i was out shooting with some friends, one guy has a very light rem 700 noodle barrel in 7MM rem. mag. he fires upwards of 6 shots one after anothe @200yds. i could see the group open up after shot 3 by shot 6 it had ran the group size to about 12" i told him it was hitting all over cause the barrell was too hot my other pal chimmed in and said heat has absolutely no affect on bullet impact an argument ensued and i finally gave up. (only case its his gun range) am i wrong?

No, you are not wrong. Most people shooting 12 inch groups are looking for just about any excuse they can find to blame it on...what was your friends reason that his groups went to shit?

I would have just given him the simple fact that as you heat metal it's going to expand and or warp. Period. Barrels are no different. However you probably already did. If that's the case... go to plan B. Plan B = realize that your friend is a ignoramous and learn to live with it.

Could be the barrel was fine and the guy flinching. If the barrel warped the impact would walk in one direction.
Good Luck!

If the steel is stress-free, then as it heats it expands uniformly, and there'd be little impact from the heat. (The bore size would increase, causing some change.) Getting to "stress-free" is the problem, even for custom makers, and such a state is very unlikely to exist in commercial-off-the-shelf rifles.

Jaywalker

quote:

(The bore size would increase, causing some change.)

Why would the bore size increase?

Poulsbo,

Your observation is valid, if whimpy barrels didn't have heat issues don't you think the Army and the Marine Corps would be using them on sniper rifles and saving the weight? They are real serious about trimming weight that the soldier packs around nowdays, if it was viable the M24's and the Corps custom builds would be sporting light tubes.

These are some of the few bolt action rifles that must work, and many shots are expected, most other applicatons don't factor in having to fire a couple of full mags in quick sucession. A DG rifle must work but a single mag is the norm not two or three ( unless your culling water buffalos or something )

The thinner barrels are free to vibrate more than the thick ones. Also for every shot the same heat is generated in either barrel, but the thinner one gets hotter, because of less mass to absorb the BTUs. Since the thinner barrel gets hotter the vibrations change more with the greater expansion of the steel. There will be a pop quiz on this tomorrow, so take notes.

quote:

Originally posted by Walker:

quote:

(The bore size would increase, causing some change.)

Why would the bore size increase?

The steel expands radially. Same reason you heat a tight fitting barrel band and let it cool after installing; it shrinks onto the barrel as it cools. Someone always starts the argument that the steel "swells" so the bore must shrink. Not so, at least not enough to overcome the amount of radial expansion.

It varies... my son's 223 gets less accurate as the barrel heats up with handloads, but more accurate as the barrel heats up with ex-mil ammo... he uses it in Military Rifle (30+ shots in about 5 mins) the barrel is way too hot to touch after that...

I have plenty of ultralight barrels that shoot extremely well... for three shots. Then they start to track a bit. However, I would conjecture that the real problem (assuming the barrel is more or less stress relieved properly) is how the sight picture through the scope is affected by the heat coming off the barrel. Lighter barrel means this happens all the sooner...

I understand the barrel band ID expanding and the same for light weight barrels but since steel will expand equally in all directions wouldn't the bore diameter shrink if the bore diameter was equal or less than the thickness of the barrel wall.

quote:

Originally posted by Walker:

quote:

(The bore size would increase, causing some change.)

Why would the bore size increase?

quote:

The steel expands radially. Same reason you heat a tight fitting barrel band and let it cool after installing; it shrinks onto the barrel as it cools. Someone always starts the argument that the steel "swells" so the bore must shrink. Not so, at least not enough to overcome the amount of radial expansion.

True. Think of it as a flat plate of steel, perhaps 4x4, with a hole in the middle. Heat it, and both the plate and the hole get bigger. College engineering labs always ask the question, too. They demonstate it with a ring and a sphere. The sphere won't pass through the ring until after the ring is heated.

Jaywalker

quote:

I understand the barrel band ID expanding and the same for light weight barrels but since steel will expand equally in all directions wouldn't the bore diameter shrink if the bore diameter was equal or less than the thickness of the barrel wall.

It doesn't matter how thick the steel is,both the bore diameter and the overall diameter will increase.Part of my job used to be to install nozzle sleeves into heavy turbine casings.The outside diameter of the nozzle sleeves is about .003" larger than the inside diameter of the turbine casing.We heated the turbine casings to high temperature and the nozzle sleeves then slid easily into place.

Think of it this way; a thin cylinder like stubblejumper is talking about is a flat plate in a different shape. The plate gets longer as it heats and the only way to accomodate the additional length of the plate is to enlarge the radius of the hole. The plate is as thick (in this relationship) as the circumference of the tube.

Got it. Thanks

i'm going shooting with him again this weekend and i'm gonna bury him if this conversation comes up again. i'll make a cheat sheet on my hand. but on the serious side i thank you for all the input.

quote:

Originally posted by Jaywalker:
If the steel is stress-free, then as it heats it expands uniformly, and there'd be little impact from the heat.

Anyone who has ever shortened a barrel knows that a bore does not run exactly dead center through the barrel. They tend to wander off center here and there.

