Thread: Projectile vs barrel length

Similar to @rambo-6mmrem question but different,
Is there a calculation for max projectile weight for barrel length?
For example, 6.5 Creedmoor, 1 in 8" twist, 18" barrel

Is it better to run 120, 130, 140, 143 ELD-X etc

Essentially i would like the best projectile/powder without pooing up the barrel with deposits

Cheers

Calculate the max length that your twist rate will stableise, then look up a table of bullet lengths in your cal, match length to weight and job done.

1:8 would do well with all those weights I imagine.
I've got a 20" 6.5CM Howa Alpine that I think is 1:8, I shoot 1/2 or better groups with 123 Amax's and thats with between 38gr and 42gr's of 2208 (ie a 10% powder spread)

I have found more twist is better than less.

Litz says more RPM = higher BC, & the velocity penalty is typically less than the usual ES.

Obviously in extreme situations too much twist is less than desirable, ie .224 40 grain @ 4500 fps in 1-8 or .375 350gr SMKs @ 3400 fps with 1-7 exit twist

No, what has a calculation is twist rate for projectile weight. Even shooting a heavy projectile with the right twist rate the barrel length will only determinate how fast you will need to "push the projectile" to achieve the velocity that will keep your projectile efficient out to long or extreme ranges.
Apparently, depending on how far you need to shoot the faster projectiles will go they will be supersonic for longer and efficient to the desired distance, cutting or having a short barrel length will only say you will need a "hotter" load to make your rig efficient.

My 0.05$c

Mac

Heavier calibers, such .308 upwards will have rifle especifications different for the purpose they are build for, a varmint 308 is likely to have a slower twist rate to push lighter projectiles faster at shot and medium range(bush or small fields(up to 300mt max?)), other 308 version designated to further distances the shooter will desire a faster twist so they can shoot heavier projectiles with less athmosferic influence in its path.
A heavier caliber such the .375 mentioned by KiwiGreg is categorised by me as a special caliber and as you can imagine it is designed to hit things at a very long distance. These calibers are designed with barrel length and twist rate ad equated to the purpose of the projectiles being used to reach those distances. Many people wanting a rifle to an especific purpose (long-extreme range) normally build the rig to suit their needs, a .308 or 300WM with 1:10 or 1:9 twist, a Creedmoor with 1:7(not really need, just example) a .243 normally 1:10 with a 1:8 or 1:7", a 7mm normally 1:9.5" to a 1:8.5" and etc.
Do a quick google about magnums and ultra magnums caliber, apparently the ultra magnums have a bit faster twist than the normal magnums.

Again, I hope I am of some help.

Agree faster twist generally better but don't go crazy on the fast twist direction. In terms of projectiles it also depends very much on your intended use - gongs vs game & what size game and range. The terminal performance of different pills and weights can be very different. If you're after a target bullet then this is of no consequence just focus on max BC & max velocity and find pill that you boom stick likes. If game and especially large game is the target then this will narrow down your choice quite considerably for 6.5 calibre. My own choice for 6.5 is 140gn partition for up close and 140 gn AMAX for +300 yrds.

Thanks for the replies. I probably should have had powder choice in my original post as well!

I suppose one of my concerns is that trying 2208 with 100gr Nosler BT, 120 AMAX and 129HSP the barrel had a light grey coating deposited at the crown.
While using 2209 and 140gr Interlocks and 143 ELD-X the deposit on the crown was black and a heavy scale that could be chipped off with my finger nail but took some considerable effort. This occurred with various powder charges!

Originally Posted by Fireflite

Thanks for the replies. I probably should have had powder choice in my original post as well!

I suppose one of my concerns is that trying 2208 with 100gr Nosler BT, 120 AMAX and 129HSP the barrel had a light grey coating deposited at the crown.
While using 2209 and 140gr Interlocks and 143 ELD-X the deposit on the crown was black and a heavy scale that could be chipped off with my finger nail but took some considerable effort. This occurred with various powder charges!

That is another conversation that I will "TRY" to help later.

Originally Posted by Kiwi Greg

I have found more twist is better than less.

Litz says more RPM = higher BC, & the velocity penalty is typically less than the usual ES.

Obviously in extreme situations too much twist is less than desirable, ie .224 40 grain @ 4500 fps in 1-8 or .375 350gr SMKs @ 3400 fps with 1-7 exit twist

I thought Litz said that too fast spin and you get large spin drift, but that the effect of spin drift disappears at longer distances i.e.gradually corrects itself over distance.

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.260 creedmore is a short action. My thought process would be this
-28" barrel max.
-1/8 twist because you need that to stabilise berger vld.
-if you want more mv then go the ackley improved route
-get a no turn neck. that is the best decision ive ever made.
What overall weight can you cope with? My rifle weighs a shitload but i dont care. Its pushing 140s as fast as possible without destroying brass but the barrel is 29" long and thick to stop it flexing. With a 260 you can match my 6.5x55 mv with a 26" shorter barrel and AI chamber. Getting a no turn neck should make it cleaner, barrel length needs to match the burn rate of your chosen powder. I'd use 2209 in a 260 for the medium burn rate and mid length barrel. Id use ELD-M for target and berger vld hunters for hunting. Those ELD-X dont beat the m's unless you can can jam enough powder in the case. That fancy BC is useless to me as my max load cant push them fast enough (2795fts)

Considering only velocity, the best powder burn rate is governed by the projectile and the chamber not barrel length. The "faster powder for shorter barrels" is mostly myth.
There may be other advantages in reduced muzzel pressure etc with a quicker burn rate though.

So considering all the feedback, is 2209 the best option or would another power be better? Or do I just run a better cleaning regime?

your observation of grey deposit at crown - are you running a suppressor or brake by any chance ?
either one, but especially suppressor, can cause excessive carbon build up, most easily detected at the crown or muzzle but the gunk will be thru out the barrel. Your cleaning regime needs to take this into account. From my experience I would not expect a lot of difference between ADI powders like 2208 & 2209 in carbon build up, but could be better/worse if you changed brand of powders.

Maybe a good read regards barrel length and velocities.

6.5 Creedmoor- Effect of Barrel Length on Velocity: Cutting up a Creedmoor! | Rifleshooter.com