BARREL EXPANSION or CONTRACTION??

[camac]

Guys,
We have had several topics on this but I thought I would do a little testing to try and clarify in my mind what is happening as there has been several valid and plausible explanations on both sides. I will post the results before discussing too much further.

Please note this is a one off test so far and I hope to repeat it but we should all treat this as unconfirmed yet. I just want to try and draw in a few other people to do a similar test and or comments.

I took 8 rounds down to the range with my reamer bushings and 6.5*284. Loaded with 4831SC and 142 SMK. This is an old savage barrel that has been recut and recrowned SS and approx 1 inch diameter at muzzle.

Before firing the first shot the 0.2566 bushing would not fit in the muzzle but the .2564 would.
I fired my first cold bore shot and then tested the bore diameter again.
SURPRISE - the 2564 wouldn't fit anymore (too tight) but the 2562 would (This shot chronoed at 2874fps).
I quickly chambered and fired another round (no delay with round in chamber so minimum case heating). This one Chronoed at 2942 and once again the bore size was still smaller than original but 2562 still fitted.

The next shots were 2957, 2948, 2962 and at around this point (around shot 4 or 5) the barrel started opening up again to the point even the 2566 fitted in - I didn't have any larger bushings to see how large it really got. I also noted that after about round 4 or 5 the outside temp of the barrel was starting to warm up to the touch.

The first few shots did indeed appear to "restrict" the barrel before then opening up as the heat got to the outside of the barrel. Velocity rose during contraction but stayed there even when barrel started to open up again....

 

[RonAKA]

It is possible you are are seeing a very transient effect. Under slow nearly steady state conditions, if a plate with a hole in it is heated, then the hole grows bigger just as if it was solid steel. But if you very quickly (how long does a shot take?) heated just the inside layer of steel, then it is going to be constrained by the cold, much bigger, and unexpanded surrounding steel. The only choice it has is to expand inward and temporarily shrink the hole.

I would not have expected it to be that noticeable, but I believe you that it is possible.

 

[highmaster82]

be very carful doing this,about 30 years ago a friend of mine made up some bore gages to do the same thing ,left one in the bore too long ,it stuck tight had to heat barrel with a torch to remove it ,wound up cutting off and re crown . BW Davis

 

[spclark]

RichB, I believe he's speaking of bushings used with piloted reamers: a given reamer can be used with a pilot specifically selected to best fit a barrel to be chambered. Since different barrels may have different bore / groove dimensions, having a set of bushings in graduated diameters allows picking the one that best fits a given bore.

Using such a bushing at the muzzle end to check for changes in bore dimension is a novel approach, albeit not without risk as BW Davis just pointed out.

A similar tool would be a muzzle gauge used to check bore condition of service rifles: in this case a tapered gauge is engraved with a series of graduated rings. Inserting the gauge into a given muzzle indicated the degree of bore wear according to which ring is left exposed.

 

[camac]

Point taken BW and I had thought a little on this- I figure the barrel expands as the heat continues to conduct out through the diameter of the barrel, so immediately after the shot is the safest time to do this. Definately not whilst cooling and contracting. It is also an old barrel that I really am only using for experimenting now. A recut and crown doesn't matter too much. I would like to repeat with bushings in chamber end but I need larger bushings to do so.

Yes RichB, spclark is right, I have full sets of piloted reamer bushings that go up in 0.0002" increments. I also didn't post the last couple of velocities which kept climbing (and is a typical pattern for this rifle) steadying out at 2970.


RonaAKA - this is the theory that I was trying to test. It has been proposed by a few that this is the case. Stainless Steel is a very poor conductor of heat and the heat takes some time to get to the outside. All metals expand when heated and the "shear forces" or constraining effects of the outside of the barrel are greater than the "hoop" expanding forces of the inner heated area such that the metal can only go in.
As the heat travels out, the ouside of the barrel starts to expand as well and the whole lot gets larger.

Thanks for the input guys - keep it coming as It is really interesting to me.

Of particular interest. Why does velocity continue to rise or at least stay steady as barrel starts to expand again? - I could imagine a rising pressure as barrel constricts but then lowering as it starts to expand again?? and a similar pattern with velocity?
Cam

 

[Tozguy]

It would be plausible that a 40000 to 60000 psi (or thereabouts) punch on the tail of a bullet is enough to swage the bullet to fit the bore even as the bore gets larger from heat expansion. The increased velocity would come from the powder burning faster as the 'combustion chamber' ie the barrel gets hotter. Anyway, that's my story and I'm sticking with it

 

[Greg110]

Reduced friction?

