# What's that thing on the barrel of a tank? **URL:** \ **Category:** Factual Questions\ **Created:** [March 22, 2003, 3:51pm UTC](https://boards.straightdope.com/t/whats-that-thing-on-the-barrel-of-a-tank/163373 "2003-03-22T15:51:04Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![Scruloose](https://avatars.discourse-cdn.com/v4/letter/s/e68b1a/32.png) [@Scruloose](https://boards.straightdope.com/u/Scruloose)\ **Post date:** [March 22, 2003, 3:51pm UTC](https://boards.straightdope.com/t/whats-that-thing-on-the-barrel-of-a-tank/163373/1 "2003-03-22T15:51:04Z")
If you look at a [tank](http://www.kamiura.com/m1a1.JPG), about halfway down the barrel, there is an odd shaped “collar” on it. Perhaps it is on other artillery pieces as well. What is that thing? ---
**Author:** ![MikeG](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mikeg/32/4329_2.png) [@MikeG](https://boards.straightdope.com/u/MikeG)\ **Post date:** [March 22, 2003, 3:53pm UTC](https://boards.straightdope.com/t/whats-that-thing-on-the-barrel-of-a-tank/163373/2 "2003-03-22T15:53:24Z")
While waiting for **Tripler** to respond, I think it is called a compensator. I could speculate as to what it compensates for, but I’ll wait to hear from Trip. This bit of trivia is based soley on my teenage model building endeavors and could be TOTALLY wrong of course:) ---
**Author:** ![R.P.McMurphy](https://avatars.discourse-cdn.com/v4/letter/r/7993a0/32.png) [@R.P.McMurphy](https://boards.straightdope.com/u/R.P.McMurphy)\ **Post date:** [March 22, 2003, 4:25pm UTC](https://boards.straightdope.com/t/whats-that-thing-on-the-barrel-of-a-tank/163373/3 "2003-03-22T16:25:15Z")
Just a guess but I would be willing to bet that it is part of the mechanism to minimize recoil when a shell is fired. Remember the principle of “equal and opposite reaction”? With the power of those cannons you don’t want to be sending the tank in reverse evertime a shell is fired. ---
**Author:** ![UncleBill](https://avatars.discourse-cdn.com/v4/letter/u/d26b3c/32.png) [@UncleBill](https://boards.straightdope.com/u/UncleBill)\ **Post date:** [March 22, 2003, 4:32pm UTC](https://boards.straightdope.com/t/whats-that-thing-on-the-barrel-of-a-tank/163373/4 "2003-03-22T16:32:43Z")
While I wonder why an Air Force Engineer would know, the old **Tripster** may surprise us. Paging **Ex-Tank** … ---
**Author:** ![MikeG](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mikeg/32/4329_2.png) [@MikeG](https://boards.straightdope.com/u/MikeG)\ **Post date:** [March 22, 2003, 4:37pm UTC](https://boards.straightdope.com/t/whats-that-thing-on-the-barrel-of-a-tank/163373/5 "2003-03-22T16:37:21Z")
DOH!:smack: I’m sitting here thinking “ExTank will be along shortly to give up Tha’ Dope” and I go and type Trip’s name in. mea culpa ---
**Author:** ![ChalkPit](https://avatars.discourse-cdn.com/v4/letter/c/a88e4f/32.png) [@ChalkPit](https://boards.straightdope.com/u/ChalkPit)\ **Post date:** [March 22, 2003, 5:34pm UTC](https://boards.straightdope.com/t/whats-that-thing-on-the-barrel-of-a-tank/163373/6 "2003-03-22T17:34:52Z")
I think its the ‘bore evacuator’, which helps clear out fumes, and assists with balancing and recoil of the gun. Not sure, as I’m no tanker. ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [March 22, 2003, 6:30pm UTC](https://boards.straightdope.com/t/whats-that-thing-on-the-barrel-of-a-tank/163373/7 "2003-03-22T18:30:15Z")
According to my friend, a former Marine, it is the equivalent of a pipe coupler. It is a section with an interrupted thread that allows removing the front barrel extension so that the gun can be used in cramped quarters such as forests and narrow city streets. The barrel with the extension is so long that it would be virtually useless in such situations. Removing the barrel extension reduces range and accuracy but long range and extreme accuracy at long range isn’t needed in cities and forests. This is possible with current tank cannons because they are smoothbore so there is no precision rifling that has to match up at the joint. The shell is fin stabilized and is propelled through the barrel by a sabot. The sabot expands under pressure, like the old-time Minie Ball, to seal the barrel and prevent blowby of the propellant gas. It has a lot of aerodynamic drag so it slows down rapidly after leaving the barrel and the shell goes on its way. ---
