Twist rate for coated bullets

[puddleglum]

Hello all,

Would I be able to shoot powder coated cast bullets reasonably well out of a .338 Federal with a 1:7 twist?

I am going to start building a bolt action rifle for suppressed shooting, both supersonic and subsonic. I am leaning toward a 338 Federal since there is a lot of established load data, and I can also take advantage of the 8.6 Blackout shenanigans that are afoot, and use some of the LOOOONG heavy expanding subsonic bullets that are being made. I would be looking at a faster twist rate to use those bullets; Maker recommends a 1:6.5 or faster twist for their 350g bullets. I am thinking that fast of a twist rate wouldn't be ideal for cast. I messed around with the Berger stability calculator, and it seems that that 350g copper bullet is still stable at 1:8, but it is getting pretty close to unstable. A 1:7 twist might be a decent compromise to shoot supersonic j-words, super sonic copper solids, and the subsonic expanding copper solids.

I am curious if I could expect to have decent results lobbing coated lead with that twist rate. I would probably like to be able to do 200-220 grain supersonic cast as well 300g+ subsonics.

Is this reasonable with powder coated boolits, or am I trying to make a pony do too many tricks?

Thanks,
-pg

[charlie b]

Don't know but may be worth a try.

Keep in mind most cast bullets are significantly shorter than the jacketed so will stabilize with a longer twist (and/or slower vel). The critical part is how fast will the bullet be spinning at the muzzle. So, it is a function of twist rate and velocity.

.30 cal the limit for a 10 twist is somewhere between 2100fps and 2400fps depending on a lot of things. At 8 twist it is probably a bit lower. Powder coat helps a little it seems, but, not that much.

[BLAHUT]

Don't know but may be worth a try.

Keep in mind most cast bullets are significantly shorter than the jacketed so will stabilize with a longer twist (and/or slower vel). The critical part is how fast will the bullet be spinning at the muzzle. So, it is a function of twist rate and velocity.

.30 cal the limit for a 10 twist is somewhere between 2100fps and 2400fps depending on a lot of things. At 8 twist it is probably a bit lower. Powder coat helps a little it seems, but, not that much.

A lots depends on how hard your bullets are cast at ? You may have to play with load to see if they are stripping or are they following the twist ? Speed can have a large effect on stability ? A lot of cast bullets will do better, more accurate, at lower villosity's / speed ?
That is where the fun is at - learning ??? I shoot a lot of cast in pure lead at about 1200 FPS, any faster and accuracy suffers ?? IMHO.
Powder coating is just the lube......

[popper]

Faster twist requires a tougherbullet. With regard to lube groove or not in PC bullets, the grooves will tend to move center of gravity forward toward the center of pressure, increasing(?) stability. 1:7 in 300 BO works so it should work for 338. Copper solids will be much longer than lead. Length is used in calculations vs weight as it concerns the distance between pressure center and weight center. This is important as the force (lever) acting on rotational momentum attempts to cause precession (gyro stuff) causing corkscrew path or worst case, tumbling and bullet destruction. In a nutshell, rotations momentum is not hard to get with a heavy bullet and slower twist but high surface (side?) area will cause more 'lever' effect. Those old hollow base mini bullets solved a problem.

[JonB_in_Glencoe]

If popper says it'll work, then I'm sure it will. But he kinda lost me at precession

I kinda hope you try some non-PC boolits along side your PC coated boolits.
Especially if you heat treat your boolits, to make them less likely to strip.
>>>And depending on boolit design, If you are seating the boolit nose into the lands, you'll likely have COAL issues, just like most casters that are new to PC and new to castboolit Rifle ammo reloading.

[charlie b]

I have not had a problem with stripping in a 10 twist .308, even at 2450fps. Just plain Lyman#2 alloy. PC or lubed they did the same thing. The bullet grooves were nice and clean. But, they didn't hit near the target. The rotational path was 'interesting'.

