243 - 55gr/6mm CM - 70gr for Varmint

[J_Impact]

Forgive my ignorance... not stoking the fire on the 243/6mm CM pissing match. Just looking to get educated.

I’m looking at picking up a new rifle for coyote hunting that I can eventually passing on to my daughter so that she can use it for hunting deer in Alberta.

Why am I not seeing factory lower grain ammo like 55gr for 243 not in a 6mm CM round? The lowest I’m seeing is 70gr...

 

[rifleman762]

I did some quick number crunching based on velocities and typical twist rates. The 55gr bullets will run about 3900 fps from either of those cartridges. A 1:8 twist barrel (6mm Creedmoor) will spin that bullet at 351,000 rpm. A 1:9.25 twist (243 Winchester) will spin 303568 rpm. Some very thin-jacket varmint bullets will "poof" in mid-air as low as 260,000 rpm but most V-Max and Varmageddon bullets are reliable up to around 300,000 rpm, give or take. Reliable as in they will make it to the target without exploding in mid-air.

So what about that 70gr 6mm Creedmoor? Hmmm... The Nosler 70gr Varmageddon 6mm Creedmoor ammo runs 3550 fps. In a 1:8 twist again that gives 320,000 rpm. Still pushing the envelope a bit but I'm sure Nosler has tested it to make sure it will work . But 350,000 rpm? Those bullets aren't going to make it to the coyote.

Long story short: bullet RPM limitations. Most folks never run into this issue because the ultra-fast varmint cartridges always had slow twist rates, like the 220 Swift with a 1:14 or 22-250 at 1:12.

 

[J_Impact]

I did some quick number crunching based on velocities and typical twist rates. The 55gr bullets will run about 3900 fps from either of those cartridges. A 1:8 twist barrel (6mm Creedmoor) will spin that bullet at 351,000 rpm. A 1:9.25 twist (243 Winchester) will spin 303568 rpm. Some very thin-jacket varmint bullets will "poof" in mid-air as low as 260,000 rpm but most V-Max and Varmageddon bullets are reliable up to around 300,000 rpm, give or take. Reliable as in they will make it to the target without exploding in mid-air.

So what about that 70gr 6mm Creedmoor? Hmmm... The Nosler 70gr Varmageddon 6mm Creedmoor ammo runs 3550 fps. In a 1:8 twist again that gives 320,000 rpm. Still pushing the envelope a bit but I'm sure Nosler has tested it to make sure it will work . But 350,000 rpm? Those bullets aren't going to make it to the coyote.

Long story short: bullet RPM limitations. Most folks never run into this issue because the ultra-fast varmint cartridges always had slow twist rates, like the 220 Swift with a 1:14 or 22-250 at 1:12.

Mind blown... thank you for the clear explanation!

 

[rifleman762]

Mind blown... thank you for the clear explanation!

You're welcome . I had to figure this stuff out when I tried to use a 50gr bullet in a 1:7 twist 22 Creedmoor (I actually use fireformed 22-250 brass). It doesn't work well unless you slow down the bullet to the point where you may as well use a 223. Luckily the bullets made it past the chronograph before they turned into smoke.