Subject: [Q] rifle barrels, heat, change in point of impact, Rem 740

From: [email-masked] (Gospozhyn)
Date: 16 Sep 1994 18:35:47 -0400

Hello, I recently picked up a Remington 740 rifle in .308, and although I
haven't had a chance to fire it (since it is at home and I am at school) I
was told that this rifle tends to shift point of aim after multiple rapid
shots because of barrel heating. 

This has me curious.  What is the actual mechanism relating barrel heat to
change of impact?  Is there a book or article that fully explains this and
its ramifications?  I imagine the relevent factors (for steel) are barrel
mass, rigidity, and surface area and heat output of the cartridge.  Where
can I find equations for this stuff? 

And what about solutions (in addition to the obvious one of replacing the
barrel)?  Two that come to mind are aluminum radiators or something and
(this is a long shot) that stuff they sometimes out in computer cases as a
heat sink.  But I can't tell if such ideas are even remotely feasible
because I don't know enough about it, especially the numbers.  Info, recs?

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From: [email-masked] (Steve Rayner)
Date: Mon, 26 Sep 1994 02:59:36 GMT

In a previous article, [email-masked] (PAUL) says:

> how could it be possible for the barrel to "bend " if it is heated uniformly
>all the way around? The thermal growth would be lengthwise , assuming the cross
>section is a fairly reasonable tube. I tried to find the "coefficent of thermal
>growth" for steel
>in my old copy of Machinery's Handbook. It is too old! I know that the factor
>given for aluminum is .005" over 5' of linear length over 10 degrees. If memory
>serves me correctly steel is approximatly 1/3 of that figure.check your library
>for Machinery'
>s to see.
>

The answer to this is simple.  When a rifle barrel is bored it is seldom true.
the barrel is bent slightly to true up the bore before the outside is
machined.  All of this sets up strains in the barrel, when the barrel gets
hot expansion is uneven due to the built in strains, and the gun barrel bends
a few thou.  About 1 in 1000 have a true bore the first time, hence the
legend.

-- 
                                Steve Rayner.

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From: [email-masked] (James W. Swonger)
Date: Sun, 25 Sep 1994 17:54:09 GMT

>section is a fairly reasonable tube. I tried to find the "coefficent of thermal

 While the heat may be provided uniformly, it is not removed uniformly.
The barrel is usually sitting with the upper side exposed to air, and
the bottom nested inside a casing (some guns uniformly enclose the
barrel; none of mine, however). A differential cooling has the same
result as differential heating, i.e. a bending moment. The barrel
will become something like a pair of arcs, parallel, with the
radius of curvature inversely proportional to the difference in
length between the two sides.

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