bullet lenght and key holes

thres · Posted: Wed Apr 16, 2008 4:13 am Post subject: bullet lenght and key holes

hey does any one know what length sierra 65 gr bullets are. my bro tryed loading some of them and got some key holes. guessing he got the wrong twist rate. going to work out via the greenlaw formula what weight his gun would prefer.

SingleShotLover · Posted: Wed Apr 16, 2008 4:54 am Post subject: Re: bullet lenght and key holes

What caliber is the rifle in question? If it is a .224 caliber (.223, 22/250 etc.) the twist is probably too slow to stabilize the bullet. .223s generally have a 1/12" twist while 22/250s traditionally have a 1/14" twist rate. While either will stabilize bullets up to around 60 grains, anything longer can easily become unstable and key-hole. Many newer rifles are coming out with faster twist rates (notably Savage) that allow heavier (read "longer") bullets to be used in certain rifles.

Pumpkinslinger · Posted: Wed Apr 16, 2008 8:37 am Post subject: Re: bullet lenght and key holes

If he loaded some why can't you just measure the length...

If you have Excel I can send you a file to calculate Greenhill twist or length. It's just the basic formula and doesn't factor in velocity over 2000 ft/sec. Over 2,000 ft/sec you can get by with a slightly slower twist than the Greenhill formula calculates.

SwampFox · Posted: Wed Apr 16, 2008 11:00 am Post subject: Re: bullet lenght and key holes

Thres,
In looking at the Sierra bullet data base, the only bullet close to 65 grains is the 63 grain Sierra .244 Varmiter. It is .76 inches in length with a .231 BC over 2,800 fps and .235 over 2,000 fps.
The data could be out dated but should be close if Sierra has in fact introduced a 65. The Sierra .243 bullets are 60 and 70 gr.
Best,
Ed

SingleShotLover · Posted: Thu Apr 17, 2008 4:50 am Post subject: Re: bullet lenght and key holes

An interesting formula for determining optimum twist rate is as follows:

180 divided by the length of the bullet in calibers multiplied by the bullet diameter.

Example: If a Hornady 55 grain 22-caliber SP (.224 caliber) bullet is .701" long

.701 (length) / .224 (diameter) = 3.129 (length of bullet in calibers)
180 / 3.129 = 57.53
57.53 X .224 = 12.89" (1 turn in 12.9 inches)

For anticipated velocities under 3,000 fps, the factor of 150 is usually more accurate than the factor of 180.