[puddleglum]

If popper says it'll work, then I'm sure it will. But he kinda lost me at precession

I kinda hope you try some non-PC boolits along side your PC coated boolits.
Especially if you heat treat your boolits, to make them less likely to strip.
>>>And depending on boolit design, If you are seating the boolit nose into the lands, you'll likely have COAL issues, just like most casters that are new to PC and new to castboolit Rifle ammo reloading.

The main goal of this rifle will be a suppressor host. So any loads I develop for it will be going through a can. My understanding is that lubed lead is a no go with a suppressor due to lead build up, but PC is fine.

[JonB_in_Glencoe]

The main goal of this rifle will be a suppressor host. So any loads I develop for it will be going through a can. My understanding is that lubed lead is a no go with a suppressor due to lead build up, but PC is fine.

Oh, I missed that in your OP.
Yes, I've heard that also.

[popper]

Gyro precession = a spinning cylinder wants to stay in the same path but applying force to it tries to get it to move at 90 degrees to axis of motion.
Saw a posted video yrs ago, guy shooting 22 slo-mo filming. Normal trajectory, then slight damage to nose - corkscrew path. More damage and corkscrew till it would take off in wild angle. I found base damage did the same - circle pattern on target. More rotational momentum, more stable - until the center of pressure (wind force) overtakes the rotational. Axis of spin pretty much constant, Angle of wind force changes (center of pressure moves), that force acts on bullet and changes axis of spin.

[bsev81]

I almost exclusively shoot coated cast in my 338 Spectre's with 1:6.5 twist. Mostly subsonic, no issue stabilizing 315gr down to 740 fps. I haven't done much with supers but 200gr checked and coated flew fine, I only got them up to 1690 fps before moving onto another project. Best to find a powder that will give you decent case fill but keep the velocity on the slower side for the Federal with your supers.

[puddleglum]

With regard to lube groove or not in PC bullets, the grooves will tend to move center of gravity forward toward the center of pressure, increasing(?) stability.

That is very interesting. I am leaning away from going with a fast twist rate on the rifle and maybe going bigger bore and something more cast friendly. So when I am looking at molds, something with grooves, while not necessary for lubrication, will potentially be more stable.

[charlie b]

I would not worry about stability unless you go to a slower twist. Most mfg rifles have a faster twist for longer jacketed bullet stability. Cast bullets at 'higher' velocities are then spun 'too fast' and they can open up groups.

So, cast rifles is kind of a balancing act centered around the rotation rate of the bullet at the muzzle. A slower twist means you can shoot them at higher vel. But, too slow a twist and you lose stability. A fast twist means increased stability but limits how fast you can push them.

Picking a bore size is similar. For me the sweet spot is .30cal (I do not care for a lot of recoil). Others it is .338, .35, .45, etc. All depends on what you want to do with it.

[puddleglum]

I almost exclusively shoot coated cast in my 338 Spectre's with 1:6.5 twist. Mostly subsonic, no issue stabilizing 315gr down to 740 fps. I haven't done much with supers but 200gr checked and coated flew fine, I only got them up to 1690 fps before moving onto another project. Best to find a powder that will give you decent case fill but keep the velocity on the slower side for the Federal with your supers.

What mold are you using for the 315?

Thanks,
-pg

[bsev81]

What mold are you using for the 315?

Thanks,
-pg

https://noebulletmolds.com/site/prod...39-315-fn-bz1/

[mnewcomb59]

I think the 7 inch twist would really hurt your chances for supersonic success. If you wanna shoot heavy subsonics, why stop at .338? You could shoot a 395 grain bullet in a 16" twist 45 Colt, for example with Western 8.0 data. You could probably have the same mag capacity in a semiauto with 45 BM and slap a fast twist barrel on that to shoot 600 grain lead HPs. I think for cast bullets the 338 Fed would shine with full power loads with a 14" twist. 180-220 grain at full jacketed speeds would be possible. It would be like a flatter shooting, slightly more aerodynamic 358 Win shooting 30-06 weights and speeds.