Does A 1:7 Twist AR-15 Overstabilize 55 Grain Bullets?
BY Herschel Smith
1 year, 5 months ago
I missed this, but our friend Andy at Practical Accuracy did a video on whether a 1:7 twist barrel does well with lighter bullets.
The answer? It depends on the ammunition. Ammunition selection seems to be the king-maker on whether you do well at the range.
On November 9, 2022 at 6:51 am, Joe Blow said:
I had heard this from some old timers, though it was never explained ‘why’, its just what the old timers said to do. I did it with my AR, worked for me: Wolf 72 grain fmj or hp. “Try every box/brand/size/weight bullet you can find, your gun will like one of them, then shoot nothing but that.”
Theres a youtuber goes by tibosaurus rex (or something like that) who has a lot of good videos discussing long distance shooting. He talks about rifle harmonics, and how it is bedded, and hand grips, and points of contact, slings, and… a whole lot of different factors determine how your gun will shoot. Hand-loaders noodled it out on paper with a chronograph. I sat down on a 100 yd benchrest with about 20 different boxes/brands of ammo (over 3 range trips). The heavier bullets did better (24″ barrel 1:8 twist), lighter ones (55gr) didn’t even find paper! In the end, the Wolf rounds were repeatable, reliable, and cheap! Stack it deep!
On November 9, 2022 at 8:46 am, MN Steel said:
This holds true for every rifle if you plan to use pre-rolls. I had a Mini-14 Ranch Rifle with the skinny barrel that sprayed a cone of fire with everything I fed it, until I tried Win White Box 5.56 and it would cloverleaf until the barrel heated.
If you roll your own or know someone that can, there are nearly endless possibilities to drive tacks.
One caveat: if you plan on sharing during a time of tension, buy ball ammo for the needy because ot will at least feed and go bang; that home-rolled HP/PSP/OTM/Hog Hammer might get hung and jammed if the feed ramps aren’t the same angle or polish as yours.
I’ll take something that goes bang and goes in the right direction over playing SPORTS all day every time.
On November 9, 2022 at 10:19 am, Latigo Morgan said:
Definitely fit your ammo to your rifle. I was sitting with about 1/2 a case of Hornady black box 77 gr. in 5.56 and wound up clicking on a youtube video that just completely trashed that round.
Well, damn, did I buy a pig in a poke? It was the only pig in the poke at the time I bought it, so there wasn’t much choice. Took it out to the range and it was a sub-MOA round out of my 1:7 FN barrel on a PSA kit. Not too flippin’ shabby. Just wish I had more of it.
On November 9, 2022 at 2:04 pm, James said:
oh gosh. i was able to shoot 3gun under a timer at up to 600yrds with M193 ball with both a 1/7 16″ NATO chamber and a 18″ 1/7 Wylde chamber. Handload or don’t use shitty ammo.
On November 9, 2022 at 6:20 pm, Paul B said:
Finding ammo your gun likes is always key. That is why some people hand load. But buying something off the rack is double good.
On November 10, 2022 at 1:26 am, Georgiaboy61 said:
It is possible to over-spin varmint loads with thin walls, which are designed that way to fragment dramatically at the small game (varmint) target. Many of these loads are designed to be shot at muzzle velocities well in excess of 3,000 fps, and in the case of cartridges like 22-250 and 220 Swift, approaching 4,000 fps. Apologies for not being able to find the reference from which that came, but it has been some years since I saw it.
It is unlikely to “over-spin” garden-variety 55-grain M193 Ball/FMJ even using a fairly quick twist such as a 1:7 or even a 1:6.5. If it is true mil-spec M80 Ball, then it is designed to military specifications set by NATO, and likely to be durable and tough in hard use, not at all like varmint ammunition which is designed to fragment easily when encountering a target.
Potential over-spin does occur in one circumstance, namely ELR shooting. Those who use rifles to shoot long range and extreme long-range have to think about some of the same problems artillerymen do, namely choosing the correct spin rate to cause stability in flight, but not so much spin that the projectile remains nose-up in the terminal phase of its flight and therefore does not impact at the target in the proper nose-first attitude required to activate a contact fuse, for example.
Conversely, the projectile has to retain enough rpms of spin to remain in the nose-first attitude, and not “swap ends,” as it might be prone to do otherwise. That depends on the weight distribution of the projectile and whether it is tail-heavy or not, and if so, to what degree. Spin stabilization and what students of physics will recognize as the “right hand rule” keep spinning projectiles of this kind oriented properly.
On November 10, 2022 at 1:33 am, Georgiaboy61 said:
It bears mentioning, too, that every rifle and barrel is different. Whereas conventional wisdom may predict that a 1:9 twist will not adequately stabilize .224-caliber bullets in the 70-80-grain range, some barrels will in fact handle them fine, or at least some of the loads in the 70-77-grain range. That same conventional wisdom also predicts that 1:7 twist barrels are too fast for 55-grain loads – which therefore ought not to print accurately. But there are plenty of 1:7 barrels which handle that weight/type of load just fine.
To second what others have stated, you simply need to do real-world empirical testing at the range and in the field to determine which loads and precise flavors of ammo your rifle and barrel prefer and which don’t work as well.
One last nugget of information provided to me by a mentor: Sometimes, a borderline unstable load can be stabilized adequately (I’m speaking of reloading here) by pumping up the MV to the higher end of the safe range for that cartridge, load and firearm. This works well in bolt-action rifles, but in self-loading gas guns also. Bolt guns since you have a bit more free-play in terms of how hard you can push things safely and responsibly, due to the inherent strength of the design.
On November 10, 2022 at 12:33 pm, RHT447 said:
Good video. Clear and concise.
Just to pile on, I have a Winchester M-70 Heavy Varmint in .223, 26-inch stainless fluted barrel, 1/9 twist, H.S. Precision stock. I bought it new. One of my handloads is Hornady 75 gr A-max bullets (which I molly coat) over a case full of Varget. Shoots 3/8-inch center-to-center @ 200 yds.
Also have an AR HBAR I built for NRA competition. 20-inch barrel, same 1/9 twist. I tried the above load in this rilfe and to borrow a phrase, cone of fire. Some friends at the time suggested trying Reloader 17 (IIRC) but I did not pursue it further.
Another rifle is an M1A super match built by TLC Gunworks back when they were in Clovis, CA. It also shoots bug holes, and likes 190 gr Sierra Match Kings. The original plan was to use a Krieger barrel with a 1/10 twist, but they were out of stock, so I wound up with a barrel from Maremont. One day I got a wild hair and decided to see what the Maremont twist actually was, using a tight brush on a rotating cleaning rod. The twist measured closer to 1/12.