Pro an con of a 32” barrel over a 30” barrel

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Bluza
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Pro an con of a 32” barrel over a 30” barrel

#1 Postby Bluza » Sat May 15, 2021 8:39 am

Ok so I am very green in fclass and pretty much in big bore as well

My first build Is to go into F standard FTR
I am getting very confused as the standard is 28” to 32” barrels in 308
My question the pro and con on each and why and what twist ?
What I have read the 30” with a twist rate 1:11.25 is the ideal setup

Thanks for giving me your time.
Will

cheech
Posts: 384
Joined: Wed Jul 15, 2009 11:10 pm

Re: Pro an con of a 32” barrel over a 30” barrel

#2 Postby cheech » Sat May 15, 2021 9:58 am

Firstly good to see your interest in Fclass

The first thing to do is get down to NRAA range and speak to shooters of each different classes within Fclass , as building a rifle for each class has its differences to be successful there is lots to learn and I would run out of room on this page

3 classes are,
Open
FTR
Standard

Barrel length and twist and profile , will be slightly different with preferred projectiles , also has major weight influences when conforming to rules of equipment

But to answer your question , longer barrel gives more velocity , 1:11.25 twist May stabilise 185gn bullets.

Best of luck with information gathering

williada
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Re: Pro an con of a 32” barrel over a 30” barrel

#3 Postby williada » Sat May 15, 2021 3:39 pm

Welcome to the forum Bluza. Cost is the biggest consideration for new shooters. The next issue to determine is, what distances you are most likely to shoot on a regular basis? Some barrels, both profile and length are less suited to 1000 yard shooting. This distance takes a lot of experience to master and it is a distance the bullets must retain a velocity above the transonic zone, i.e. a speed where the bullets are dropping from supersonic speed to a speed below the speed of sound. It is a very unstable part of a bullet trajectory if that occurs and for .308's its not so much an issue for distances less than 1000 yards which a suitable twist rate can accommodate.

It is my belief that grouping ability is more important than speed, but sufficient speed is desirable at times when there is atmospheric turbulence such as that in strong winds blowing over steep mounds etc. but within the scope of FTR or F standard, group is what counts and learning to read conditions and responding to them with appropriate sight movements is the key. Firstly recognize that there is a change in conditions. Stick with one rifle and learn its personality.

The rifle weight is the most restrictive factor on barrel choice, in terms of barrel profile, length and twist rate. Barrels that are too heavy and long can can give vertical if the rig has too much weight forward.

Unlike benchrest shooting over short range, efficiencies have to be gained with an efficient powder burn over longer strings. An inefficient powder burn contributes to fouling and loose shots. Faster powders must be used in shorter barrels such as 2206H. Shorter barrels are stiffer and so the muzzle moves less and are usually better suited to OCW tuning. Look that up on the internet for detail. I would not go less than 26" for long range shooting. However, those using longer barrels are using the faster powder in 308, but watching carefully for pressure. Their goal is group size, not speed.

Another tuning style is an harmonic tune and this is where barrel length is critical. Longer barrels are far easier to tune for long range shooting using this method because the barrel vibrations are longer compared to a short stiff barrel. Some years ago while testing from a machine rest for the NRAA using factory ammunition and barrels capable of holding harmonic tune with an efficient burn many barrels of different internals both in size and twist rate, I found 31.5 inches was the most accurate and common across many barrels.

What is important to know is that harmonic tune is strictly speed/length related and nodal points are related in physics to harmonics. Of course past the muzzle tuners alter tune not by changing resonance by the internal volume as the tuner barrel is fixed and calculated by manufacture. These tuners condition the atmosphere so as to stabilize gases around the bullet base. Harmonic tuning is achieved by varying seating depth because frequencies which are really tiny and can be thought of as whether an instrument is operating at a sharp or flat. Only small movements in seating depth are required to remain in tune established from load development.

There is a trend in TR to go longer and skinny up to 35 inches. This allows for efficient powder, finding a nice harmonic tune but are usually back bored for conditioning the atmosphere around the boat tail bullet. Bullet choice and speed will of course determine the effective twist rate. However the twist rate that is said to be desirable for gyroscopic stability on many ballistic programs is 1.5. However it is my view this factor is a little short for extreme distance. Also beware the BC changes with speed and over distance heavier bullets maintain their energy longer. A 223 might look to have a BC the same as a 308 at short range but is disadvantaged at long range.

The benefit of a longer barrel is being able to get more use out of it by re-chambering or crowning as it wears.

A tuner in the main, effectively alters the barrel lift as a damper would. It is not an harmonic tuner because it does not alter bore volume in its strictest meaning. What it does rely on is taking advantage of the the natural variation between loads such as extreme spread of velocity in what is effectively a compensation tune. In a positively compensating rifle (even a small amount) the fast bullet get out before the muzzle lifts too high whereas a slow bullet exits the barrel when the muzzle lift is higher. So the slow bullet trajectory is higher than the fast bullet. Gravity then takes effect over distance and at a specific distance all the bullets intersect to form a tight group. This is known as the compensation point. Of course neutral compensation or negative compensation can exist too. Longer barrels are more conducive to compensation tuning. Percy Pavey once told me that the POMs altered the powder years ago at Bisley, so the Australians cut their barrels back to achieve the desired compensation. Inches are very important.

