6.8mm Twist


PDA






Wolfen
December 22, 2007, 12:25 AM
Is 12 inch twist, three land too slow for 110gr - 120gr in the 6.8mm Rem?

What is best?

If you enjoyed reading about "6.8mm Twist" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!
230RN
December 22, 2007, 04:05 AM
I've been curious about this question since it popped up last night and I've been waiting for someone more expert than I to answer.

So, since nobody else is anwering. I'll dip my toe in.

It would appear from googling around that the standard twists for this cartridge are 1 in 10" and 1 in 11" with a 115 grain bullet.

If that's the case, it looks like yes, 1 in 12" is a little slow. It looks like your 1-12" twist is set up for a lighter-bullet load.

Just for grins, with a 1 in 10" twist and the 2800 feet per second velocity for the standard load, the RPM of the bullet out of the barrel is 201,600 RPM. I love throwing bullet RPM in since it always leads to further discussion.

Three lands?

Hm.

I've heard of five lands (I believe the Smelly Elly [SMLE] had five lands and grooves) and four lands (pretty common) and two lands (rushed-production Springfield 03-A3 rifles had only two lands and grooves), but I never heard of three lands and grooves.

Not that it's impossible, just that I never heard of it. No theoretical reason why there can't be barrels with three lands and grooves, but it's a new one on me.

If any of the above is wrong, please advise.

rbernie
December 22, 2007, 10:58 AM
A 1:12" twist will almost certainly stabilize any 110gr-115gr bullets available of any construction. It may not stabilize a largely-copper bullet over 130gr, but it probably would.

As far as most people can tell, a 1:10" twist is actually a wee bit faster than needed. The current trend seems to be to move to a slower twist to gain velocity.

230RN
December 22, 2007, 11:31 AM
Ah, better information. Thank you, rbernie. If I may speculate more, apart from the slower twist allowing higher velocity, would it not also encourage tumbling on penetration of the target?

And can you comment on the three lands and grooves? Are they doing that as well? With today's rifling methods, it would seem that it's not a matter of cutting costs --as in running a single cutter through a bore only three times instead of four or more times. So what might be a reason for going to three lands and grooves?

As I commented, 03-A3 barrels, which were cut rifling, were done with two lands and grooves to cut costs during the rush of production in WWII.

Or was Wolfen mistaken on this?

rbernie
December 22, 2007, 11:45 AM
Tumbling is a function of the balance of the bullet, i.e. how much the bullet is rear-weight biased. The more to the rear the CG of the bullet is, the faster it will tumble. The speed at impact or the RPM at impact are far lesser variables than weight distribution.

The number of lands/grooves has little effect upon the stability of the bullet. It does effect the amount of pressure needed to engrave the bullet into the rifling, how well the bullet seals the bore against the combustion gasses, how quickly the rifling can be made, and how quickly the barrel wears.

In general, fewer lands reduces the pressure required to 'start' the bullet in the rifling and should be faster to make (if cut rather than forged or button-engraved), but wears faster.

Wolfen
December 23, 2007, 04:29 PM
230RN & rbernie

Thanks to each of you for your interest and comments.

I started using the cut 3 land/grove barrels back in the late 70's when I started building Unlimited Silhouette pistols and Rifles. I purchased the "Blanks" from the GREEK in Tulsa. I found them to be the best for me. Accuracy and longevity were outstanding.

There are a few top rated barrel makers that cut three land/grove barrels today.

I'm not real clear on it now, but as I recall, the Greek told me they allowed slower twist, better control and better wear than other configuration.

If you enjoyed reading about "6.8mm Twist" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!