Twist rate

[Shadow 300]

Hello everyone,
I just joined in hopes of learning some things about the .300 Whisper cartridge. I recently ordered a 7.62 Phantom suppressor from YHM. The reason is that I have always wanted to build a bolt action in .300 Whisper and use a suppressor. The can came in to my dealer and I'm just about to start the form 4 wait. The first thing I really need to know is what the twist rate must be in order to stabilize this bullet. I have heard anything slower than 1 in 8" will not stabilize a subsonic bullet this heavy with the result of possible baffle strikes. Someone else told me bullet stabilization is dependant on length of bullet relative to length of barrel. What is the truth? Thanks for your help.

[Malaga]

1:8 will work real good....

[Expatriot]

The first thing I really need to know is what the twist rate must be in order to stabilize this bullet. I have heard anything slower than 1 in 8" will not stabilize a subsonic bullet this heavy with the result of possible baffle strikes.

1:8 is only needed for the really heavy or really long bullets. 1:10 will stabilize most bullets under 200gr. It is easily possible to build subsonic loads with 150gr bullets. There is really no reason to use the ultra heavy bullets.

Someone else told me bullet stabilization is dependant on length of bullet relative to length of barrel.

I'm not sure what the length of the barrel has to do with bullet stabilization?

[Hoser]

A 1-8 will work for everything up to 240 Sierras.

I am using a 8 twist in my bolt gun with no problems at all.

[Spook]

Heavy bullets are typically higher in BC and bullets with higher BC's resist wind deflection better than light bullets and maintain downrange velocity better than light bullets.... Given those parameters its less difficult to shoot heavier bullets more accurately the further downrange the target gets.... something to consider when working with a caliber that doesnt offer a lot of bullet expansion on live targets.

That being said if you know your target will be close in and/or you know that you'll never shoot anything heavier than 220gr , a 10 twist will work fine.

[amafrank]

I'd like to point out that the 1:8 twist rate allows you to not only shoot the heavies quite accurately (220-240gr) but will work quite well with the lightweights as well. We have fired 100gr plinkers with good results in the same guns we were shooting 240gr matchkings. The converse however is not the case. I have done a repair on a suppressor for a customer who learned the hard way that you don't want to shoot the 220's out of a 1:10 twist barrel subsonic. Wiped out 3 baffles and the endcap. If you are planning on benchrest shooting of the 300 than you should really pick a particular bullet you intend to shoot and choose the twist rate for that but otherwise choose a twist for the heaviest bullet you shoot and the lighter ones will work too. Overstabilizing of bullets is not normally a problem unless you are shooting extreme ranges, are doing indirect fire at extreme range or if you are shooting very thin jackets at high velocity. The faster twist will spin the bullets up to very high rpm if you get the velocity up. The 100gr plinkers at 2600fps were spinning pretty fast but not enough to blow the jackets up. Some varmint shooters find that they blow up the bullets by overdriving them and there are those who have posted on these boards that this is a concern for our 300's as well. I don't think that is the case however as we cannot load enough powder in the case to get the velocity that will do that. The bullets we use are not superthin jacketed either so the strength is there and the loads are not. Bottom line here is that if you intend to shoot subsonic than the heavy bullets are best for that. They carry the velocity longer and the energy as well. 1:8 is the best rate for those bullets. If your interest is strictly supersonics than 1:10 is fine.
You must choose.......


Frank

[Expatriot]

Heavy bullets are typically higher in BC and bullets with higher BC's resist wind deflection better than light bullets and maintain downrange velocity better than light bullets....

The published BC figures are only valid within a narrow velocity range. None of those BC figures are even close to reality once we go below the speed of sound.

Subsonics have a very short range and do not tend to bleed velocity very quickly regardless of weight.

[Alleycat]

BC adjusted for 1800 fps and below. It is all relative. Each bullets BC will be affected in the same way. Stabilization has to do with the length of the bullet relative to its diameter. The shape and construction have an affect as well. Some bullets have the center of gravity shifted one way or the other. Examples would be a hollow point Vs a soft point. All of this is a problem already solved 1 in 8 is the best choice. If you do 1 in 8 you can shoot any thing well. If you do a 1 in 10 you may get 10 to 20 fps with the lighter bullets, but you will be limited in your bullet weights.