 

[camac]

Thanks Guys,
Both plausible explanations - a change in friction coefficient with temperature or combustion chamber temperature would both offer increase in velocity and pressure with temp. Barrel diameter may well have little to do with it but more likely it is complex and involves all of the above with some playing a bigger role than others.
The other one going through my head as a possibility:- at the chamber end of the barrel the heat generated and exposed to inside of barrel bore is much greater than at muzzle. Also the barrel diameter is greater. Therefore what I measured at the muzzle bore diameter contraction could be far less than what goes on closer to the chamber. Possibly the contraction at the chamber end might continue for some time or number of shots longer than muzzle?? I will try and investigate this further but I need some bigger bushings or try with one of my newer barrels??

 

[spclark]

Possibly the contraction at the chamber end might continue for some time or number of shots longer than muzzle??

Seems probable to me. Such a condition may swage bullets to a diameter somewhat less than optimum for bore diameter closer to the muzzle, perhaps to a degree typical bullet obturation wouldn't adequately compensate for.

The base expands under pressure while a majority of the bearing surface is reduced, leading to less friction but potential for non-concentric bullet axis relative to the bore?

Pretty finicky conditions for non-rocket scientists to get a grip on. Makes for an interesting discussion though.

 

[camac]

Pretty finicky conditions for non-rocket scientists to get a grip on. Makes for an interesting discussion though.

You're not wrong - It has even confused a few of the "rocket scientists" and PhD's who work on thick wall chambers. They don't seem to have a consensus of opinion as to what is actually happening. Some swear black and blue that contraction in bore diameter can't happen, others think it can. My slow old brain can "imagine" it happening but I really know nothing. Could be a very interesting study for a PhD candidate though. Pressure, velocity, temperature and bore diameters??? What is really happening in there?? - Surely the US army would know??? Surely somebody has studied it somewhere???

First stepfor me would be to confirm my first set of observations (anyone else want to try please do - just head the above warnings and only use bushings whilst barrel is warming up.) and secondly try and rig up some bushings on rods to check bore just in front of chamber too.

 

[Tozguy]

Couldn't count the number of times I have put a ball bearing in the freezer while heating the case it is going into before installing it. An 'interference' fit becomes an easy drop in. Soooo, simply can't imagine a bore shrinking in diameter as the barrel heats up.

 

[Rust]

I've worked with parts and equipment that require that they be stored in a temperature controlled work room for up to 48 hours before measurement is even attempted, against certified masters using instruments which routinely measure to .00001", that is one one-hundred thousandth not an accidental extra zero.

Upon initial heating of a thick walled cylindrical shape, the inner diameter will decrease. This is especially apparent if heated from the inside. As the heating becomes uniform throughout the work piece, and given a little time for exerything to move towards eqiulibrium, the inner diameter starts to expand towards its final dimensions for a given temperature.

A rule of thumb is that steel expands a little less than .001" per linear inch per one hundred degrees F.

The idea is that the metal in a cylinder will try to expand axially (little effect on diameter) radially and circumferentially. The last two interfere with eah other a bit, and it is the radial component which will give an initial decrease in diameter, albeit a very slight decrease.

This from forty years of actual field experience with real equipment, no theories need apply.

 

[Tozguy]

Rust, much appreciate the explanation. Just wondering if you could venture a ball park figure as to how much initial bore shrinkage would be predicted from a 100 deg F increase to a 6mm heavy contour chrome-moly barrel. Also, do we understand that the bore diameter would expand back beyond the starting point as the barrel temperature gradient between inside and outside evens out to a higher level?

 

[RDavies]

It all sounds logical to me. At work I am involved with a lot of fitting large bearings to mining equipment and have seen effects of heating large and small areas in different ways. I often got wierd looks when I get them to do the opposite of what they expected. When heating a large frame with small (well up to 12"), heating the inside always shrunk the bore. I only got them to use heat if we could get the heat on the outside, otherwise we used bush/pin shrinking. Another thing we proved time and time again, was that when shrinking parts with liquid nitrogen, it was always easier to get the bushes in if we kept the bearings in a freezer for a few hours before putting in the liquid nitrogen until long after it stopped "boiling". The metal took a while to react fully.
As for the velocity climbing up, as the throat and barrel got some more carbon fouling, and the lubricating effect of solvent in the pores, the friction would climb.

