Barrel Twist and ammo selection [Archive] - Glock Talk

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arushus
10-25-2012, 09:52
So I got rid of my 1-9 twist barrel and am installing an upper with a 1-7 twist barrel. Thing is, I have a bunch of m193 and other 55gr ammo saved up that Im worried wont shoot well out of my new barrel. Are my worries unfounded? Or should I be getting rid of my 55gr ammo and buying heavier stuff. Obviously from now on Ill be buying 62gr and heavier ammo, unless there arent any issues with shooting 55gr ammo in a 1-7 twist barrel...

Any help would be most appreciated! Thanks!

arushus
10-25-2012, 11:19
Nevermind, found my answer over on m4carbine.net. Apparently a 1-7 twist is fine for bullets as light as 40gr (it is a bullet's length that matters, not necessarily weight), and a bullet being "overstabilized" is a myth for all intents and purposes.

Gunnut 45/454
10-25-2012, 13:50
Yep- though I've found my 1:7's don't care much for bullets below 50 grs. When I load them slow they work pretty good -max charge not so much.:upeyes:

SCSU74
10-25-2012, 14:06
Depends on the gun and on the distance. My 6920 hates 55g past 50 yds. It shoots low right, when I switch to our duty round (62g) they are dead on


Sent from my iPhone... which probably auto-corrected something wrong

fnfalman
10-25-2012, 15:05
A 1:7 will group OK with 55-gr M193. Might not give it a benchrest group, but for practical purposes, it'd be fine. Though I've seen some sweet handload Nosler Ballistic Tip loads that went through a lot of different rifles with all sorts of rifling twists just fine too.

Shoot the gun and ammo first to see if there's even a problem.

K. Foster
10-25-2012, 15:36
You won’t get your best accuracy with 55g but it’ll be fine for plinking or training.

oldman11
10-25-2012, 15:45
Why go from a 1-9 to a 1-7 twist to begin with?

Jungle Work
10-25-2012, 16:17
I've been shooting 1:7 twist Colts since the mid 80s with both 55grn and 62grn bullets.
I have seen no appriciable difference in accuracy. What does effect the bullet is distance, wind and barrel lenght.

smokin762
10-25-2012, 17:14
So I got rid of my 1-9 twist barrel and am installing an upper with a 1-7 twist barrel. Thing is, I have a bunch of m193 and other 55gr ammo saved up that Im worried wont shoot well out of my new barrel. Are my worries unfounded? Or should I be getting rid of my 55gr ammo and buying heavier stuff. Obviously from now on Ill be buying 62gr and heavier ammo, unless there arent any issues with shooting 55gr ammo in a 1-7 twist barrel...

Any help would be most appreciated! Thanks!

Did you try heavier bullets in your 1/9 barrel before you changed it for a 1/7 barrel? :dunno:

I shoot Hornady Superformance 75 grain ammunition in my 1/9 barrel without any problems. With a scope, they are tight groups. With Iron Sights, not as much as I want. But that is on me, not the rifle twist.

Big Bird
10-25-2012, 17:17
I can keep M193 ammo to 3/4-1" groups with BUIS at 50 yards in my 1-7 twist PSA barrels.

K. Foster
10-25-2012, 20:14
I can keep M193 ammo to 3/4-1" groups with BUIS at 50 yards in my 1-7 twist PSA barrels.

Have you shot any groups with heavier bullets. How do they compare?

WoodenPlank
10-25-2012, 22:01
Have you shot any groups with heavier bullets. How do they compare?

At 50 yards, I doubt it'll make a difference. I've shot 1" groups at 50 with my Aimpoint, using XM193, XM855 and 5.56 TAP 75grn back to back, with relatively minimal shift in POI.

cowboy1964
10-25-2012, 22:46
55gr ain't great for long-range work anyway. 1:7 for me from now on because I want to be able to use the 77gr OTM suckers.

arushus
10-25-2012, 23:38
I didnt change barrels just to go from 1-9 twist to 1-7, It just happened that my old cheap upper wqs 1-9, and the new Spikes Tactical CHF upper Im getting is 1-7...

K. Foster
10-26-2012, 08:10
At 50 yards, I doubt it'll make a difference. I've shot 1" groups at 50 with my Aimpoint, using XM193, XM855 and 5.56 TAP 75grn back to back, with relatively minimal shift in POI.

Yeah, I didn’t word my post correctly.:embarassed: What I meant to ask BB is if he has shot that gun with heavier bullets at longer distances and what groups he got then.
As a comparison to the xm193.

Matthew Courtney
10-26-2012, 09:32
Nevermind, found my answer over on m4carbine.net. Apparently a 1-7 twist is fine for bullets as light as 40gr (it is a bullet's length that matters, not necessarily weight), and a bullet being "overstabilized" is a myth for all intents and purposes.

Bullet overstabilization is not a myth. It is a very real occurance with lighter, shorter bullets at high velocities from fast twist barrels. It simply is not a factor with 5.56 projectiles from a 1/7 twist barrels inside of 300 yards. Stretch those 40 gr bullets out to 500 yards and the bullet will no longer be pointing in the direction of travel. It will be pointing in the direction of departure because, being overstabilized it cannot turn downward, or they will turn downward at varying points in the flight path causing more vertical dispersion than heavier bullets.

