View Full Version : Rate of twist
Sorry, new to the AR and I've been doing a LOT of reading since purchasing my first AR on rate of twist. Seems the general consensus is kind of two sided. Faster rates are recommended for carbine length barrels and faster rate of twist is recommended for heavier bullets. I guess my question is pretty specific. For my new purchase, a 16 inch barrel with a 1 in 7 twist I seem recommendations in the 60 to 80 grain bullet weight (depending on source). Sounds like the 62gr m855 would be pretty ideal. How much of a problem would going to a lighter m193 type round be? Assuming this wouldn't really matter at short and medium ranges? At what range does it become a factor? Or does it?
ColtSeavers
09-20-17, 18:22
The short, short version.
1:7 will shoot everything.
1:8 will shoot everything except possibly the berger VLDs and AMAX 80 grainers.
Bullet construction and shape are very important factors along with barrel twist rate and, as always, every barrel is an individual case study.
If you don't reload 80+ gr bullets, even 1:8 will be just fine.
I haven't had any issues at short to mid range using 55 grain with my 16" 1/7. I can't comment on long distance shooting with it though.
Sounds great, thanks for the affirmation!
The short, short version.
1:8 will shoot everything.
1:7 will shoot everything except possibly the berger VLDs and AMAX 80 grainers.
Bullet construction and shape are very important factors along with barrel twist rate and, as always, every barrel is an individual case study.
If you don't reload 80gr bullets, 1:7 will be just fine.
I think you got that backwards:
1:7 will shoot everything.
1:8 will shoot everything except really long bullets.
(According to Bergers (http://www.bergerbullets.com/twist-rate-calculator/) the 80 gr VLD should be good in a 1-8 at 2650 fps, but 1-7 is preferred.... the 80gr Long Range BT and longer need 1-7)
The shorter the bullet (generally lighter) the slower the twist you can use, but faster twists shouldn't hurt anything.
It depends on what kind of accuracy you expect. I shot 80g bullets out to 600 yards with a 1:8 twist barrel with success. The standard for my match load in a match barrel was under 2" at 300 yards or I would not use the load/barrel combination in a match. I could meet that goal with a 1:7 barrel with 80g and heavier projectiles. I was never able to meet that goal with 55g - 64g projectiles.
Shooting reduced course matches at 200 yards, my 1:7 carbine did not shoot 50g-64g bullets well enough to shoot a Master score. If under 10" at 300 yards is acceptable, a 1:7 carbine will shoot 55g-64g projectiles at an acceptable level.
Three things I have learned from barrel selection and reloading match ammunition: 1) barrel twist rate is a recommendation for a particular projectile weight, but bearing surface has a lot more to do with accuracy than bullet weight. 2) the barrel manufacturer has a great deal of influence on how well a barrel will shoot. 3) you won't know how it shoots until you actually shoot groups on paper.
The big question is "what kind of accuracy do you expect and at what distances?"
I use ballistics calculators as a guide. I've based barrel and projectile selection on results at known distances. I prefer a 1:8 twist rate for projectile weights of 80g and lighter. A 1:8 twist barrel with a quality 80g projectile will get the job done out to 600 yards.
ColtSeavers
09-20-17, 22:08
I think you got that backwards:
1:7 will shoot everything.
1:8 will shoot everything except really long bullets.
(According to Bergers (http://www.bergerbullets.com/twist-rate-calculator/) the 80 gr VLD should be good in a 1-8 at 2650 fps, but 1-7 is preferred.... the 80gr Long Range BT and longer need 1-7)
The shorter the bullet (generally lighter) the slower the twist you can use, but faster twists shouldn't hurt anything.
Absolutely no idea how I got my head that far up into my fourth point of contact, thank you for the course correction.
If you are shooting m855 or m193 there is no noticeable difference. As said, even with match grade ammo, you *may* see a difference between 1-7 and 1-8. Iirc most 1/7 twist barrels are actually 1:7.7.
If I was specing a general purpose barrel, I would require it to be 1:7 or 1:8, as long as it's either, other factors become more important, like is the vendor reliable, durability, etc.
Keep in mind, a shorty 1:7 barrel will shoot like crap but a good 1:7 or 1:8 barrel will shoot 77gr or less bullets well (down to at least 50gr vmax).
If you are shooting m855 or m193 there is no noticeable difference. As said, even with match grade ammo, you *may* see a difference between 1-7 and 1-8. Iirc most 1/7 twist barrels are actually 1:7.7.
If I was specing a general purpose barrel, I would require it to be 1:7 or 1:8, as long as it's either, other factors become more important, like is the vendor reliable, durability, etc.
Keep in mind, a shorty 1:7 barrel will shoot like crap but a good 1:7 or 1:8 barrel will shoot 77gr or less bullets well (down to at least 50gr vmax).
What size groups were you able to shoot at 300 yards with a 1:7 twist barrel and 50g V-Max? How did the groups compare to a 1:8 twist barrel?
