Reloading velocity issue: I can't figure that one out!!!

[seabast]

All of the twist rates for the 243 from Nosler show 1 in 10", Seabast, do you have a gun that has a non-standard twist rate? I assume not.

9.25" I believe

[Gun Nut]

I do not see how twist rate will change velocity, it is the pressure behind the bullet pushing it out the barrel, if it was a smooth bore that sealed up on the bullet the velocity would be the same, twist rate causes difference in stability and therefore accuracy, not velocity.

Throat length, chamber size, throat erosion, rifling depth/bore condition, all of those things can alter velocity, again you pulled an article that says a massive change of 400fps.

The throat length can be seen when you compare 30-30 rifles and 6.5x55mm rifles. The savage 340 in 30-30 has a long throat and develops lower chamber pressure than the Winchester model 94, less pressure and therefore less velocity. The 6.5x55mm military barrels are known to have long throats to accommodate long 160gr RN bullets. This lets a short bullet like a 100gr or 120gr push out a bit before hitting the lands and essentially creates a larger pressure vessel for the powder to burn in, this lowers the pressure and velocity compared to a shorter throat barrel. This being said the throat is always the same for the same rifle and does not seem to occur the same way with new rifles based around the SAAMI chamber dimensions.

Fox I have two Model 336 Marlin 30-30's one built before micro-grooving, the other with micro-grooving. The older handle cast bullets like a charm. The one with the micro-grooving scatters cast all over the map. What I’ve found to date is that the older rifle is extremely tight chambered and bullets have to be sized to .308 or they won’t chamber. In the newer rifle I’m able to chamber .309 sized bullet with no effort, and I’ve even contemplated picking up a lube/sizer to try a .310 size bullets. This current thread has reignited my thoughts on what might be causing the micro-groove rifle to scatter cast about. I was blaming it on the depth of the micro-grooves .002 as opposed to traditional grove depth of .004. It may not be the micro-grooving at all, but rather a substantial drop in projectile velocity do to the larger throat diameter. The spin on a bullet is the product of two variable the rifling and the velocity at which the bullet travels through the rifling. The loads I use to propel the cast bullet pushes it close to 2000 fps and works well in the older Marlin. In the newer Marlin with it lager diameter throat the velocity could be falling of substantially so the bullet is not traveling fast enough for the rifling to provide adequate spin to stabilize it. I once had stability problems with a cast .303 bullet, it would continually key-hole with the fast burning powder load given in the manual. Then I read somewhere about using a reduced load of medium burning power, so I tried it and it straighten the bullet right out. Since than I’ve pretty much stuck to using the medium burning powdes when shooting cast.

You don't stop hunting because you grow old. You grow old because you stop hunting.
- Gun Nut

[seabast]

In the .243 Win. I prefer to use IMR 4064. Having said that it may be possible that the barrel on your .243 may have a different twist ratio then the gun used to establish the velocities in the manual, The variation in the twist ratio can result in a slow or fast barrel, which affects muzzle velocity. Projectile with longer bearing walls tend to give better performance in rifles with a fast twist, whereas projectile with short bearing wall give better performance in firearms with a slow twist. That aside, why is it important to match manual ballistics? In my mind, the important think is to find a load that gives consistent performance in your particular firearm, regardless of velocity. Actually lower velocity bullet that provide for consisted performance, will extend the barrel life of your firearm. Just some thoughts.

You don't stop hunting because you grow old. You grow old because you stop hunting.
- Gun Nut

Thanks for the information.

To answer your question, it's because this is the first time a load is so far away from the MV book and I'm trying to figure out is I'm doing something wrong or not. Everything started trying to match the performance of the Hornady 75g....

[Fox]

Fox I have two Model 336 Marlin 30-30's one built before micro-grooving, the other with micro-grooving. The older handle cast bullets like a charm. The one with the micro-grooving scatters cast all over the map. What I’ve found to date is that the older rifle is extremely tight chambered and bullets have to be sized to .308 or they won’t chamber. In the newer rifle I’m able to chamber .309 sized bullet with no effort, and I’ve even contemplated picking up a lube/sizer to try a .310 size bullets. This current thread has reignited my thoughts on what might be causing the micro-groove rifle to scatter cast about. I was blaming it on the depth of the micro-grooves .002 as opposed to traditional grove depth of .004. It may not be the micro-grooving at all, but rather a substantial drop in projectile velocity do to the larger throat diameter. The spin on a bullet is the product of two variable the rifling and the velocity at which the bullet travels through the rifling. The loads I use to propel the cast bullet pushes it close to 2000 fps and works well in the older Marlin. In the newer Marlin with it lager diameter throat the velocity could be falling of substantially so the bullet is not traveling fast enough for the rifling to provide adequate spin to stabilize it. I once had stability problems with a cast .303 bullet, it would continually key-hole with the fast burning powder load given in the manual. Then I read somewhere about using a reduced load of medium burning power, so I tried it and it straighten the bullet right out. Since than I’ve pretty much stuck to using the medium burning powdes when shooting cast.

You don't stop hunting because you grow old. You grow old because you stop hunting.
- Gun Nut

The micro-groove issue is a known one to cast shooters, the lead is a lot softer, even with a hard alloy, than the jacketed bullet. the cast lead bullet is sliding over the rifling rather than cutting into the rifling. The micro groove is known though to get more velocity from the same jacketed bullet as the older traditional rifling and also do that at a lower pressure, there is less resistance to the pushing into the rifling.

I think in this case the only way to compare apples to apples is to shoot the old one with factory jacketed over a chronograph and the same with the new one. Then shoot identical cast bullets in both over the chronograph and compare, your velocity difference between the 2 loads will probably be very close, meaning if the micro groove is 100fps higher with cast over the traditional then it will probably be the same with the cast, assume the bullet sizes are the same and the loads are the same.

The throat can have an impact for sure, but size your cast to .308 or .309 and try them and see, you will not know based on where the bullet hits compared to what the gun was sighted in for, numbers are the only things you can measure.