 

[camac]

Thanks guys for the input.
Tozguy I also put pins and bushings in freezers for a while and sometimes bearing housings in the oven but in these cases it is always a fairly even, slow heating.
The Stainless Steel barrel metals are very poor conductors so the difference in temperature from inside to outside is very significant. I could imagine around 100 degrees F between inside and outside pretty easily (if not much more) immediately after firing which would then start to dissipate and spread outwards. There is approximatelly 0.38 inches of steel so 2-4 ten thousandths contraction could be in the right ball park - thanks for those figures Rust.

 

[Rust]

A rapid increase in heat applied from the bore will give the greatest contraction of bore diameter, and a thicker walled barrel will show a greater degree simply because of the higher differential temperature from the bore to the outer surface. Under any normal firing conditions I'd venture from .0002"-.0003". Might get more tyring to imitate a machine gun.

IMO a better quality barrel displays fewer quirks from heating up, whether from loosing stiffness or from dimensional changes. By better barrel I mean one with no localized residual stresses from the barrel making process, with the bore centered in the barrel and everything nice and straight.

 

[Steve Blair]

IMO a better quality barrel displays fewer quirks from heating up, whether from loosing (sic) stiffness or from dimensional changes.

Reduction in Modulus of Elasticity within the temperature ranges discussed is not significant. Up to about 600°f there is less than 10% modulus reduction in common barrel steels and the material farthest from the bore is the largest contributor to stiffness. Why do you think that any observable barrel stiffness reduction would occur?

If a machine gun barrel is fired to red heat, significant stiffness loss will occur. I have done that myself and had a bullet impact the flash suppressor on an M60 as a result. No one would be able to hold a conventional rifle with a barrel in that condition and the stock would likely ignite.

 

[Tozguy]

Just a note for those interested, in Guns of the week, week 92, a carbon wrapped steel barrel is showcased. Towards the end of the article there is some discussion of heat dissipation and an interesting note from the editor about heat in the last few inches of the barrel (muzzle end) sometimes causing bullet failure. Been trying to figure that one out ever since. Might relate to this thread but then maybe not.

 

[Tozguy]

Camac, I hope I am not overdoing it with your thread but the subject has been on my mind for over a year. Changes in bore diameter from barrel heating might not explain the fliers. Literature research has turned up that variation in bore friction has little effect on muzzle velocity. Along these lines, this source also states that the velocity drop associated with moly coated bullets does NOT come from lower friction in the bore. It comes from the vaporisation of the moly. The vaporation of moly reduces the temperature of the combustion gases thereby reducing pressure.

Much has been written about the importance of bullet gyroscopic balance and the alignment of the bullet with the bore. A different source has stated that excess muzzle heat causes bullet failure. Add in our own observations that copper fouling is often visible in the muzzle and I ask does this mean that barrel heat creates fliers when it starts to melt bullet jacket and upset its balance? Maybe the bore scope boys have some thoughts on where and when copper fouling occurs with respect to barrel heat (I hope).

 

[spclark]

The vaporation of moly reduces the temperature of the combustion gases thereby reducing pressure.

Glad you brought that up! I was gonna ask otherwise. So using moly-coated bullets somehow affects propellant combustion in a manner similar to the retardants propellant manufacturers apply to their products. I have to wonder if the additional sulfur component of moly when burned tends to increase the corrosive potential from using moly. I know some shooters who add moly powder to their propellant in addition to applying it to bullets.

I haven't noticed as significant a change in MV after switching to hBN powder for bullet lube as when I'd used moly. I switched because of the sulfur in moly (and DANZAC) and because hBN is much cleaner & easier to use in practice.

Add in our own observations that copper fouling is often visible in the muzzle and I ask does this mean that barrel heat creates fliers when it starts to melt bullet jacket and upset its balance? Maybe the bore scope boys have some thoughts on where and when copper fouling occurs with respect to barrel heat (I hope).

What copper fouling occurs in my barrels (.223, 6 & 6.5mm, .308) invariably is limited to the last 6 to 10 inches of the bore, obviously where the bullets have reached maximum velocity. As long as I clean them at the end of a day's shooting (80-100 rounds) and am mindful of removing what fouling I can see with my Hawkeye, after 300 - 400 rounds the fouling ceases to be evident.