175 grain SMK with 1:12 rifling

Status
Not open for further replies.
1

1-1 Banger

Member
Joined
Aug 26, 2012
Messages
104
http://riflemansjournal.blogspot.de/2010/06/ballistics-heavy-bullets-in-113-twist.html

I read here and an earlier post I put up that my 175s would more than likely stabilize out of a barrel with 1:12 rifling in my 308. I'm probably worrying for nothing but will the slower twist have adverse affects at longer range as opposed to using say 168 grain rounds? I haven't gotten to shoot the rifle much, I bought it on leave, got a VERY rough zero, then had to pack it up. I haven't seen let alone fired it for going on a year and a half now so I've started doubting it.
 
Everything that Mr. Salazar says in his blog indicates that it will work. Thanks for posting that, it is an interesting read.

One point that I don't quite understand is this:

Accordingly, MV plays a very small role in gyroscopic stability.
Click to expand...

Obviously, the faster a bullet travels in any given twist barrel, the faster it spins. Mr. Salazar claims that the forces acting on the bullet nose at higher velocities cancel out the stability gained by the faster spin. I suppose that could be true.

My personal experience is with 80 gr. SMK's in .223. Long story short, my 20" 1:9 Savage 10PC won't stabilize that bullet, but another shooters Savage Varmint configuration with a 24" 1:9 barrel will stabilize it nicely. Prior to reading the blog, I had concluded that the greater velocity he got from the extra barrel length allowed the bullet to stabilize.

Laphroaig
 
Obviously, the faster a bullet travels in any given twist barrel, the faster it spins. Mr. Salazar claims that the forces acting on the bullet nose at higher velocities cancel out the stability gained by the faster spin. I suppose that could be true.
Click to expand...

What this means is basically that if you have a round spinning too fast, it will be just as unstable as a round that was understabilized by rifling that is too slow. Think of the bullet like a top, spin it too slow and it falls over, spin it too fast and it flies around erratically because of the forces acting on it's axis. The gyroscopic axis he refers to is an imaginary line drawn from the tip of the bullet, straight through it and extending through the bottom of it. For it to be stabilized properly, it must spin around that imaginary line without wobbling off of it. Spun to fast out of fast rifling, the round will begin to yaw horizontally and vertically around that axis instead of spinning straight.
 
What this means is basically that if you have a round spinning too fast, it will be just as unstable as a round that was understabilized by rifling that is too slow. Think of the bullet like a top, spin it too slow and it falls over, spin it too fast and it flies around erratically because of the forces acting on it's axis.
Click to expand...

Yeah, but there's a wide latitude before you get to the point of a bullet being spun too fast. With bullet weights and twist rates considered normal for a particular cartridge, you aren't going to reach that point while staying within SAAMI pressure specs.

Don
 
About 15 years ago I bought a 40X from The Remington custom shop. It was a left hand Palma single shot. Instead of being labeled .308 it was stamped 7.62. The twist was 11.25. With 175's it would group consistently 3/8" 5 shots at 200 yards. Back then, most long range shooters chose Varget as the powder.
 
Status
Not open for further replies.
Back
Top