Starting a .308 ELR build

[Scotchmo]

Yes, the 2nd calculator is just an approximation.... The Twist calculator shows what happens around Mach 1 much better.... The worst stability is just below Mach 1, requiring the fastest twist.... If a bullet is stable there, it will work anywhere....

Bob

And that’s where I like to see a 1.5 stability factor. Right at that inflection point.

[rsterne]

Can't argue against that!.... Pretty much guaranteed plenty of stability and therefore the best BC at whatever velocity you choose.... One thing to consider, though, is that the Kolbe Twist Calculator is probably somewhat conservative at the "below Mach" inflection point when working with boattails.... Miles Morris tells me you can probably get away with a slower twist on a BBT than the Kolbe minimum, and actual testing by quite a few shooters confirms that.... Imagine if you will rounding off the lower spike on the curve starting at 1000 fps, dipping about half as far as the spike, and then rejoining the vertical line at about the height of the 1000 fps point.... Something like this....



In other words, about a 10% slower twist than the inflection point.... Unfortunately, each slug will be a bit different, but it's something to keep in mind if you are wondering if a given slug will be stable in a given twist....

Bob

[Scotchmo]

Bob,

Also, in a marginal twist rate situation, a slower muzzle velocity could help stability. The graph for that example shows 1:7.5 twist for 1060fps but only a 1:9 twist for 900fps.

My preference for ELR is 1000fps+, but I know some others having great success out to 300.yds at under 900fps.

[Scotchmo]

I got my 73” TJ barrel liner prepped and tested.

Performance definitely better. With the regulator gauge reading 3000psi (it reads high, so actual pressure closer to 2900psi). 171gr at 1060fps and 151gr at 1090fps.

I used a 1” OD aluminum sleeve over the barrel and that saved about 8lb compared to the steel sleeve. Accuracy was about 2moa at 100m. So I’ll be finishing the 1” OD steel sleeve to compare. Might just be the barrel prep, so will also work on that some more.

Another part of this project for 2025 will be an attempt at making my own barrels. I ordered and received a button to form 1:8 twist .308 rifling. I’m going to convert my hydraulic pipe bender into a press for pushing the button. There will likely be some trial and error experimentation.