Barrel thickness and rapid fire accuracy?

[Gino]

Okay, I've got a few questions for all you riflemen! How much does the point of impact change as a rifle barrel heats up? I'm mostly interested in bolt action rifles. What is the difference between an average hunting rifle and a bull barrel sniper rifle? Does the average Savage 1100/Win M70/Rem M700 hunting rifle have an appreciable problem with this?
Are most WW 2 bolt action battle rifles immune to this?

Thanks for any imput,
Gino, who isn't really a good enough shot to figure this out for himself (And none of my rifles are scoped!)

 

[Art Eatman]

Gino: Working backwards on your questions: WW I & WW II battle rifles pretty much all had barrel-bands clamping the stock to the barrel. (So did turn-of-the-century stuff.)

The barrel-steel heats up at a different rate than the wood of the stock. This means that as the barrel lengthens and enlarges its diameter oh-so-slightly, the forces between the wood and the steel vary and change the vibrations of the barrel. The changes in the vibrations of the barrel from shot to shot as the gun heats up will affect both the group size and the point of impact.

A free-floated barrel, not touching the wood of the stock forward of the receiver ring, will not be affected as it heats up.

A bull-barrel, having more mass, will heat up more slowly than a sporter-barrel, and is stiffer. But even a bull-barrel will eventually be affected, if it touches the wood of the stock.

For those situations where only one shot is expected to be made, and that first shot is all-important, I see no point to a bull-barrel.

As an example: I bought a Sako Vixen, .243, with a Mannlicher stock. If I allowed cool-down time between shots, it would group around 5/8". If I shot a five shot string, each succeeding shot would be one inch above the previous hit. No lateral dispersion of note.

Disassembling the gun, I discovered that the stock was two-piece, with a barrel-band hiding the "join" some six inches back from the muzzle. I threw all that front crap away, cut the stock back and rebedded it, and now regularly shoot around 1/2" groups.

Hope this helps...Art

 

[James K]

Remember the "steps" in the '98 Mauser barrel? Those were to allow the barrel to expand (lengthen) as it became hot. The stock was cut away forward of each step to allow expansion.

Change of impact, as Art says, is usually due to barrel interaction with the stock, but sometimes internal stresses in the barrel itself can cause a barrel to "walk" its shots.

While "sniper" rifles are usually made with heavy barrels and "hunting" rifles with light, barrel heat is not usually a factor in either. Nothing I have ever heard or read about snipers indicates that rapid fire was ever used, and the same goes for hunters.

In those cases, heavy barrels give weight for steady aim in fixed positions, while light barrels are used for rifles that will be carried long distances.

One of the most accurate barrels I ever saw would fire 7.62 National Match into 1/2 inch at 100 yds., shot after shot. The muzzle diameter was .52" by my caliper. The gun weighed 6.3 lbs. Go figure!

Jim

 

[Art Eatman]

Gino: Going back to that part of your question aabout different brands of rifles, I'd venture that many of the differences would be as much luck as anything.

Today's equipment is so good, compared to 40 years ago. A "low cost" or "low grade" gun whose barrel was the first one rifled with a new "cutter" could be better than a "high grade" barrel rifled with a nearly-worn-out cutter. Differences in accuracy might come down to how often does a given manufacturer replace worn cutting tools and bits. Other than this sort of wear, computer-controlled equipment provides an incredibe degree of accurate repeatability.

Referring to Jim Keenan's comment about stresses, the latest trick is cryogenics. The barrel is cooled down to that sort of temperature, minus 250 F or more, and warmed back up at a controlled rate. This relieves internal stresses in the steel caused by the original machining operations. The result is less effect from heat (twisting, warping, bending, whatever) as a string of shots is fired.

As for the rifling itself, you can find as many arguments as there are barrel-makers and gunsmiths--and maybe shooters! I have a Weatherby in .30-'06, made in Germany around 1970. It probably has around 4,000 rounds through it. It still shoots 1/2" to 3/4" groups. Obvious question: If 4,000 rounds does not "wear out the barrel", but must have caused some wear and rounding of the original sharp edges of the lands, what effect could there be if a rifling-cutter is new or worn? Damfino.

I guess that's why I harp on the bedding. In 20 or thirty different rifles of different alleged qualities, my re-bedding has always made an improvement.

"Arcane magic, My Son", said the Old Fart.