I'm not an engineer, but I'm pretty certain that when a barrel such as this starts to heat up, it will start moving in the direction where there is the least amount of resistance. Hopefully there will be enough clearance between the barrel and forearm to avoid contact when this occurs.

malm,

Can't argue with a guy who's shortened more barrels than I have (!), except, I wonder if that bore-wandering isn't a reflection of the stresses that bent a barrel, then had it straightened for mounting and sale.

I would have thought for a barrel which was drilled and then bored, the bore would be straight, even if the drilled hole had been crooked at first - unless the entire barrel later bent.

I read at one point that Marlin sorted its rimfire barrels by how closely the bored barrel bore exited the center of the blank. The closer to the center, the more likely the barrel would be a "heavy" barrel; the more off center, the more likely the barrel would be a "sporter weight." Since they don't stress-relieve at all, after button-rifling, the heavier barrel benefits by not having steel removed asymetrically along the length and having a greater mass of steel to reduce the effect of stresses.

On the wandering-bore barrels can you tell how they were made? It's dificult for me to see how a hammer-forged barrel could have reflected that, except by stresses, since the barrel is formed around a straight mandrell (lots of stresses there, so maybe there's some later machining). Also, the only folks who are making cut-rifled barrels (Krieger, et al) probably care very deeply about a wantering bore. Button-rifled barrels would most tend to exhibit the wandering-bore syndrome, I would think, particularly those that have been cut down to sporter dimension.

What do you think?

Jaywalker

Sometimes it's best to just smile, keep your mouth shut and continue with what you were doing before the conversation came up. (Southern style )


Terry

Mr. Jaywalker:

Barrels are not "bored" after drilling in the sense of a single point boring cut that is going to be straight no matter what the pilot hole does. The barrels are reamed after drilling and the reamer will follow the hole. The rifle bore never gets any straighter than it was when drilled, unless the barrel is straightened.

Howsomever, the gun drill itself is a "single point" cutter, being ground to cut with only one lip. The effect is that the drill has a remarkable tendency to continue in the direction it started in. It is truly amazing just how straight of a hole a properly set up gun drill will make.

I think that the biggest cause of the "wandering hole" syndrome is problems with the blank itself. In a gun drilling machine it is the barrel that spins whilst the drill is fixed, and the rotational speed is quite high. The blank is held at each end in chucks or bushings.

If a blank has a small bow in it, this bow is going to become greater when the blank is spun at high speed because of the imbalance of the blank. Now we have a arc in the spinning blank and we drill a straight hole though this arched blank. When the hole is done and the spindle stopped, the blank returns to its natural, straighter shape and the once straight hole becomes crooked.

I do not think that button rifled bores are more prone to wandering than cut rifle bores. It is just a matter of the care involved in making the barrel. Cheap barrels are buttoned, because it is the way it can be done cheaply and I would expect more straightness problems with quickly made cheap barrels. High quality buttoned barrels are going to be as straight as any other type of rifling. Lilja or PacNor barrels are going to be as straight as a Krieger. Straight is all about the hole, not about the rifling.

Glenn Fewless

Glenn, Good to know about the reaming vs boring. That's for drilled/reamed blanks that are buttoned. How about the hammer-forged barrels - do they wander?

Jaywalker

Most of the factory production rifles with thin barrels are bedded with a pressure point near the end of the stock. When they heat up they tend to bend away from this pressure point. Most of them tend to shoot higher as they get hot, but they may also move to the left or right if the pressure point is slightly off center.

There are a lot of potential variables that can come into play reguarding such issues. Barrels need to be stress relieved, at least partially, because of the composition of the steel itself which can greatly vary, especially when comparing every barrel under the sun.

Those who have read P.O. ackleys chapter about testing various millsurp actions can get a good idea of how much steel itself can have huge variations that go back to the origin of the steel and various refining processes.

Then there is the quality of barrel manufacturing, which includes heat treating and stress relieving, as well as the amount of meat on the barrel. In a nutshell, heat, like many other things, will probably not effect the accuracy any two rifles the same. But it can certianly have an influence on all of them.

quote:

Originally posted by elkhunter:
Most of the factory production rifles with thin barrels are bedded with a pressure point near the end of the stock. When they heat up they tend to bend away from this pressure point. Most of them tend to shoot higher as they get hot, but they may also move to the left or right if the pressure point is slightly off center.

Yeah, check and see if your buddy's barrel is free floated or not.

The portion of the barrel that is covered by the stock will cool less rapidly than rest exposed to the air. No matter how uniform the steel it can warp if it's not uniformly heated.

The LW 2100 light varmint taper 257 will still make a one hole group with the barrel too hot to hang on to.
http://www.brownells.com/aspx/NS/store/productdetail.aspx?p=15610&s=

If his groups went to 12 in. it isn't likely to be from a change due to heat, at least 10-11 inches of it won't be. Also as some of you don't think the bore size increases, why would someone loosen up a press fitted part by applying heat? Mechanics and gunsmiths do this every day of the week.