6mm Creedmore, light bullets and fast twists

[Mr. F]

Hey guys, so im having a barrel spun in 6 Creed but lately I've only been able to get my hands on lighter .243s (110 SMKs and and Bergers 105 VLDs). Purpose of this stick is steel shooting and will see most use in the high altitude/high heat of the UT summer.

Im planning on having it finished long at 28-29" and i already have a 7 twist blank. The 110 SMKs have a published recommended twist rate of 7T by Sierra the rest of the stuff i have does not have such a disclaimer

The question is, should i go with the 7T that i have on hand and roll the dice with the rest of the bullets or am i just overthinking this whole thing and the 7T will work fine with the non Sierra bullets even though they are going to have a significant higher RPM coming out of a 29"?

Im not planning to hot rod this either, as soon as i find good accuracy above 2900fps im good.

Going with the Saami Freebore spec too of .183.

Any suggestions are appreciated.

Thanks

 

[alamo5000]

I just ordered a 6mm ARC barrel and I went 1:7 twist.

That said lots of people are getting stability with as slow as 1:7.8.

It's not an apples to apples comparison but it's close enough. I personally chose to have a bit faster of a twist rate because I tend to like to shoot heavier bullets.

Generally speaking I don't think there is a lot of merit to "over spinning" a bullet but I am always open to learning new stuff. In this case you aren't going down to some crazy fast twist or anything like that. 7 vs 7.5 I don't think will make a massive difference considering your barrel will be more than long enough.

Again though I opted for 7 just because I could.

 

[Mr. F]

Yeah, 6 ARC could be 300-400fps slower than 6 Creed with the same 105 pill tho.

 

[alamo5000]

I have a theory on twist rates and such. I'm constantly learning so maybe someone will chime in with an authoritative view.

Twist needed is a function of bullet geometry (shape) and velocity (which is also a function of barrel length). If the bullet is longer and has longer bearing surfaces generally speaking it will need a faster twist.

That said there is a limit on things. I speculate that early examples of the valkyrie show this. The heavy bullets were too long and very skinny with thin jackets. Then they tried to send those long thin bullets down fast twist rates and the physical material that the bullet was made of couldn't handle the stress.

Really you need to figure out roughly what velocity range and a general idea of what types of bullets you plan to shoot.

From all of my reading on the subject unless you plan to shoot heavy bullets like 115 grain or whatever there is no need to go 1:7 but again I don't think it will matter a whole lot. It might have some complicated effect on the bullet at extended ranges but even then I don't know if that's a valid thing.

 

[TheOfficeT-Rex]

You're overthinking it.

Yes, they'll be a bit overspun, but that doesn't mean they'll disintegrate as soon as you hit 300k RPM. If those are the bullets you can get, run them at low 2900s.

For reference, MV*720/Twist = RPM. In your example, you'd be right at 298k RPM. IIRC, Berger recommends keeping it below 300k, and beyond that, as slow as you can spin it while still keeping >1,5 SG (to minimize any effect of jacket thickness or uneven radial weight distribution, without being so slow you degrade the BC).

If it really worries you, this is the perfect excuse to put a fancy new left hand gain twist mod400bb carbon fibre bartlein on order while you continue to scrounge for heavier 6mm bullets for your 7 twist.

 

[TheOfficeT-Rex]

I have a theory on twist rates and such. I'm constantly learning so maybe someone will chime in with an authoritative view.

Twist needed is a function of bullet geometry (shape) and velocity (which is also a function of barrel length). If the bullet is longer and has longer bearing surfaces generally speaking it will need a faster twist.

That said there is a limit on things. I speculate that early examples of the valkyrie show this. The heavy bullets were too long and very skinny with thin jackets. Then they tried to send those long thin bullets down fast twist rates and the physical material that the bullet was made of couldn't handle the stress.

Really you need to figure out roughly what velocity range and a general idea of what types of bullets you plan to shoot.

From all of my reading on the subject unless you plan to shoot heavy bullets like 115 grain or whatever there is no need to go 1:7 but again I don't think it will matter a whole lot. It might have some complicated effect on the bullet at extended ranges but even then I don't know if that's a valid thing.

You're on the right track...
Twist required is primarily driven by bullet length, and to a much lesser extent, shape. Longer bullet for a given calibre = faster twist required. I think Litz goes into more depth in one of the No BS BC podcasts.

I can't comment on the valkyrie specifically, but most bullet blow ups happen with thin jacketed, non bonded match or varmint bullets at high velocities, which drives a higher bullet RPM. Considering a 224 sends them at almost 3000 fps, I can see how a 6 twist barrel will make them shed jackets.