6 cm twist rates (7 vs 7.5) and elevation

[bojangles808]

I had a question about twist rates. Looking to shoot factory 108 eldm out of a 24 inch 6cm Tikka prefit. With 7 twist I did the RPM calculations with 2950 fps and id be about 303k which sounds bad. 7.5 twist is 283k. Does that improve at elevation or get worse and what's the role of elevation? Majority of my shooting is around 0-1500 elevation but sometimes do hunt 7k plus. Does any special throat have an effcect in things (I see PVA offers different 6mm throat). I can't shoot suppressed so want to stick with 24 inch for maximum velocity but still struggling to understand this stability and don't want to spend all this money on a prefit only to have bullets dsinegerate. Trying to find optimum setup

 

[solarshooter]

Required twist rate to stabilize a bullet goes down (ie longer twist) as air density goes down/elevation increases. If you're good at sea level you're good at elevation. 7.5 should be plenty for all but the heaviest of the heavy 6s. Certainly the 108. There are some downsides to spinning a bullet faster than needed.

As far as throat, main reason is to "optimize" seating depth for different bullet lengths. I have a 6CM with a saami chamber and 108 Berger EHs sit with the boattail right at the shoulder neck junction, and 0.040" jump, at 2.800" COAL. I see no reason to change any of this - it's literally what the cartridge was designed around.

 

[wind gypsy]

7 or 7.5, either works. I've spec'd 7.5 for the 3 tubes ive ordered to date. I'd probably lean that way with a 24" barrel.

 

[sickles107]

All the research I did last year lead me to 7.5 for a 20" 6cm. Can't remember the exact details of the why, but it works well for me pushing 108 eldm, average 7kDA

 

[bojangles808]

There are some downsides to spinning a bullet faster than needed.

Thank you. That's mostly what I'm worried about is 303k too much spin if that's what I'm gusing with 24n barrel, 2950 fps, 7tw, factory 108 eldm? And if it's borderline does it get worse at elevation? I don't want the bullet to disintegrate. Embarrassing as I've already ordered the barrel and didn't give it more thought til now.

 

[solarshooter]

Not sure if I can tell you an exact RPM limit for a given bullet, but I think there was some discussion of it in the 6UM thread: https://rokslide.com/forums/threads/the-6um.290585/. Elevation doesn't come in to the bullet disintegration discussion, only the stability/twist rate discussion (see earlier comment). My gut is that you're fine either way but you could search that thread for info on bullet disintegration vs twist rate and speed. Number and depths of grooves can also play in to disintegration (fewer deeper grooves bite into the jacket deeper).

 

[HGL]

I've got a 1-7" twist 6 CM, shoots the 80 grain sierras just fine. I am at 150' sea level here, if that makes a difference.

 

[khuber84]

2950 is fairly slow for a 24" 6cm. You'll be able to run 3050 easy. Def go with 7.5tw. 7.5tw is plenty to stabilize pretty much everything, cept maybe a 115 vld or 110 smk at low altitude in cool temps.

 

[z987k]

Unless you're running some really thin jacket varmint bullets, you're not going to over twist them.
Put the bullets you want to shoot into the jbm stability calc and keep it above 1.5.

 

[khuber84]

I know my seekins ph2 is 8tw. It's shot everything 108-115gr just fine running 2950-3100 from it's 24" barrel. My DA here when not winter time is typically 3-6K, winter it'll get down to 1500-2K but still been stable at 400y on paper.

 

[bojangles808]

I know my seekins ph2 is 8tw. It's shot everything 108-115gr just fine running 2950-3100 from it's 24" barrel. My DA here when not winter time is typically 3-6K, winter it'll get down to 1500-2K but still been stable at 400y on paper.

Yea I think 7.5 and 8 twist definitely fine just not sure if I screwed up by ordering a 7.

Unless you're running some really thin jacket varmint bullets, you're not going to over twist them.
Put the bullets you want to shoot into the jbm stability calc and keep it above 1.5.

Well it's about 2.4 on the JBM with 7 twist and 2.1 with the 7.5 tw for comparison. That's not too much spin? I guess I will give it a go and see what happens

 

[BULLBLASTER]

Ive got 700 rounds on a 7 tw with no ill affect. 115 bergers at 2950 have been very accurate.

 

[Lawnboi]

Blowing up bullets is more a result of the grooves and bore diameter.

Iv got a lot of rounds through 7 twist 6 and 22 cal with zero issues using 6 groove .219/.237 bore barrels.

 

[atmat]

Either is fine. Pick your poison.

 

[khuber84]

Blowing up bullets is more a result of the grooves and bore diameter.

Iv got a lot of rounds through 7 twist 6 and 22 cal with zero issues using 6 groove .219/.237 bore barrels.

The non-tight bore diameter, makes a huge difference. Def found that to make a difference especially in overbore 22 cal stuff.

 

[HighUintas]

There are some reports on the net of the 108 blowing up at over 300k rpm. However, those may have just been the make of the barrel (lands, groove config + roughness) that caused it. Not sure. If you call Hornady, they may tell you what max rpm the 108 can reliably handle.

I've been looking at this too as I've been considering a 6cm and want a slightly longer barrel for that free speed.

 

[Antares]

I'm having an ACE barrel made for a Tikka to shoot the factory 108 ELDM load. It's at Kampfeld's getting chambered and threaded currently. After talking with ACE, these are the specs we settled on:

0.236" diameter
6 groove
1:8 twist
18" finished length

Not sure if that helps...

 

[Wrench]

I'm running a 7tw xc at 2985 and it's been great. I never shoot under 2500asl and haven't been above 5k with it.

 

[khuber84]

I'm having an ACE barrel made for a Tikka to shoot the factory 108 ELDM load. It's at Kampfeld's getting chambered and threaded currently. After talking with ACE, these are the specs we settled on:

0.236" diameter
6 groove
1:8 twist
18" finished length

Not sure if that helps...

I have a few their 237 bore 4gr and they shoot great. I have a 237 bore 6gr 8tw coming for my 6prc.

 

[SouthPaw]

I have a 7tw 24" 6cm barrel on the way from PVA. It will be my 2025 hunting barrel. They only offer 7tw for Tikka prefits, but the bore diameter and groove give me comfort that stuff won't blow up. I have a pile of 106tap to shoot through it. We'll see.