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Ordered some .224, 55 grain spitzer bullets from Sierra Bullets in Sedalia, Mo. When I got the order I noticed on the front of the box it was stated: Medium Velocity bullets. Having never seen this before I E mailed Sierra and asked them about the phraseology. I was informed that the bullet was not to be used for velocities over 3600 fps and the barrel twist was to be slower. None of my reloading manuals approaches 3600 fps in a 55 gr. Does anyone have an explanation for this?
 

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It has to do with the bullets construction and its capabilities not published load data for any particular .22 calibre cartridge. There are bullets made for varmit hunting that have different jacket construction and again different velocities fall within the capabilities of those bullets as well. The twist rate reference takes this into consideration as well as to present the best optimum performance to avoid what is known as over stabilization of the bullet which can create some foul flying bullets if the twist rate is too fast up to and including key holing or jacket seperation in the worst cases.
 

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It is not the "Over Stabilization" that makes for the inaccuracy but the RPM that the fast twist gives the bullet. If the construction is lighter, as you might get for older bullet designs, then the centrifugal force causes the bullet to disintegrate in flight to various degrees. Some never make it to the target.

"Over Stabilization" is a misnomer. There is a minimum rotational speed that is needed for a bullet of a certain diameter and length as calculated by the Grenhill formula or several other more complex formulas. Bullets rotated faster than that are often termed "Over Stabilized". Such excessive rotational speed does not effect stability but does determine if the centrifugal forces will destroy or distort the bullet in flight.

There are loads for the 55 gr bullet in 22-250 and 22 Jaybird as well as 223 Winchester Super Short Magnum that exceed Sierras 3600 FPS velocity limit. Heed this warning. I have had 223 bullets in my fast twist AR explode bullets on their way to the target. They where older design mundane hunting bullets not intended for the fast twist and high velocity. I stepped up to 75gr bullets and no more exploding bullets. I was able to knock down the velocity of the lesser bullets and get them to the target in-tack but the accuracy was not there.

LDBennett
 

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LD is right on the money (as usual :)).
Those bullets are just built with a lighter jacket and primarily designed to function as designed at .223 velocities out of faster twist barrels. If you exceed that, there is a very good possibility that they will do as LD said...start to disintegrate as soon as they leave the barrel.
They're kind of a middle ground between the "standard" bullet jacket and the thin-jacketed varmint (Blitz, SX, TNT, etc) bullet styles.
 

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Since the heavier bullets are expected to be spun up to high RPM's by the faster twist required for stability, the bullet construction is made to sustain higher RPM's. Some more modern lighter bullets have jackets that do fine at the high RPM's. But a common hunting bullet in 223 may not survive. Reloading manuals by the bullet manufactures tell you what really works in what twist rate barrels (usually, but if not then give them a call).

LDBennett
 
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