I have a question, I am about to shorten my 20-inch barrel down to 18 for the SPR project and the diameter under the hand guards is very large, like .920 or so. The barrel is quite heavy and I want to skin it down a bit in the weight department. Now rather than just turning it down to .820 or so I thought why not cut some cooling fins into it ala 1928 Thompson barrels? Now my question is will this create some weird barrel harmonics that will affect accuracy? I know that fluting is the principle method of reducing weight but no one around her can do fluting.  Opinions?

IPSC_GUY sends

I think it would be harder to find someone who does fins,  It shouldn't affect accuracy too much, but unlike flutes it doesn't help accuracy either.

The RROC used a finned aluminum heat sink, which is a better idea, as aluminum transfers heat faster than steel to air and steel transfers faster through contact.

It has to be close toleranced though and a tight fit to be effective.

JP ennterprise allready makes aluminum heat sinks for their barrels but they are fluted and not finned and only for JP's odd barrel contours.

you should be able to send your barrel(or upper) back to the manufacturer and they can flute your barrel and refinish it for a fee.

Post an inquirey into this on the Industry board

You don't want to go fins, you want fluting. Thompson SMG's are a slow .45 cartridge and low pressures compared to the barrel/cartridge weapon you have with dif. type everything in pressures, etc.. Fins go around the barrel, fluting runs horizontally. You neeeeeed and get the extra strength with fluting. Micro fluting is the way to go on that barrel, and you can call around to see who will do it for a price you want to pay.
Good shootin, Jack

Ok, this kinda makes me angry, like clips and mags, but it isn't a personal thing.  Fluting does not increase strength.  It is impossible to gain strength by taking material away.  Micro fluting sounds cool, make a bunch of small cuts with ball endmill.  Decorative, kinda like the hammer forged finish on some ruger 10/22s.

What is wrong with turning it down though?  Are you making an SPR or more of a varmint type gun that need to be heavy?

Yes, quite a bit of confusion about fluted barrels.

A fluted barrel will be stiffer than a non fluted barrel [b]of the same weight[/b] given both barrels are made of the same material.

Ponder that one.

You cannot remove material and make a part stronger.

Fins will not give the strength that fluting do when the same amount of metal is removed. The direction of the cutting is the opposite and there is not as great a dia. left on fins as on fluting. Micro is not a catch word, it provides more surfaces for cooling, and many wider ribs running up and down in a horizontal manor. To get angry at proven performance and basic engineering is not going change those facts.
Good shootin, Jack  

I always thought fluting added rigidity in the same way an I-Beam supports more weight than a round bar.  BM engineers say that fluting will actually increase accuracy slightly by increasing rigidity and reducing barrel whip and harmonics.

I believe them.

Oh and I forgot one of the main reasons why the aluminum fins are a better idea is that the huge amount of metal you add, barely increases weight, unlike finning a 1in or 1.25inch barrel.  If you go for a custom heat sink, I would take an HBAR and turn it down to about .70in or so (except for forward of the gas block and then add a finned aluminum heat sink about 1-1.25in OD. rear of the handguards (channel for gas tube clearance of course) and give it screws to tighten down like a scope ring.  If it is done properly you would have an awesome settup. Put that under a SIR and cooling will be the last problem you have to worry about.

Fluting may increase surface area by about, 180% but finning could probably do several times that.

[url]http://home.sprynet.com/~frfrog/miscellb.htm#fluted

http://www.riflebarrels.com/articles/barrel_making/rigidity_benchrest_rifles.htm

http://www.probed2000.com/flute.htm[/url]







Fluting is used for the same reason that "I" beams are used in construction, to save weight. An "I" beam is stiffer than a round bar of [b]the same weight[/b] due to the increased surface area of the "I" beam.

This is a bit off-topic, but they are called "wide flange beams" not "I" beams.  If you think there isn't a difference, try calling it a "gun" and not a "rifle" when talking to a drill instructor! [abused]

Saying that a fluted barrel is stiffer is a bit misleading, and I'll use the wide flange beam as an example.  If you had a solid shaft of steel and a wide flange with the same veritcal and horizontal dimensions, the solid shaft will deflect (bend) more due to the force of gravity.  In effect, the weight of the shaft itself is pulling it down because there is more material for gravity to pull on.  

Essentially (and this is simplified considerably) what makes the cross section of the steel stiff (i.e. minimize deflection) is the veritcal and horizontal dimensions and the [bold]profile[/bold] of it, not the mere amount of material.  

I would take issue with the comment that the I beam is stiffer due to the increased surface area.  However, I think we all are on the same page as to why fluting makes the barrel stiffer- more efficient profile.

I thought there was something undesireable about milling flutes as opposed to having them ground.

Any thoughts?

Wouldnt you have to stress relieve and totally re-heatreat the whole damn thing

Cutting the barrel, making it shorter, will increase rigidity. [;D]

True, the rigidity decreases with the cube of the barrel's length. But, I didn't say you cannot remove material and make a barrel stiffer, I said:



TopCrest, either milled or ground the concerns are the heat generated and the stresses imparted to the barrel.

A truly well made barrel (with some exceptions) will have all of its external profile finished before being reamed and rifled. That's expensive tho due to the potential scrap rate. The next best thing is to stress relieve the barrels.

So will fluting impart stress into the barrel? and if it does how does one relievethe stress afterward?

IPSC_GUY

Yes, if it isn't done correctly.  Slow, light cuts, and turn the barrel often is the best way.  It is possible that any stresses would not show up until you heat the barrel up a little bit, and then it walks.  It all depends, you probably won't have any problems, it is just that a round barrel profile is easier to do, and with minimal chance for stresses.

If you absolutely need it done right I think technically fluting first and boreing and rifleing second is best, but that is only offered by Krieger as far as I know.

Then a stress relieveing helps, Cryogenic treatment is nice.

But A BM with a Cryo would probably shoot great too.  my BM fluted Dissy HBAR is doing 3/4MOA with 52grain in it's 1/9in twist. I'm not really happy with that I think there is a way to do better, I will load my own ammo when I get around to it and accuracy should improve. (Hopefully). It could be a while though, I am saving for some new suppressors and don't want to take a dent out of that.