Starting a .308 ELR build

[Scotchmo]

I finally got the dual bottle arrangement built. After breaking 2 bits on some stainless all-thread, I made the banjo bolts from two 10.9 (similar to grade 8 ) M18x1.5 bolts. I would have preferred 8.8 (general purpose), but M18x1.5 bolts have a limited selection. I also bought a set of cobalt drill bits to make it a little easier. I decided to use 1/8” bits rather than smaller to reduce the chance of breakage. For safety, I prefer smaller passages on the bottle side of the regulator. I repurposed two 3d printer nozzles as restrictor jets. I have foster quick connects on the regulator inlet and outlet. That makes assembly/disassembly of the airgun much simpler, but one of them has a leak, so I still need to deal with that. I think it looks better with the 49” barrel. There’s room in the case, so I might add a big moderator but then it will be back to looking too long.

I hope to get back to the range next Friday or Saturday.

[sb327]

Dual tanks looks good.

I think the shorter barrel looks better as well. It’s still an extreme length without getting into the ‘too’ long realm.

Will be curious to see how it does ‘out there’.

Dave

[Scotchmo]

I was able to get to the range today and do some testing with the shorter barrel (was 61”, now 49”).

First the bad news. It lost about 60fps in muzzle velocity. Prior, muzzle velocities at 3500psi with the 151gr and 171gr were 1106fps and 1076fps. Now they are 1045fps and 1014fps. Even with changes to my spreadsheet including reducing the coefficient of friction in the barrel, that velocity loss is still more than expected. I’ll need to look into why later on.

Now the good news. The integrated Praxair regulator is very consistent. In 22 shots with the 171gr, I got an average velocity of 1014.0fps with a standard deviation of 1.98fps.

I shot four groups of 5 shots each at 100m. One was about 1.4moa, but the other three were:
0.99moa
1.04moa
1.06moa
So, accuracy is a little better now.

Why?
Lower velocity?
Shorter barrel?
More weight (heavy reg and twin bottles)?
Weight distribution?
Reduced velocity spread?

With the new solenoid, valve opening is still unreliable when over 3600psi. So I’ll use 3500psi for now. So I’m limited to 1014fps with the 171gr. Less than I originally wanted, but I’ll live with it for now.

The 3500psi set pressure doesn’t give near as many shots either.

I might have TJ make me another 61” barrel in the future. Probably go with a carbon sleeve next time.

Also considering getting a .257 in 1:10 TJ barrel rather than the .308 1:10, but then I’d have to get Arsenal or Noe to make me a mold for a 3.5 or 4 caliber long BBT slug.

[Scotchmo]

I had originally used Foster quick disconnects to install the regulator. Not the best for a semi-permanent, semi-rigid installation.

I had to educate myself on the various swivel fittings that might work. I ordered a few likely candidate to look at.

First I considered flange seals. They use an o-ring seal. But too bulky. Better for high flow rates, but that’s not needed for a plenum refresh.

I also looked at ORFS (o-ring face seal).

I looked at NPSM. This is an adaptation of NPT to a conical metal to metal seal.

I ended up using JIC connection. The primary USA conical metal to metal seal, with a little higher pressure rating than NPSM.

These connections are more typically used in hydraulics but can work for gases too. Time will tell.

[Motorhead]

Absolutely on FM Flare to M taper compression fittings !  As i recall these are rated when in stainless far above the pressure we use / see in our PCP's.
I grew up playing with these as my father worked in Rocketry hydraulics always having boxes of misc type fittings he had taken off site having at home he used for far less critical applications.
I too cobbled stuff together playing as his rather ignorant mini-me.   Lol

[Scotchmo]

Brennan JIC in these small sizes are rated at 6000psi for steel (what I got), and 7200psi for stainless.

There are higher rated, higher cost AN versions with tighter tolerances. Might have been what you played with. Can be used interchangeably with JIC but at the lower rating when combined.

JIC, industrial and automotive
AN, aerospace and military

I’m familiar with pipe threads, but these other fittings are mostly new to me. I was aware of them but not familiar.

[Scotchmo]

Now that I’m willing to accept a lower velocity with the 171gr .308, I tested more for best groups today. Cambria range at 2000ft elevation. I tested down to about 975fps. 1000 to 1010fps range was looking good. Best 1005fps group was about 0.64moa C-to-C, at 100yds. Vertical spread was only 0.25moa. I used a front bipod and rear squeeze bag.

I don’t remember ever doing better than that at 100yds.

[sb327]

That’s looking really good, Scott. Thumbs up.

Dave

[rsterne]

WOW, just WOW, Scott !!!!

Velocity is cool, but accuracy is everything !!!! .... Looks like Arsenal Molds are pretty decent, HUH?.... 

BTW, TJ's have a 7" twist available in .257 cal, I had them make a mandrel for it years ago for the 3.5 & 4.0 cal. long BBT's.... and NOE have molds for both for the original designs.... I don't know if a 10" twist would handle the 4.0 cal, but probably would the 3.5 cal.... I know for a fact both tumble in a 14".... have not tried them in a 10" yet....

Bob

[Scotchmo]

WOW, just WOW, Scott !!!!

Velocity is cool, but accuracy is everything !!!! .... Looks like Arsenal Molds are pretty decent, HUH?.... 

