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In general, as you increase the weight for a given diameter you increase the length. The length of the bullet is what is used to determine the proper twist rate need to stabalize that bullet.
According to my computer programs, the optimum twist rate for the following Sierra .224 bullets applies:
53 gr MK 10.75
55 gr Var 10.45
60 gr Var 9.52
63 gr Var 9.9 This bullet is shorter than the 60
69 gr MK 8.3
My experiance is that you can get away with one number plus or minus from the optimum rate. However when you go up or down two, things may get real "if'y".
As to how bad accuracy is effected by using a .224 bullet past the weight to twist ratio that will suport that bullet weight, that depends on the distance and use. A full auto AR or 16 with a 1-12 twist and 69 gr bullets might be great at 25 yards. But do not expect head shots at 300 meters. You might see oblong holes in paper as close as 50 meters in such an instance.
The original 223/5.56 round in the M-16 which had such a nasty reputation for bad wounds was a 55 gr FMJ bullet (shorter than the 55 Var and same length as 53 gr MK) in a 1-12 twist barrel. The bullet was barely stable, but it remained barely stable out to 300 meters.
Ed
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