Reddit Discussion Thread About Barrel Twist Rate
BY Herschel Smith5 years, 2 months ago
My 7 twist LWRC keyholes 55gr stuff. So I’d say that’s pretty bad for accuracy. Try a box out of your gun, yourself.
Yes, it’s possible to overstabilize bullets. As I’ve said before, the 1:7 twist rate was designed for a heavier round in order to stabilize the tracer rounds at 63.7 grains. It’s not optimal for 55-grain rounds.
If people would read the pages of TCJ, they would know things like this.
On January 2, 2019 at 5:40 am, Edwin said:
It’s not just projectile weight that must be considered when evaluating twist rate, but also projectile velocity, and all viewed in the context of “what am I trying to accomplish.” It’s possible to over-stabilize a 55 gr projectile expelled at maximum velocity for the projectile weight, and have that same projectile almost perfectly stabilized when expelled at a lower velocity.
It’s an “antique” caliber, but 220 Swift can provide a great, if expensive, ballistic education when one starts juggling projectile weight, projectile design, velocities, twist rates and zero distances (achieving the desired results sometimes means having to buy barrels in 6-packs…..).
“Acceptable accuracy” is a definable goal, and never as simple a subject as many would like it to be.
On January 2, 2019 at 7:09 pm, Georgiaboy61 said:
Re: “My 7 twist LWRC keyholes 55gr stuff. So I’d say that’s pretty bad for accuracy. Try a box out of your gun, yourself.”
Over-stabilization is definitely a possibility, but it may not be the only explanation for what the owner of that LWRC is seeing.
Jacketed projectiles, such as full-metal jacket military-specification bullets with a lead core surrounded by a copper alloy (gilded metal) jacket, are generally stoutly-constructed enough to stand up to the very high rpms generated by a faster twist rate barrel such as a 1:7 RHT.
On the other hand, some hunting and varmint loads, as well as some match-grade OTM loads, may not stand up well to the centrifugal force of extremely high rates of spin – and may disintegrate and fly apart during flight, or become unstable and key-hole on the target.
It isn’t simply bullet weight which determines the rate of optimum twist, but the length of the projectile as well as its design. Since nearly all bullets are designed in such a way as to be tail-heavy, i.e., have their centers of gravity behind the midline, sufficient spin is required to keep the projectile in the proper orientation in flight.
Varmint bullets, in particular, are designed to fragment easily when encountering a small game (varmint-sized) animal, rather than for the deep penetration required of some hunting bullets for larger prey species. Since varmint bullets are often hollow and have thin walls, they are more-prone to disintegrating in flight due to excessive spin.
OTM-style match bullets are made by drawing up petals of gilding metal (copper alloy) to surround and enclose a lead or other core, coming together at the pointed tip or metplat. Although more-stoutly constructed than varmint loads, it is possible to cause an OTM design to disintegrate in flight via over-stabilization. This effect has become rarer in recent years because of advances in ammunition design and greater attention by ammunition manufacturers to the use of strict QC manufacturing measures as well as high-quality starting materials.
Bonded or solid-metal bullets, on the other hand, which are designed to be extremely tough for enhanced penetration against large and/or dangerous game, after firing at high velocity from magnum or other long-action cartridge cases – are very hard to damage via over-spinning them.
As previously mentioned, over-stabilization may not be the only factor in play with the specific user in question.
The barrel may be damaged in some manner. For some reason – manufacturing defect, abuse by a previous owner or a retailer, etc. – the barrel may be damaged internally. Some of the rifling may be missing or worn-away, or perhaps the manufacturer did not cut the correct rate of twist into the barrel.
Seasoned reloaders know to slug their bores to check manufacturer specs for twist rates; it is not at all uncommon for stated rates to be markedly different from those actually found on the rifle. You may think you have a 1:7 but actually have a 1:8 or 1:9, and so on.
If there is damage to the crown of the barrel, it may be affecting POI on your targets, perhaps causing the key-holing seen. The crown is the last part of the barrel the projectile “sees,” and as such has a large influence on bullet flight.
Defective or damaged ammunition can’t be ruled out, either. If there was a defect in the manufacturing of the ammunition used, i.e., such as inaccuracies in weighing the bullets and components, or irregularities in concentricity, these would also tend to cause atypical results on target.
Twist rates are funny things anyway. A 1:9 RHT may stabilize a 75-grain OTM bullet when fired from a bolt-action with a 24″ barrel and sufficient velocity to generate the needed spin stabilization. The exactly same bullet/cartridge combination, however, when fired from a shorter barrel of the same 1:9 twist and with lower MV, may be inadequate to provide stability in flight.
Conversely, a barrel which would be expected to provide good or even just acceptable results, does not perform for some reason, with a specific load or with all loads.
Every barrel is different. The only way to discover what yours “likes” is to determine its preferences empirically, which is to say by experimenting with different loads by different manufacturers under different sets of conditions. Be scientific, keep careful notes, collect your data and you should eventually be able to form some reasonably accurate conclusions.
If you can’t sort out what your particular rifle or carbine likes, and there is no rhyme or reason to your data, then perhaps it is time to send it back to the manufacturer under warranty (you did send in your warranty card, didn’t you?), to let them sort it out.
P.S. – Many manufacturers will not honor a warranty if you have had the FA in question worked on by an outside gunsmith or armorer – i.e., one not on their staff. Moreover, some manufacturer’s warranties are voided by the use of reloads. Do your homework…know before you go!