**Author:** ![Tripler](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripler/32/21807_2.png) [@Tripler](https://boards.straightdope.com/u/Tripler)\ **Post date:** [March 22, 2003, 7:24pm UTC](https://boards.straightdope.com/t/whats-that-thing-on-the-barrel-of-a-tank/163373/8 "2003-03-22T19:24:23Z")
> [@](#): > > \*Originally posted by David Simmons \* > \*\*According to my friend, a former Marine, it is the equivalent of a pipe coupler. It is a section with an interrupted thread that allows removing the front barrel extension so that the gun can be used in cramped quarters such as forests and narrow city streets. The barrel with the extension is so long that it would be virtually useless in such situations. Removing the barrel extension reduces range and accuracy but long range and extreme accuracy at long range isn’t needed in cities and forests. > \*\* Heh? What?!? This is a new one to me. . . IANAMBISWIW\*, from what I’ve read in the past, the “blister” halfway down the barrel is in fact a bore evacuator, allowing for some of the gas pressure to do whatever it has to do to make the Army happy. > [@](#): > > \*[Quoth FAS.org](http://www.fas.org/man/dod-101/sys/land/120.htm) \* > While the combustible case is thoroughly consumed during firing, further assurance of a clean tube and chamber for each successive round is provided by a cannon bore evacuator. The evacuator functions as a large vacuum device, sweeping all portions of the barrel interior free of the most minute particles. All 120mm rounds have a propellant containment device that prevents propellant spillage if the combustible cartridge case is damaged. There _is_ a laser compensator on the tip of the barrel muzzle, which detects ‘bending’ of the barrel due to heat, stress and strain, etc. This compensator feeds information to the gunnery computer and adjusts the aim point accordingly to keep it true. But **UncleBill** is right. I just clean snow off of runways and dream about playing with tanks and stuff. 😃 Tripler - I Am Not A Marine, But I Sometimes Wish I Were – they have cool toys. ---
**Author:** ![Trucido](https://avatars.discourse-cdn.com/v4/letter/t/ea5d25/32.png) [@Trucido](https://boards.straightdope.com/u/Trucido)\ **Post date:** [March 22, 2003, 7:27pm UTC](https://boards.straightdope.com/t/whats-that-thing-on-the-barrel-of-a-tank/163373/9 "2003-03-22T19:27:54Z")
The device in question is usually called a fume extractor. It is most definitely not a coupler. Tank main guns most emphatically do not come apart as you described, barring catastrophic failure. Your friend is either mistaken or pulling your leg. ---
**Author:** ![Trucido](https://avatars.discourse-cdn.com/v4/letter/t/ea5d25/32.png) [@Trucido](https://boards.straightdope.com/u/Trucido)\ **Post date:** [March 22, 2003, 7:27pm UTC](https://boards.straightdope.com/t/whats-that-thing-on-the-barrel-of-a-tank/163373/10 "2003-03-22T19:27:54Z")
The device in question is usually called a fume extractor. It is most definitely not a coupler. Tank main guns most emphatically do not come apart as you described, barring catastrophic failure. Your friend is either mistaken or pulling your leg. ---
**Author:** ![mangeorge](https://avatars.discourse-cdn.com/v4/letter/m/d78d45/32.png) [@mangeorge](https://boards.straightdope.com/u/mangeorge)\ **Post date:** [March 22, 2003, 7:34pm UTC](https://boards.straightdope.com/t/whats-that-thing-on-the-barrel-of-a-tank/163373/11 "2003-03-22T19:34:47Z")
Why most emphatically? Sounds to me like **David Simmons** , or his friend, might have a pretty good idea here. Peace, mangeorge ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [March 22, 2003, 7:44pm UTC](https://boards.straightdope.com/t/whats-that-thing-on-the-barrel-of-a-tank/163373/12 "2003-03-22T19:44:37Z")