Gunnut 45/454
10-26-2012, 10:02
arushus
I guess it really depends on what your uses are. If your just plinking /trianing to shoot at short ranges 0-100 yards then you'll not see the difference of using light bullets. I don't only use my AR's for HD/SD they hunt as well so I need accuracy out to 400 yards. My 1:7' twist give me minute of man accuracy out 300 yards with 55gr FMJ probably further but my eyes are not twenty anymore. So that's my personnal limit especially with irons. Scoped I can still get out there. But normally when I know longer then 200 yards shots will be the norm I go up in bullet wt 69gr or more.:supergrin:

TangoFoxtrot
10-27-2012, 04:56
I have a 1:8 on my S&W and shoot 55 gr with great accuracy.

swinokur
10-27-2012, 06:33
A chart.

cowboy1964
10-27-2012, 12:45
A chart.

I disagree with that chart. I don't think a 1:9 twist can shoot a 62gr bullet any more accurately than a 1:7 twist. Besides, it's not the weight, it's the bullet length. Hornady's 75gr bullets supposedly shoot just fine in 1:9 twists. That's the claim anyway. I realize we have to generalize to weight but that chart is overly restrictive, IMO.

arushus
10-27-2012, 12:55
Bullet overstabilization is not a myth. It is a very real occurance with lighter, shorter bullets at high velocities from fast twist barrels. It simply is not a factor with 5.56 projectiles from a 1/7 twist barrels inside of 300 yards. Stretch those 40 gr bullets out to 500 yards and the bullet will no longer be pointing in the direction of travel. It will be pointing in the direction of departure because, being overstabilized it cannot turn downward, or they will turn downward at varying points in the flight path causing more vertical dispersion than heavier bullets.

Quite right, it is a very real and very possible occurance. I just meant it isnt an issue with most any .223 load within most ranges.

Thank you for the correction!

Matthew Courtney
10-27-2012, 15:08
A chart.

There is quite a bit more overlap with 1:8 and 1:9 twist barrels than the chart indicates. In addition, A 1:9 is not optimal for 40 gr .223 bullets of typical construction. A 1:12 would likely be better.

One must keep in mind that air density plays a huge role and it is affected by temperature, humidity, and barometric pressure. Also, twist is only relevant to the extent that it affects RPM and muzzle velocity affects RPM as well. A 77 gr that exits a 16 inch 1:8 barrel at 2600 fps turns 234,000 rpm while a 40 gr that leaves a 20 inch 1:8 at 3650 fps turns 328,500 rpm. From the same 1:8 twist, we see a 40% increase in RPM due to the faster velocity. That 40 gr from a 1:12 turns 219,000 rpm, a mere 7 % decrease in RPM compared to the 77 gr from the 1:8, even though the twist is 33% slower.

At the end of the day, there is much more to it than weight, or even length, and twist. Velocity affects RPM and barrel length affects velocity. The bullet makers usually give the best guidance on the optimal twist rates for their bullets at various velocities.

Fortunately, there is a lot of range in which most bullets are stable. If you want to defend yourself and punch paper @ > 600 yards, get a 1/8 or 1/7. If you want to defend yourself and shoot varmints, get a 1:9. The tipped varmint bullets are doing very well from 1/8 twist barrels, giving them more versatility than they had with traditional varmint loads.

WoodenPlank
10-27-2012, 18:48
There is quite a bit more overlap with 1:8 and 1:9 twist barrels than the chart indicates. In addition, A 1:9 is not optimal for 40 gr .223 bullets of typical construction. A 1:12 would likely be better.

One must keep in mind that air density plays a huge role and it is affected by temperature, humidity, and barometric pressure. Also, twist is only relevant to the extent that it affects RPM and muzzle velocity affects RPM as well. A 77 gr that exits a 16 inch 1:8 barrel at 2600 fps turns 234,000 rpm while a 40 gr that leaves a 20 inch 1:8 at 3650 fps turns 328,500 rpm. From the same 1:8 twist, we see a 40% increase in RPM due to the faster velocity. That 40 gr from a 1:12 turns 219,000 rpm, a mere 7 % decrease in RPM compared to the 77 gr from the 1:8, even though the twist is 33% slower.

At the end of the day, there is much more to it than weight, or even length, and twist. Velocity affects RPM and barrel length affects velocity. The bullet makers usually give the best guidance on the optimal twist rates for their bullets at various velocities.

Fortunately, there is a lot of range in which most bullets are stable. If you want to defend yourself and punch paper @ > 600 yards, get a 1/8 or 1/7. If you want to defend yourself and shoot varmints, get a 1:9. The tipped varmint bullets are doing very well from 1/8 twist barrels, giving them more versatility than they had with traditional varmint loads.

This is why 1/7 is needed in SBRs.

Jeff82
10-27-2012, 19:16
http://img.photobucket.com/albums/v165/Jeff82/Cartridges/193-a2.jpg

Matthew Courtney
10-27-2012, 20:25
http://img.photobucket.com/albums/v165/Jeff82/Cartridges/193-a2.jpg


Please note that the M16A1 has a 1:12 twist and the A2 has a 1:7 twist.

arushus
10-28-2012, 07:49
What are the distances of the bottom two targets?

Matthew Courtney
10-28-2012, 08:06
What are the distances of the bottom two targets?

If it from the study I read, itis 100 Yards?

arushus
10-28-2012, 08:08
If it from the study I read, itis 100 Yards?

That chart is confusing...

Matthew Courtney
10-28-2012, 08:37
That chart is confusing...

All it does is offer a graphic illustration of the fact that a 1:7 twist barrel shoots m855 better than a 1:12 twist barrel without giving up accuracy with m193.