Originally Posted by
BHuij
Hopefully I've posted this in the right subforum.
I'm trying to get my head wrapped around the relationship between twist rates, projectile diameter/composition/mass, and accuracy. I've done a bunch of reading and I think I have the basics down, but I came here to get clarification and correction if I've got something wrong.
Here's what I think I've learned:
-A smaller diameter projectile must spin faster to stabilize. Spinning too fast or too slow will cause inaccuracy
-A heavier projectile must spin faster to stabilize.
-A harder and smoother projectile (i.e. j-word vs cast lead) can spin faster before destabilizing and losing accuracy (hence we can't push boolits as fast as we can push bullets)
As an example, I have a Mosin Nagant I recently started reloading for. I'm shooting Lee 185 grain round nose GCs out of it. The 1-10 barrel twist means I hit maximum accuracy with this projectile as I approach 1800 FPS. Some informal load workup testing has confirmed this. I'm not actually chronoing, but it's easy to graph a line between the minimum load velocity and maximum load velocity in my literature, and sure enough, my most accurate load comes at about 1730 FPS.
Supposedly the accuracy falloff from pushing this boolit faster comes from the fact that WW alloy lead destabilizes as it spins faster than whatever RPM is achieved at 1800FPS. However, if I decrease the mass of the projectile, i.e. by going to a 155gr mold, I have to spin even slower to properly stabilize, which translates to an even lighter load becoming necessary (maybe somewhere in the neighborhood of 1500 FPS or so?).
Of course, factory ammo comes out at well over 2000 FPS to my knowledge. It certainly kicks like a mule compared to the mouse fart 23.1 grain load of IMR 4227 I'm getting best accuracy out of with my homebrew boolits.
Basically, due to the 1-10 twist of the rifle, if I want to shoot cast rather than buying j-word ammo, I'm basically capped at 1800 FPS unless I want to sacrifice considerable accuracy. Theoretically I could go to a heavier projectile (200 grains +?) if I wanted to be able to shoot faster, but I don't see an extra 15 or 20 grains of mass getting me even up to 2000FPS before my accuracy craps out. Is this all correct?
The reason I ask is because I'm a fairly bad shot with a rifle, and even casting and reloading for my MN, I'm still spending close to $15 to shoot 50 rounds every time I go to the range. I'm looking to get a rifle that I can reload for dirt cheap, where I can go and shoot hundreds of rounds for practice while keeping costs way down. 22LR is an obvious choice for economy (minus reloadability), but I'd like something I can get out a little further so I can practice shooting at 200 yards + for cheap.
I've looked into several small-caliber centerfire rifles, like .17 Hornet, .22 Hornet, etc. Something where the lead and powder cost per round become almost negligible and the price to shoot comes down to barely more than a primer. Unfortunately, it looks like most of these calibers are made to be well over 3000 FPS, often into the 4000+ FPS territory. If I can't push my MN past 1800 FPS, how fast can I reasonably expect to push a lead .17 projectile before it destabilizes? I've even heard stories of smaller projectiles spinning so fast that they completely disintegrate a few yards out of the muzzle.
I'd like to find something that costs pennies per round to shoot, which indicates small caliber. But also something where I'm not neutering the round's speed or accuracy by shooting lead instead of buying factory projectiles. Kinda seems like if that's what I want, I need a long barrel with a very slow twist, which probably puts me into "super expensive custom rifle" territory, defeating the purpose of trying to shoot cheap.
Any and all feedback is welcome. Thanks in advance!