Renegade04
09-21-17, 07:53
https://i.imgur.com/wiuhctk.jpg
Will an 80gr VLD fit in a PMag? My understanding was that 1/8 could stabilize anything that would fit in a pmag except possibly in arctic conditions.
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What size groups were you able to shoot at 300 yards with a 1:7 twist barrel and 50g V-Max? How did the groups compare to a 1:8 twist barrel?
1.3" at 100yds. Chrome lined barrel and 1-4x scope.
300yds I've only done once with 50gr and it was keeping within a 5.5" group.
I would not make much to do with that weight at 300yd as it's a poor bullet choice.
sundance435
10-03-17, 11:53
Sorry to be a little specific here, but I finally bought a PWS Mk1 Mod 2 after considering it for a long while (won't lie, current events sparked the decision) and I have a decent amount of 77gr OTM on hand (mostly IMI). Any issues with that weight in the 1:8 barrel?
Thanks
Sorry to be a little specific here, but I finally bought a PWS Mk1 Mod 2 after considering it for a long while (won't lie, current events sparked the decision) and I have a decent amount of 77gr OTM on hand (mostly IMI). Any issues with that weight in the 1:8 barrel?
Thanks
I think you'll be fine with that combination. I have a Sionics rifle with their 16" chrome lined, medium contour 1:8 barrel rand I've shot fine with it using BH factory reloaded 77gr OTM out to 600 yards.
sundance435
10-03-17, 13:42
I think you'll be fine with that combination. I have a Sionics rifle with their 16" chrome lined, medium contour 1:8 barrel rand I've shot fine with it using BH factory reloaded 77gr OTM out to 600 yards.
Thanks. I've seen reports on the other forum that so-and-so's PWS hates 77gr, loves 55gr., hates SS109, etc., just all over the place. I realize it comes down a lot to the actual ammo/brand itself, but I've never owned an AR with anything but a 1:7 barrel.
Thanks. I've seen reports on the other forum that so-and-so's PWS hates 77gr, loves 55gr., hates SS109, etc., just all over the place. I realize it comes down a lot to the actual ammo/brand itself, but I've never owned an AR with anything but a 1:7 barrel.
We can give you a general idea on how it should shoot, but you really won't know until you shoot groups with your rifle. I owned a 1:7 twist carbine that shot great groups with 55g Hornady V-Max and mediocre groups with 75g ammunition. My 1:8 twist carbines shot both well.
Let us know how your rig shoots 77g OTM at 300 yards.
As others have stated, you should be fine to shoot just about anything. Now, assuming specifically what your rifle will like is a different story. V-Max and FGGM are always a pretty safe bet to do well out of a good majority of barrels. Outside of match grade loads, range/mil-surp/plinking variants are a mixed bag in terms of grouping, precision and accuracy.
You should be able to find a specific brand or two that shoot relatively well for either m193 or m855. At the end of the day, those loads were never meant for extreme precision. Pat Rogers gave a speech (https://m.youtube.com/watch?v=0Ej-xCW_YQs) where he explained in pretty simple terms what's realistic to expect out of a traditional AR and mil-surp ammo. If nothing else is gained from this thread, set aside an hour and watch that whole video. Great discussion of the AR platform that's absolutely worth the time. Sadly it was his last before passing.
Again, you shouldn't have any concern with using just about any weight bullet out there. Especially the traditional 55 and 62gr loads. Just buy a bunch of different brands and have fun testing them out. While not guaranteed, finding a non-match round that can hang around 2 MOA fairly consistently, out of a given barrel, isn't all that uncommon.
That's the bottom line. But, if you're up for some more "food for thought" regarding bullet weights and twist rates, feel free to read on...
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As others have indicated, weight is only part of the equation. Length is equally important. The reason weight is used so often is pretty simple. Generally, the heavier the bullet, the longer it will be. So, it's just quick and easy to go off of weight. However, while it's a pretty "safe" rule of thumb, there are certainly exceptions. An A-Max with a polymer tip, or an all copper solid like the TSX, will be longer than a SMK of equal or slightly greater weight. On the flip side, a DRT projectile with a powder core is going to be shorter than a SMK of equal or lesser weight.
When it comes to twist rates, what's stamped on the barrel or listed in the spec sheet may not be completely accurate. A 1:7" might be more like a 1:7.5", and a 1:9" might be more like a 1:8.5". If you're ever bored, you can measure the twist rate of your barrels with a cleaning rod (https://m.youtube.com/watch?v=cdoCNZBlRGQ). It's not foolproof. But doing it a handful of times and finding the average will give you a pretty good idea. Definitely unnecessary for your current barrel in question. But, it's a knowledge worth having in the event you run into a barrel with no other indication of twist rate.
The whole point of this appendix is twofold. First, I'm a weirdo who enjoys writing. Second, to illustrate that weight and twist are a good way to make some informed assumptions; but certainty requires testing the actual barrel/bullet combo on hand.
Enjoy the new rifle!
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