Barrel length, twist rate and accuracy

Disco Bob

Disco Bob

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If a 18" barrel was taken down to 9 inch, how would the accuracy of the projectile be affected? How much would it be off by say 50 yards? 100 yards?

Since the barrel length is basically taken down by 50% from the orig length and the orig twist rate, how will this disturb the spin rate of the projectile leaving the barrel?

Is there a math calculation for this type of equation ? length of barrel to twist rate to weight of projectile?

If anyone out there can shine a light on this topic I would really appreciate it
 
Cutting a barrel down that much will completely change the performance, and I don't think you will be able to predict it.

The spin rate of the bullet won't change, it will still be spinning at the same revolutions per minute. However, you will have a much slower velocity, and this will impact the stability of the bullet in flight.

https://bergerbullets.com/twist-rate-calculator/
 
Correction, the RPMs will be very slightly lower for the shorter barrel, but the overall reduction in velocity will have the greater effect
 
cosmic said:
Isnt RPM of the bullet proportional to linear velocity? I'm having trouble understanding otherwise....
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Bullet RPM is absolutely a linear relationship between velocity and twist rate. Half the velocity is half the RPM. Halving barrel length will absolutely affect all performance parameters of the rifle. Theoretically accuracy should be better, but the huge sacrifice in ballistics might make achieving that accuracy difficult.
 
Blastattack said:
Bullet RPM is absolutely a linear relationship between velocity and twist rate. Half the velocity is half the RPM. Halving barrel length will absolutely affect all performance parameters of the rifle. Theoretically accuracy should be better, but the huge sacrifice in ballistics might make achieving that accuracy difficult.
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Are the accuracy minimal, lets say at 50 yards? and massive differences at 75 yards and beyond? Thank you for your reply..
 
Disco Bob said:
Are the accuracy minimal, lets say at 50 yards? and massive differences at 75 yards and beyond? Thank you for your reply..
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Y'know, if you want good answers, you have to ask good questions.

You are busy asking, pretty much, "How long is string?" or such unanswerable question. (well actually, the answer to the string question is "This long!", but I digress! :) )

If you have a particular firearm and caliber in mind, say so, but with it all in generalities like you are asking, the amount of useful information you are going to get is pretty minimal.

The only thing that can be said in your 50 vs. 75 yards scenario, is, It Depends! Depends on the cartridge used, depends on the bullet used, depends on the sighting system used, depends on the barrel quality, depends on the quality of the barrel work, depends depends depends.

Length only affects the speed, if the barrel is not at or near the optimum length for the cartridge in question. Too long a barrel and the velocity drops, due to drag. Too short a barrel and the powder does not burn completely, and you lose velocity.

Accuracy is related to stability of the bullet in flight, so if you need a heavier bullet to stabilize at a lower velocity, you need a faster twist rate.

Your questions as asked, are pretty much useless for getting anything resembling a useful answer.
 
Good answer trevj.

I think the string is an inch or two longer...

...and I believe we are not allowed to take an 18" barrel down to 9 inches anyway...
 
I agree. A good answer. In general terms though, your answer raises more questions, for me anyway. I have an RPR .223 Rem with a 20" barrel with 1:7 twist. Right now, I use 75 gr HPBT, load 20 thous off the L&G, and achieve approx 2815 fps. My aim is to see what I can do more with my RPR in F Class shooting. What can I do to improve consistency at distance? I had thought of swapping out to a longer and heavier barrel with a similar twist rate and a 90 gr HPBT with a higher BC. I know it's likely to affect chamber pressure and I'll probably have to modify powder loads, but is it worth the effort in the end? I'm laughing to myself while I type this because there are so many variables involved (the string is very long) and it has taken me years of trial and error with my old go to F Class rifle which has a 30" barrel with a 1:13 twist rate and fires 308 155.5 Bergers. I'm also now considering getting an RPR in 308 to develop over time. I like the RPR because they are comfortable, easy to modify and I have all this 308 brass I've collected over the years. The only thing is that the RPR in 308 comes with a 20" barrel with 1:8 twist rate and eventually I would probably consider getting a longer barrel. Same issue. Any advice would be welcome.
 
There is [or was] a Co here in NZ that made some very good chassis type rifles long before chassis systems became popular - SSRNZ. Very accurate, top quality rifles. They did a lot of research on barrel length, particularly in 308 cos that sent a decent sized projectile capable of killing a deer. They cut the barrels right back to 10" and the rifles were very very accurate all the way down to the 10" Using standard powders resulted in the expected velocity loss. But if reloaded with some of the faster powders, they were able to retain plenty of velocity. IIRC 2600fps was achievable in a 10" barrel. So if you do your homework, think outside the square, and use a bit of nouse, its is possible to cut your barrel real short and retain plenty of velocity. The trade off is barrel life - fast powders in short barrels wreak havoc on your barrel.
 
You can have a bolt rifle with a barrel of any length provided it was made that way at the factory and the overall lenghth is greater than 26 inches. A gun-smith is not allowed to cut a previously installed factory barrel below 18 inches. I had this conversation with the R.C.M.P. registry a few years ago. You can install a shorter than 18 inch barrel on an action provided there is a paper-trail showing that the barrel was shipped from the factory in the shorter length . To save a couple of inches in length is probably not worth the scutiny if you have to prove it was not shortened below a standard factor offering.
 
transumer said:
I can’t find the regs that apply to cut/crown a 22” bolt rifle to 16” similar to the Ruger Scout rifle. Any ideas?
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Yeah. Keep looking.

Bill C-68.
Try to remember. The laws were written by people who did not like guns, and mostly only knew about them in terms of "guns is bad, m'kay!"

Lots of nonsensical contradictions.
 
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