Shorter barrel = higher fps?!

[wraithen]

So the wife and I were watching american guns last night and the smiths had an arguement about whether shortening the barrel 2" (as requested by the customer) would increase or decrease the fps. They chrono'd it and it was 50 fps faster with 2" less barrel. How is this possible? This goes against everything i've read!

[buckfvr]

maybe it was too long for the caliber......leaving the barrel after it had started down on the pressure curve....rather than exiting at the peak.

[jamesjett]

What caliber and how long was the barrel before it was shortened?

[Huntbear]

You almost need to be a physics/calculus specialist to do the math.  It would depend on the caliber, and what the original length of barrel it was to begin with.

At some point you get quickly diminishing rewards as barrel length increases depending on caliber.  So yes, this can happen.  some rounds were actually designed with 20-22 in. barrels in mind.   

[wraithen]

I'm assuming .45 but they never clarified. I believe the barrel started at 20 and was chopped to 18 or 18 to 16

[buckfvr]

Yep....all the above

[Huntbear]

I'm assuming .45 but they never clarified. I believe the barrel started at 20 and was chopped to 18 or 18 to 16

.45 LC?  .45ACP?  45-70?    The first two are pistol rounds and would perform much better in a shorter barrel.  45-70 ?  Not really.

[wraithen]

.45 acp, thanks, makes sense now. didn't realize the pressure spike could be that quick.

[TheSkyBuster]

I havent watched that show lately, but the last one I saw they modified a mossberg ATR for a guy to hunt Alligators.  He paid them big bucks to "build" him the Rifle.      I only saw one problem with it.   He's a Lefty. 




http://dsc.discovery.com/tv-shows/american-guns/videos/gator-gun.htm

[Bob33]

Once all the powder has burned, acceleration stops. 

[wraithen]

Once all the powder has burned, acceleration stops.

I got that, but I didn't know that deceleration would begin in the barrel more so than in the air.

[YellowDog]

I havent watched that show lately, but the last one I saw they modified a mossberg ATR for a guy to hunt Alligators.  He paid them big bucks to "build" him the Rifle.      I only saw one problem with it.   He's a Lefty. 




http://dsc.discovery.com/tv-shows/american-guns/videos/gator-gun.htm

LOL  I saw that episode and that guy got screwed.  He paid like $8,000 + for a slightly modified shotgun and a fairly basic handgun and the frickin shotgun was for a righty and he was left handed.   

[elkinrutdrivemenuts]

What are you talking about? I thought that show was about  his smokin hot daughter?

[wraithen]

The one with the giraffe neck that wears a pushup bra that puts em around her neck?   

[Bob33]

Once all the powder has burned, acceleration stops.

I got that, but I didn't know that deceleration would begin in the barrel more so than in the air.

There is more friction in a barrel than air. 

[david0]

http://www.bellmtcs.com/store/index.php?cid=580

Here is an article about shorter barrels for the TC Encore. He cuts them down and gets better accuracy.

[Biggerhammer]

The one with the giraffe neck that wears a pushup bra that puts em around her neck?   

Hahaha! Exactly, I've shot things cuter.

[elkinrutdrivemenuts]

The one with the giraffe neck that wears a pushup bra that puts em around her neck?   

Hahaha! Exactly, I've shot things cuter.

Maybe I need to quit drinking so much, seems like I have permanent beer goggles 

[Helix]

Three things can cause this. The first and most obvious is error in setting up the chrony.  Such as setting it too close to the muzzle blast. More docketing gas out the muzzle crossing the curing will register faster. 

The second is when you have more barrel time than it takes to burn all the powder. Most common in .22 and smaller calibers. It is actually observable with a 9mm carbine.  Barrel friction in this case can slow the round in the last part of the barrel. But this is rare in major caliber rifles.

Last and if I were going to win this argument (by cheating) would be a hotter loaded cartridge used with the shorter barrel. If this is for tv expect the producers to create drama by cheating.