A program called, "Quickload" can estimate barrel length required for harmonic tuning. Yes, more velocity is achievable in long barrels but too much is undesirable if the projectile exits before or after a sweet spot of harmonic tune. I would run with 31.5" with a twist rate around 1 in 11 as an all rounder for F Standard using 155's and above for the most adaptability. Weight limits for FTR may bring you back to 30". Between 26" and 30", I found 29" had a suitable node in the standard Palma profile.

Hope this helps.

tachyon
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Joined: Mon Jul 15, 2019 1:44 pm

Re: Pro an con of a 32” barrel over a 30” barrel

#4 Postby tachyon » Sat May 15, 2021 9:48 pm

Roughly speaking each 1” of barrel (@ ~28-32) will give you an extra 25fps in velocity.

Each “1” of twist will allow you to use bullets about 20gr heavier and be certain they will stabilise.

1:12 160gr
1:11 180gr
1:10 200gr
1:9 220gr

In 308 (or any other calibre) more bullet weight = more length and

More length = better BC = more slippery bullet with less drop and less wind drift.

The price of extra barrel length is less stiffness for any given weight so depending on the class your optimal profile and length (and thus weight) will vary.

So, a longer barrel will give you more velocity but velocity is a depreciating asset. A better BC from a heavier bullet in a faster twist does not depreciate with distance. The longer the distance you plan to shoot the more you want a higher twist rate for a longer heavier better BC bullet.

For 155 grain projectiles, ie F standard 1:12 twist will work because you must use 155gr bullets so can not benefit from a fast twist with a heavier, longer, higher VC bullet.

If you want to shoot long and heavy projectiles then 1:10 (200 grain) or 1:9 (215 grain) is about right.

While a light bullet in a fast twist will be stable accuracy will suffer a little due to the dispersion caused by spinning a slightly out of balance bullet (and they are all slightly out of balance) faster than needed.

Personally, I shoot F/TR using Berger 200.20x (200gr) bullets in a 1:10 twist 30” Kreiger 4 groove using Lapua Palma brass with Federal Match Small Rifle Primers over 43.4gr of ADI 2208 which delivers about 2635 fps with an ES of ~25 and an SD of ~6.
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Bluza
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Joined: Sun Feb 07, 2021 8:30 am
Location: Gawler

Re: Pro an con of a 32” barrel over a 30” barrel

#5 Postby Bluza » Sun May 16, 2021 6:12 pm

Thanks for the help on the info on my question
It has given me some useful insight to ponder on where to go on my build
Cheers Will

bruce moulds
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Re: Pro an con of a 32” barrel over a 30” barrel

#6 Postby bruce moulds » Mon May 17, 2021 9:30 am

other considerations.
will a 32" barreled gun fit your safe?
will it fit your carry case?
these things can just be a pain in the azz that costs a lot of money just for 50 fps velocity gain.
bruce.
"SUCH IS LIFE" Edward Kelly 11 nov 1880
http://youtu.be/YRaRCCZjdTM

pjifl
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Location: Innisfail, Far North QLD.

Re: Pro an con of a 32” barrel over a 30” barrel

#7 Postby pjifl » Tue May 18, 2021 9:29 am

Strong buffeting winds can make a extra long barrel harder to use. Mainly of interest to sling shooters.

Peter Smith.

AlanF
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Re: Pro an con of a 32” barrel over a 30” barrel

#8 Postby AlanF » Tue May 18, 2021 12:52 pm

Pro : In relation to velocity gains you can also look on it as a way of prolonging brass and barrel life - with an appropriate powder, a longer barrel will give the same velocity with less pressure.

Con : In F-Class a longer barrel will ideally have a longer stock fore-end which means further to reach front rest knobs. An inch is along way when you're near the end of your reach.

Quick
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Re: Pro an con of a 32” barrel over a 30” barrel

#9 Postby Quick » Wed May 19, 2021 4:29 pm

As your starting out, a 30in barrel in 1-12T will be fine. It will stabilise a 155gr bullet well and be accurate. Going a 32in barrel is not needed in my opinion.
Shaun aka 'Quick'
Yanchep, Western Australia

308 Win F/TR & F-S
7mm F-Open Shooter.

Barossa_222
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Re: Pro an con of a 32” barrel over a 30” barrel

#10 Postby Barossa_222 » Wed May 19, 2021 6:16 pm

You guys over there in WA always were a little bit short . . . . .

Quick
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Re: Pro an con of a 32” barrel over a 30” barrel

#11 Postby Quick » Sat May 22, 2021 3:23 pm

Short n fat is where its at mate :D
Shaun aka 'Quick'
Yanchep, Western Australia

308 Win F/TR & F-S
7mm F-Open Shooter.


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