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.

[Scotchmo]

Accuracy about the same with aluminum or steel barrel sleeve.

I got some efficiency numbers. The dual 2L bottles give about 60 shots when going from 4000 to 3000psi. And then a few more shots just off the regulator. That long barrel gives good efficiency. I’m starting to think that the slow acceleration of a heavy projectile in a very long barrel is not conducive to good accuracy. The projectile spends a long time in the barrel after the trigger is pulled.

[Scotchmo]

I’m posting an update on my progress. Well, for the last year, I’ve been having back issues (and more). I decided back in December that dealing with a “super heavy” bench gun was not good for the long term. I spent the last 6 weeks re-making the square tube in aluminum and the barrel housing out of carbon fiber. I also removed the 2lb weight in the stock. I’ve reduced the weight about 15lbs in total. That’s where it stands as of today. I could cut a couple more lbs by going to 2024 and 7075 aluminum for the plenum, valve, and end-cap. Also, could go back to one bottle. That’s for later consideration.

I plan on shooting it again (finally) on Monday.

I’ve got some more projects in the works for this year. A few of them related to this build. When I rebuilt it, I increased the maximum projectile length to 1.25”. I should now be able to fit 200gr or even heavier slugs, and 500fpe should be doable. Except, even though the plumbing and plenum is designed for 3900psi, I can’t reliably open the valve at pressures much over 3200psi. I’m otherwise happy with the electronic hammer at 3000psi to 3200psi. The balance piston is already too big in diameter, as it won’t reliably seal at 3000psi with the PEEK poppit. It leaks more than I want to deal with. So a project I’m planning is a piloted valve for this rifle. Hopefully opening the valve more easily at higher pressures, while still allowing a good seal at the valve face.

I want to try a faster twist than the current 1:10 TJ barrel, so another project I’m working on is to make my own barrels. First will be a .308 to compare with the TJ? And then a 6.5mm.

[Motorhead]

Looking forward to viewing this redesign ... perhaps Oregon in May ?

Be well my friend 

[sb327]

Will be interesting to see what pilot design approach you use. I enjoy the different ones I use.

Dave

[Scotchmo]

Looking forward to viewing this redesign ... perhaps Oregon in May ?

Be well my friend 

I do plan on using it at the Oregon match in May. Though my fallback will be to use my 7mm.

Cameron K and I are planning on shooting at the Sacramento club’s February 8th long range match. If the testing goes well on Monday, I’ll have the .308  at that match. I’ll bring my 7mm as well.

[Motorhead]

Looking forward to viewing this redesign ... perhaps Oregon in May ?

Be well my friend 

I do plan on using it at the Oregon match in May. Though my fallback will be to use my 7mm.

Cameron K and I are planning on shooting at the Sacramento club’s February 8th long range match. If the testing goes well on Monday, I’ll have the .308  at that match. I’ll bring my 7mm as well.

I just may need to join you hooligans and go lob some lead.

Scott, if needing a place to stay over ... doors always open 

[Scotchmo]

Scott S, Cameron K and I had a good time at the Sacramento club long range match a couple weeks ago. Since then, I bought a Lee mold for a 300 Blackout. The as cast slugs are 1.36” long. I can fit up to 1.25” long in the latest build. I made a jig and milled .06” off each end of a bunch of the slugs for testing. The final length was 1.243”, and finished weight was 218gr. At 3500psi, I was getting 1018fps average for a little over 500fpe. That’s a milestone. I had some partial valve luck trying to go higher and got one stuck in the barrel. Took quite a bit to knock it out with the custom extra long cleaning rod. Accuracy at 100yds was not great (about 3moa), and neither was the BC. I suspect that those slugs are too much for the 1:10 twist barrel. I did a bunch more testing of the 151gr and 171gr BBT at 1020fps to 1060fps. Still getting no better than about 2moa average with this 73” barrel.

Took it apart when I got home and plan on starting on the piloted valve tomorrow.

[rkr]

Shorter barrel gives better accuracy? I've had issues with long barrel and inadequate support, going for double CF tube around the liner and barrel band that very tightly attaches barrel to the air tube solved that. Any ways to stiffen your barrel more? Alternatively adding weight to the muzzle end could calm down the vibrations.