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I am looking to re-barrel my rifle(s). Generally speaking, original barrel is 26 inches with 1:10 twist and I want to replace with a 20 inch barrel. In theory, would I lower the twist rate in order to make up for the lost length, or leave twist the same? Thank you.
 

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Need more info....what caliber are you talking and what bullet weights are you going to be shooting. Generally if you are going to a shorter barrel, you are going to lose velocity and might want to go with a tighter twist to maintain bullet stability.
 

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Sounds like a .308 rifle? Maybe 300 WM? Either way, the 1:10" will do just find with the shorter barrel
MEL
 

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There has been a trend for heavier for caliber bullets remaining supersonic farther than lighter weight projectiles. Generally as the bullet weight goes up the twist rate gets faster. Below is the link to Shilen's twist reference. Shilen is a respected barrel maker. I have two of their barrels and match bolt faces on my AR-10 style shooters and they work very well. Generally you pick the bullet weight and then you select the twist. I had some barrels made and what I did is I sent 3 loaded rounds and I requested the distance I wanted the ogive to be from the lands. Now this distance moves as your round count gets up into the thousands hence the term, "chasing the lands." My thought was the closer I had the lands start the longer it would take for the lands to burn out. If I recall correctly I picked 15 thou as the distance to the ogive. The barrels I have are crazy accurate and forgiving. I'm not going to mention the smith but his name has been mentioned here a lot.

Welcome to Shilen Rifles, Inc.

Here is the Twist rate calculator from Berger Bullet company:

https://bergerbullets.com/twist-rate-calculator/

the down side to using the Berger calculator is that they only provide their bullets as a reference guide. I'm not an engineer so I cant comment on the accuracy of results based on other manufacturers' projectiles. I do use Berger bullets.
 
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