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Discussion Starter #1
Hello all,

I just spoke with CMMG about a 10.3" barrel for my PS90 (don't get hyped, I have the From 1)...

There is the choice of 1/7 or 1/9 twist. Apparently they both have the same accuracy, but the 1/7 is better for subsonic, supressed fire. Why not then, if I am considering supressing the weapon in the future, would I ever choose the 1/9 twist? Is the standard and ballistic off-the-shelf ammo from FN sub-sonic? If not, is the subsonic available to civilians?

Does anyone have any practical experience/comments/info on this?

Thanks,
SD
 

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FN has restricted the sub sonic from civilian sales, you will have to reload your own. from what has been said the factory SS round uses a 55 grain Sierra Game King FMJBT projo.
 

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sshink said:
Why not then, if I am considering supressing the weapon in the future, would I ever choose the 1/9 twist?
unknown. Perhaps 1/9 would tear appart certain sub-par qualty rounds? I would think that a greater twist means more accuracy, even if a marginal difference. Not sure how twist could affect "sound" though. Someone would have to explain that to me.

sshink said:
Is the standard and ballistic off-the-shelf ammo from FN sub-sonic?
No, it is high velocity. As fast as your standard .223 rounds in terms of speed out of the SBR. I foget the stat.

sshink said:
If not, is the subsonic available to civilians?
No, you will have to buy that after-market from someone that does RELOADS. That scares me.... reloads that is.

sshink said:
Does anyone have any practical experience/comments/info on this?
I am sure someone else has more input.
 

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Iread it some where that for some reason in 1/9 barrel the 55 grain boat tail rounds do not stabalize as fast and you could get baffle strikes, Flat base projos don't seam to have this problem.

If your not going to suppress the weapon now then I would get the 1/9 for general purpose shooting, That means having fun. 8) When you get the suppressor then get the 1/7 barrel for it as the barrel is now easily swapable on a PS90 SBR.
 

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1 in 7 is a faster rate of spin than 1 in 9. 1 revolution in 7 inches of barrel length compared to 1 rev in 9 inches of length 1/7 would be faster twist rate than 1/9.

I think the faster twist (1/7) would have more friction so a bit slower fps using same ammo compared to the slower twist (1/9). Maybe thats why its better for supressed?
 

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Think of it in 5.56x45 terms. The projectiles are basically the same anyways. 1/9 is better for lighter bullets. Subsonic bullets, to my knowledge, are normally heavier since they need a heavier weight to counteract the slower speed. Therefore, 1/7 would be better for the "heavier" subsonic round and 1/9 would be better for the normal velocity lighter round.
 

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If I have this figured out correctly, but I may be off the deep end as well.

I'd go with the 1 in 9 unless you will only use the SBR for subsonic heavy projectiles. While the 1 in 7 does have a faster twist, lets look at this. A 1 in 7 twist means that for every 7 inches, the projectile makes one revolution whereas in a 1 in 9 twist, it is one revolution every 9 inches.

Your short barel rifle has a barrel length of 10.3 inches. This means as the projectile leaves the barrel, it has a twist rate of around 1 1/3 revolutions every 10 inches of travel compared with a twist rate of around 1 1/8 revolutions for the 1 in 9 twist for every 10 inches of travel. Lets say you add a 10 inch supressor to the barrel. This will give you a 2 2/3 revolutions for the 1 in 7 and 2 1/4 revolutions for the 1 in 9 as it exits the supressor. While stabilization in flight is a concern for the higher weighted projectiles at lower velocities, I do not think that it is sufficient to justify buying two different barrels - one for heavy subsonic or supersonic loads, and the other for lighter supersonic loads only.

I have a 1 in 9 in my SBR and have never had a baffle strike in my supressor. Of course I use a 9mm AAC pistol supressor on both the FiveseveN and PS90 SBR. Shot grouping does not appear to be affected by using the 9mm can on the FiveseveN or PS90SBR. No keyholeing was noted either - both firing a 55g Sierra Game King boat tail projectile at just under 1120fps. I am in FLorida and very near sea level therefore I can go to 1140fps before I reach supersonic speeds. You can calculate the speed of sound at sea level by using this conversion table: http://www.srh.noaa.gov/epz/wxcalc/speedofsound.shtml

This is also a good source for calculations: http://hyperphysics.phy-astr.gsu.edu/hbase/sound/souspe3.html#c1

However, as density of the air decreases or the temperature decreases(higher altitude, cooler temps), the speed of sound changes. At sea level, I calculated it to be about 27fps every drop of 25 degrees or roughly 1 fps for every 1 degree temperature drop.

The FiveseveN also has a 1 in 9 in a 4.8" barrel meaning that the projectile only twist slightly more than 1/2 revolution as it leaves the barrel or 1 1/2 twist as it leaves the 10 inch supressor - much lower than the SBR rates of spin, however, when it reaches the same distance as the SBR projectile, the twist rate is identical, it will spin one revolution every 9 inches verses the 1 revolution every 7 inches.

While the intention is to obtain the best spin rate to prevent yaw and thus keyholeing, for a 55g supersonic bullet, the ideal twist rate is around 1 in 14 for a .224 centerfire RIFLE cartridge if the information in this link is accurate. http://www.nfa.ca/content/view/129/197/

As far as I know, the P90 comes with a 1 in 9 twist rate. It would seem to me that FN knew what they were doing when they built the P90 for use with both super and sub sonic projectiles.

You also need to consider that projectile velocities have just as much as an effect on projectile yaw as does the twist rate. So does the BC and numerous other factors. Thus it is extremely difficult to predict with certainty how one projectile at a sub sonic velocities will be different from another projectile at sub sonic velocities. The same holds true for supe sonic velocities.

Personally, I think that a 1 in 7 is too fast a twist rate for this platform unless you ONLY use subsonic loads.

My less than .02 cents.
 
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