer bullet.

Dennis said...

When firing a revolver in low light I have noticed a flash that comes fro the space between the cylinder and frame; at the top. Maybe the recess channels escaping gasses to that spot. That would the flash going down toward ones hand or ammo in the cylinder below.
????

BorderWars said...

R.A.W. has the purpose, it was designed to forestall flame cutting (which I think has more to do with particulates than strictly heat, c.f. sand blasting).

S&W and Colt both milled out the top strap with a "thumbnail," which is perhaps a legacy of the fouling cup of earlier black powder guns.

Both discontinued the thumbnail, perhaps for cost vs. benefit reasons. Most people consider flame cutting a self limiting problem.

S&W brought back a similar solution when their alloy frames proved prone to the issue, they again milled out a section but also inserted a steel plate.

Anonymous said...

Steve, did you ever decide if that rifle you were intersted in was indeed the same one in the photo in "Slash Ranch Hounds"?.....L.B.

Steve Bodio said...

Still waiting but it definitely belonged to the bros so it is likely the photo'd one.

Dennis said...

I think it is to channel excess gasses back through the cylinder and the frame so the flash doesn't go down toward the loaded ammo or your hand