Good evening ladies and gentlemen.
I am planning to put a 24" heavy barreled upper on my next build and there are options regarding twist rate. It's my understanding that the heavier the projectile, the slower the spin you want. I did a brief search in the forum but I couldn't find an answer to my question in past discussions. If any of you guys know, what's the best bullet weight for the longest range?

Generally, no.

Weight isn’t so much a factor in stability so much as projectile length is.  But, most usually longer bullets do weigh more.

Longer bullets most always have higher ballistic coefficients and more efficiently retain their velocity vs short stubby bullets.  Over extended ranges this allows the longer and initially slower bullet to have less drop than the shorter, initially faster bullet.

Longer bullets require faster twist rates to properly stabilize.

With a 24” bbl you will have maximum powder burn and therefore high velocities with any given load vs a shorter bbl.  If your plan is to sling 77gr matchkings, find a 1:7 or 1:8 bbl.  Which are both faster than a 1:9 or 1:12 twist barrel if there is any confusion there.

For longer range you will be in the 75 to 80 gr bullet range in which you will need a 1:8 or 1:7 twist. Most are loading near the lands, way past mag length. For mag length 77 Nosler CC or Sierra MK will do well out of the same twist.

A few even go as high as 90 gr bullets with 1:6.5 twist. I run a 20 inch service rifle barrel with 6.5 twist but don’t get any gain with 90’s over the 80’s with good BC’s unless I way over charge the case. A longer barrel would help.

Twist rate is more important than barrel length when considering a barrel to shoot certain bullet weights. Barrel length comes more into play with velocity.  

These charts are generalizations because they go by bullet weight and not bearing surface (the actual part of the bullet that makes contact with the barrel rifling) and the fact that every barrel is a wee bit different (but that wee bit can make a big difference in terms of accuracy).



Velocity usual goes up in relationship with barrel length until the almost all the gun powder is burned (the most efficient barrel length is affected by the powder burn rate). Too long of a barrel and the bullet will start to decelerate because all of the powder will burn up before the bullet leaves the barrel but there is still friction from the bullet being in contact with the rifling. While too short of a barrel and a lot of the powder is burned after the bullet leaves the barrel (and that energy goes into nothing more than creating a big flash and more noise). There is a law of diminishing returns the closer you get to the most efficient barrel length (the velocity increase curve starts to flatten and the rifle becomes heavier and more unwieldy).

As an example here is a video on .45 ACP barrel length with Fiocchi 230gr FMJ* and a corresponding chart.
*a different cartridge with a different powder could alter these results

Cartridge overall length of heavier/longer bullets can be a concern with magazine fed guns (i.e. as you get to the upper end bullet weight selection the cartridge can become too long to fit into a magazine or feed from it, thus making it necessary to load the cartridge directly into the chamber).

Topic Moved

1:7.

73gr ELD, 77gr TMK, 80.5gr Fullbore.

Might want a CLE throat.

With a 1:6 you may worry about your bullets disintegrating right out of the muzzle.

Wasn’t 1:7.8ish considered optimum for match barrels for a long time?

What are you doing at this "longest range"  just going out and ringing steel? Specific competition or similar? Are you wanting to mag feed or are you OK running a sled?  Hand loading or are you buying factory ammo?

Dont always look at weight, yes heavier bullets are generally longer, but even then there are exceptions.  A 55 grain copper projectile will be longer than a traditional cup and core 55 grain projectile.  Therefore it will require a faster twist.

For 80gr SMK length. Popular choice from Krieger who cut-rifles. Button-rifling tends to be slower than what's stamped in my experience, so I recommend the next fastest.

Thank you all for sharing your wisdom. I'll have to save up for my upper so I have a little while to think about the information you all have shared with me.