Name: 240 SMK
Ballistic Coeff: 0.685
Bullet Weight: 240
Velocity: 1050
Target Distance: 100
Scope Height: 1.500
Temperature: 60
Altitude: 600

Ballistic Data
~~~~~~~~~~~~~~~
Range Elevation Velocity Energy 10mph Wind Deflect
0 yds -1.50 in 1050 fps 587 fpe 0.00 in
25 yds 1.92 in 1041 fps 578 fpe 0.04 in
50 yds 3.33 in 1033 fps 568 fpe 0.20 in
75 yds 2.70 in 1024 fps 559 fpe 0.47 in
100 yds 0.00 in 1017 fps 551 fpe 0.84 in
125 yds -4.79 in 1009 fps 542 fpe 1.31 in
150 yds -11.71 in 1001 fps 534 fpe 1.85 in
175 yds -20.80 in 994 fps 527 fpe 2.51 in
200 yds -31.99 in 987 fps 519 fpe 3.18 in
225 yds -45.44 in 980 fps 512 fpe 3.99 in
250 yds -61.18 in 974 fps 505 fpe 4.93 in
275 yds -79.26 in 967 fps 498 fpe 5.98 in
300 yds -99.69 in 961 fps 492 fpe 7.13 in
325 yds -122.50 in 955 fps 486 fpe 8.38 in
350 yds -147.71 in 949 fps 480 fpe 9.73 in
375 yds -175.34 in 943 fps 474 fpe 11.16 in
400 yds -205.40 in 937 fps 468 fpe 12.67 in
425 yds -237.92 in 931 fps 462 fpe 14.26 in
450 yds -272.91 in 926 fps 457 fpe 15.92 in
475 yds -310.37 in 920 fps 451 fpe 17.64 in
500 yds -350.33 in 915 fps 446 fpe 19.43 in




Name: 150 SMK
Ballistic Coeff: 0.355
Bullet Weight: 150
Velocity: 1050
Target Distance: 100
Scope Height: 1.500
Temperature: 60
Altitude: 600

Ballistic Data
~~~~~~~~~~~~~~~
Range Elevation Velocity Energy 10mph Wind Deflect
0 yds -1.50 in 1050 fps 367 fpe 0.00 in
25 yds 1.98 in 1033 fps 356 fpe 0.10 in
50 yds 3.43 in 1018 fps 345 fpe 0.41 in
75 yds 2.78 in 1003 fps 335 fpe 0.90 in
100 yds 0.00 in 989 fps 326 fpe 1.54 in
125 yds -4.99 in 976 fps 317 fpe 2.38 in
150 yds -12.30 in 964 fps 309 fpe 3.44 in
175 yds -21.98 in 952 fps 302 fpe 4.69 in
200 yds -34.07 in 940 fps 294 fpe 6.12 in
225 yds -48.61 in 929 fps 288 fpe 7.70 in
250 yds -65.61 in 919 fps 281 fpe 9.41 in
275 yds -85.11 in 908 fps 275 fpe 11.25 in
300 yds -107.40 in 899 fps 269 fpe 13.35 in
325 yds -132.04 in 889 fps 263 fpe 15.44 in
350 yds -159.52 in 880 fps 258 fpe 17.75 in
375 yds -189.88 in 871 fps 252 fpe 20.25 in
400 yds -223.15 in 862 fps 247 fpe 22.94 in
425 yds -259.37 in 853 fps 242 fpe 25.80 in
450 yds -298.57 in 845 fps 238 fpe 28.82 in
475 yds -340.79 in 837 fps 233 fpe 31.98 in
500 yds -386.04 in 829 fps 229 fpe 35.29 in

[winmdl98]

For calculating twist rate, here is something to consider:

http://kwk.us/twist.html

Keep in mind BC specs from bullets manufactures are usually for typical supersonic velocities. Sierra provides the info for lower velocities.

[Shadow 300]

This is some very good information guys, I really appreciate it. Since I bought my first can I have fallen in love with suppressed shooting. It is a completely different experience to be able to have a normal conversation while enjoying target practice / plinking with a friend or even wife. I have a YHM 7.62 SS can at my dealer waiting on form four. My guess is I have another 10 weeks or so to go. I really do not know which weapon to start with, AR, Contender, or Rem 700 bolt gun. I of course want all three but they are not free. I see advantages to each and I have never had a contender pistol so am not sure how well I would be able to shoot it compared to a weapon with a stock so I am leaning towards the bolt gun. PDC custom has a great folding stock for the Rem 700 with a detachable magazine so that is probably what I will do first. Any thoughts / comments? Thanks Guys