BTW, TJ's have a 7" twist available in .257 cal, I had them make a mandrel for it years ago for the 3.5 & 4.0 cal. long BBT's.... and NOE have molds for both for the original designs.... I don't know if a 10" twist would handle the 4.0 cal, but probably would the 3.5 cal.... I know for a fact both tumble in a 14".... have not tried them in a 10" yet....

Bob

The 3.5 and 4.0 caliber .257 molds at NOE have been “SOLD OUT” for a long time (at least a year?)

I don’t see the 1:7 in the TJ price lists. But I heard rumors of a 1:7 and 1:8 in .257 caliber.

The 4.0 caliber long .257 has a predicted BC that is not much better than the 3.5 caliber long .308. The .257 would save on air and lead, however, it might be a safer bet to stick with the .308, 3.5 long (171gr).

[rkr]

That's the way to go, good shooting. My best results at 100M have also been around 1000-1010 fps, 1/2" ctc was the best group.

[YEMX]

Wow!  That is absolutely FANTASTIC!!!

[Scotchmo]

I had a brief session at the range today to try the .308 at 200m. I set 4 paper targets at the top of the 200m berm. Baseline was to be the Arsenal 171gr that did well at 100yds. The 222yd group was around 2moa. So I switched to the Arsenal 152gr and shot three groups. Best group shown below was 0.62moa c-to-c. That’s the best 200yd+ group that I have ever done. Wind was low. I really wanted the the higher BC 171gr to be my primary .308 projectile. But the 152gr might have the accuracy edge. And its BC is plenty good for 300yds. I also need to have another casting session and bring the NOE 152gr back fior comparison.

The outside edge of the black circle is 1moa at that distance.

[rsterne]

In terms of equivalent stability, for the same shape slug, the twist rate is proportional to the calibre.... Therefore, a .257 that is 3.5 cal. long would require a twist rate (0.257/0.308) = 0.83 times the twist required in a .308 for the same length & shape slug.... In other words, a 12" twist in .308 works like a 10" twist in .257....

I don't know about an 8", but for sure TJs have a 7" mandrel in .257 cal....

Bob

[Scotchmo]

In terms of equivalent stability, for the same shape slug, the twist rate is proportional to the calibre.... Therefore, a .257 that is 3.5 cal. long would require a twist rate (0.257/0.308) = 0.83 times the twist required in a .308 for the same length & shape slug.... In other words, a 12" twist in .308 works like a 10" twist in .257....

I don't know about an 8", but for sure TJs have a 7" mandrel in .257 cal....

Bob

And wouldn’t a .257 at 3.5 calibers long (0.900”), require the same twist a .308 at 3.5 calibers long (1.078”)?

[rsterne]

If you are talking twist rate IN CALIBRES, then yes....  but in inches the twist rate is proportional to the calibre.... so 0.83 times for a .257 compared to a .308.... ie a 10" twist in .257 is the equivalent of a 12" in .308....

Bob

[Scotchmo]

Not sure I follow.

A 2.5 caliber long .257 works well in a 1:14 twist.

Scale that up to a .308 in all dimensions and it should also work well in a 1:14 twist. True?

1:14 for any other caliber that uses that scaled projectiles that is 2.5 calibers long. Correct?

The .257 is 0.643” long. The .308 is 0.770” long.

If the .257 weighs about 78gr, the .308 will weigh about 134gr.

Proportionally, the same slug. So the same twist rate. Is that right?

Or, would the .308 use a slower twist rate in this slug instance?

[rsterne]

For a given proportion of projectile (eg. 2.5 calibres long), the minimum twist rate for stability IN CALIBRES would be the same.... However, just as the projectile in a larger caliber is longer (in inches), so is the minimum twist rate greater (in inches).... Larger calibres naturally have a longer twist rate, and vice versa....

Using the Kolbe Twist Calculator, at 1000 fps I get the following for a stability of 1.500....

.257 cal. 101 gr. 0.900" long....    8.4"
.308 cal. 176 gr. 1.078" long....   10.0"

Note both slugs are 3.5 calibres long and the same shape....  http://www.geoffrey-kolbe.com/barrel_twist.htm

Bob

[Scotchmo]

For a given proportion of projectile (eg. 2.5 calibres long), the minimum twist rate for stability IN CALIBRES would be the same.... However, just as the projectile in a larger caliber is longer (in inches), so is the minimum twist rate greater (in inches).... Larger calibres naturally have a longer twist rate, and vice versa....

Using the Kolbe Twist Calculator, at 1000 fps I get the following for a stability of 1.500....

.257 cal. 101 gr. 0.900" long....    8.4"
.308 cal. 176 gr. 1.078" long....   10.0"

Note both slugs are 3.5 calibres long and the same shape....  http://www.geoffrey-kolbe.com/barrel_twist.htm

Bob

Thanks Bob, I thought they would have the same twist rate. My assumptions were wrong. I learned something new today. I do often use the Kolbe twist form and bullet design form calculators.

http://www.geoffrey-kolbe.com/barrel_twist.htm

http://www.geoffrey-kolbe.com/drag.htm

In the second one, the derived data for twist didn’t seem to match well with the twist calculator, at sub-sonic to mach1. So I had come up with my own assumptions, without any real verification (except some empirical data).

[rsterne]

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