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How Does Twist Rate and Bullet Shape Affect Accuracy?

 

  

 

 Ballistics is a Black Art, well not really but it sometimes seems to be with so many variables involved that subtle changes to bullet weight, powder, primer, OAL, etc, etc can have dramatic results
Usually, with 9MM at minor power factor we find that great accuracy can be obtained with all weights and shapes from 115RN to 145RN/HP and, most competition shooters will have a preference for light, medium or heavy bullets that give a perceived difference in recoil – either slower/softer (heavier bullets) or snappy/quick (lighter bullets) for faster slide movement and perceived quicker follow-up shots). Now, how can you have the same perception (at identical power factor) you ask? - see what I mean about ballistics being a black art when we start talking about perceived differences). The difference really comes down to the preference of the shooter and sometimes their level of skill. Slower, heavier bullets will allow the shooter to more easily see the full recoil arc and therefore enable to “call the shot”. Lighter bullets tend to move the slide faster and can allow for a quicker follow up shot. However, if we get too caught up in the “feel” we are relegating accuracy to a secondary consideration. Without going in to how powder or other variables can affect accuracy, some individual pistols will be inherently more accurate with certain weights or shapes and this I believe is as important or an even more important factor in selecting the perfect load for your competition ammo. There’s more on the choosing heavy or light bullets at https://tigersharkballistics.com.au/pages/recoil-heavy-or-light-bullets

When we start getting into Major Power Factor for Open division pistols other things come in to play such as requiring loads that produce enough gas to work the compensator/popple holes to minimise muzzle movement in recoil. In Major PF barrel twist rate and its relation to bullet shape can become more of a factor affecting accuracy. Generally speaking, heavier (longer for calibre) bullets require a higher twist rate to optimise stability with pistol barrels being produced anywhere from a relatively slow 1:32 to a fast 1:10. Barstow Barrels are 1:16 with Schuemann offering 1:16 to 1:32. Some bullet shapes are inherently more stable and at Major PF velocities the difference in stability can be more noticeable and is evidenced in better accuracy. Tigershark “Hollow Points” are markedly different in shape to the equivalent weight round nose projectile with much more of a truncated cone shape compared to the tapering ogive of a round nose. It is thought that this shape is more inclined to stay on axis to target particularly at high velocity and may explain why many Open division pistols shoot very accurately with HP's.
Yes, it's a black art but once you find a load combination that works for you, stick with it.

 

 Accuracy testing in 38 Super using cast lead,
Tigershark 123 RN & HP's

Note that Tigershark bullets are factory rated to 1500FPS velocities higher than that can have a detrimental effect on accuracy as the bullet can become distorted and unstable. The minimum velocity in Open division for a 123GN bullet is just over 1300FPS so usually, this is not a problem.  

Testing credit to T Mason.

More information on twist rates at  http://www.schuemann.com/Portals/0/Documentation/Webfile_Barrel_Twist_Rate.pdf

September 05, 2019 by Steve Hoy

Delivery Options

Due to the increase in courier costs, we have found it necessary to raise the threshold for our Free Delivery.  From today free shipping is for orders over $249 -which pretty much covers all products bought by the box. Additionally, for those customers in regions not covered by our Free Delivery offer, we have recently added more shipping options that are at budget rates.  These rates are available now and you can be select the best rate at check-out.

August 09, 2019 by Addition Collaborator

Do You Need Any More Proof?

Check out this group submitted by Phil Wright and shot by Darren Scott with a CZ SP01 Orange. 10 shots from 15M one hole! Phil and Darren are part of the Cobram Pistol Club team heading to the VAPA state championships and all of them are using Tigershark's (no wonder with accuracy like this!).  Thanks for sharing the image Phil.

August 07, 2019 by Steve Hoy