> [@](#): > > The gun is equipped with a thermal shroud, **bore evacuator** , and a muzzle reference sensor to compensate for gun tube droop. From [this page](http://members.aol.com/panzersgt/M1A2.html) [sub]Emphasis mine.[/sub] ---
**Author:** ![Trucido](https://avatars.discourse-cdn.com/v4/letter/t/ea5d25/32.png) [@Trucido](https://boards.straightdope.com/u/Trucido)\ **Post date:** [March 22, 2003, 10:51pm UTC](https://boards.straightdope.com/t/whats-that-thing-on-the-barrel-of-a-tank/163373/13 "2003-03-22T22:51:57Z")
**Mangeorge** , I included the “most emphatically” first and foremost because it’s not the case. The Rheinmetall smoothbore on the M1A2 does not break into pieces. Doing so would seriously weaken the gun’s structural integrity, slash penetrative power, and serve no purpose. Tanks are not city fighters: the US armored forces were designed for a thrust across central Europe, not an urban meatgrinder. ---
**Author:** ![mangeorge](https://avatars.discourse-cdn.com/v4/letter/m/d78d45/32.png) [@mangeorge](https://boards.straightdope.com/u/mangeorge)\ **Post date:** [March 22, 2003, 11:04pm UTC](https://boards.straightdope.com/t/whats-that-thing-on-the-barrel-of-a-tank/163373/14 "2003-03-22T23:04:27Z")
Sorry, **Trucido**. Guess I should’ve added a “winky” there. 😉 Maybe in a movie, huh? ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [March 23, 2003, 4:58am UTC](https://boards.straightdope.com/t/whats-that-thing-on-the-barrel-of-a-tank/163373/15 "2003-03-23T04:58:26Z")
> [@](#): > > \*Originally posted by Trucido \* > \*\*The device in question is usually called a fume extractor. It is most definitely not a coupler. Tank main guns most emphatically do not come apart as you described, barring catastrophic failure. Your friend is either mistaken or pulling your leg. \*\* Well, he must be just plain wrong then because he’s a serious guy who as far as I know has never told a funny story in his life. He even went and got some pictures of him with his cannon. He was a loader. My faith in humanity, heretofore so high it was out of sight, has been smashed. ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [March 23, 2003, 5:03am UTC](https://boards.straightdope.com/t/whats-that-thing-on-the-barrel-of-a-tank/163373/16 "2003-03-23T05:03:47Z")
> [@](#): > > \*Originally posted by Trucido \* > \*\*The device in question is usually called a fume extractor. It is most definitely not a coupler. Tank main guns most emphatically do not come apart as you described, barring catastrophic failure. Your friend is either mistaken or pulling your leg. \*\* Well, he must be just plain wrong then because he’s a serious guy who as far as I know has never told a funny story in his life. He even went and got some pictures of him with his cannon. He was a loader. My faith in humanity, heretofore so high it was out of sight, has been smashed. ---
**Author:** ![Michael\_Ellis](https://avatars.discourse-cdn.com/v4/letter/m/977dab/32.png) [@Michael\_Ellis](https://boards.straightdope.com/u/Michael_Ellis)\ **Post date:** [March 23, 2003, 7:10am UTC](https://boards.straightdope.com/t/whats-that-thing-on-the-barrel-of-a-tank/163373/17 "2003-03-23T07:10:03Z")
> [@](#): > > \*Originally posted by David Simmons \* > **Well, he must be just plain wrong then because he’s a serious guy who as far as I know has never told a funny story in his life. He even went and got some pictures of him with his cannon. He was a loader.** He probably wasn’t wrong. Some cannons are constructed with multi-piece barrels. A version of the famous German 88mm Flak gun was designed with a three piece barrel (chamber section, center section and muzzle section) for easier maintenance and manufacture. (Souce: German Artillery of World War Two by Ian V. Hogg) No doubt other artillery pieces had similar design.