Starting a .308 ELR build

[mackeral5]

I haven't peeked in much lately, life and work has been too busy for the hobby this year...  This thread and build were a nice treat, as always thank you for sharing. 

The spider/piston carrier does not have to be held captive in the direction opposite the valve seat.  On my Bulldog's crude simple balanced valve the spider simply has an outer oring which prevents it from moving forward prior to filling. Once filled closing forces against the piston keeps the spider firmly in place against the bulldog's tube adapter/valve carrier. 

The chamber height adjustments shared by Bob, Dave, Matt, and others are a game changer for tuning dwell, IMO.  I start tuning with max chamber height/volume and a tiny vent--.015 or so,  as a means of finding my upper limit of energy, If I want to make less power than max, I slowly increase jet size until finding my desired upper limit.   Once that upper limit is achieved, I will decrease chamber height until achieving  the desired shot cycle.  It can be very snappy, or it can be very Cothran-like.  Then finish the tune with whatever hammer adjustments are available in the platform. 

the stem and piston are Cothran.  But the stem vent has been increased to .060 or larger.  I thread the top of the stem 6/32 and make jets from 6/32 setscrews. Moving the jet/vent to the very base of the chamber was key to making the valve more responsive to chamber height and hammer spring, IMO. 

[Scotchmo]

I did testing today with two buffer x-rings in the balance chamber. Yesterday’s and today’s sessions were shot until my 45min SCBA tank dropped below 3000psi. No failures in the valve, though I have not yet taken it back apart for visual inspection after today’s session. It was still doing fine at the end of today’s session. Velocity was a little higher than yesterday (as expected). The velocity is highly pressure dependent, and the bottle reg is not doing me any favors as it creep after sitting and recovers slow when shooting. Next session I’ll tether the plenum directly to the SCBA tank and manually fill to desired pressure. I’m looking for a high quality in-line reg to user for tethering in the future.

I did some accuracy testing with three different slugs. I used up what I had cast of the Arsenal 151 and 169gr today, so I’ll be doing a long casting session before the next outing. The Arsenal slugs I shot today were unweighed/unsorted, all lubed and sized. The NOE slugs were weight sorted to <+/- 0.5gr, lubed and sized.

It’s looking like 1100fps is too fast (at least for the 151gr) as the groups opened up quite a bit. I have not shot the 168gr that fast, and might not as shot-cycle/air usage/recoil all become objectionable. I’m now thinking that my longer term velocity settings might be in the 1050 to 1070fps range.

I shot some NUAH targets, set at 100m. Easy enough to get “Marksmen” level in most cases. Though I’ll need to get 200m and 300.m groups for a better evaluation.

For reference, my 7mm regularly does 1.5moa.

[Scotchmo]

I got my new scale that can measure over 150gr. The average weight of the Arsenal 170gr ended up at 171.1gr. I’ll keep everything from 170.6 to 171.6gr. There were not many >171.6, slugs with casting wrinkles tended to be less than 170.6gr.

[rsterne]

I don't have any data to confirm, but I think round O-rings would compress more than X-rings.... They should also be more "progresssive" in the force required, I think, as they will start out with minimum contact with the seats and each other, and have room to "spread out" into the square space they occupy.... Also, using the softest possible ones would be desirable, as the idea is to increase the "stopping distance" of the crash as far as possible....

Bob

[Scotchmo]

I got a tethering regulator and tried it out today. It is a used (2018, like new) Praxair ProStar PRS409249 for under $200. It’s a rare model from what I can tell. New, it cost about $1000. Almost 6 lbs. stainless steel body. 6000psi max input, 4500psi max output. Very stable over a wide input range.

Testing today was with a peek poppit. The Derin poppit had occasional valve lock No valve lock so far with the peek poppit. Tested up to 3600psi.

[rsterne]

NICE Reg !!!!

Bob

[sb327]

Looks to be a high quality reg. And at a steal. Nice.

Dave

[rsterne]

Scott, if you are looking for long lengths of CF tubing to sleeve barrels, try Rockwest Composites.... https://www.rockwestcomposites.com/tubing/round-tubing/round-carbon-fiber-tubing .... Here is a partial list....



They make all types, from Filament Wound to Twill Weave, and from Standard Modulas CF to Ultra High Modulas (VERY stiff).... and lengths up to 10 ft!.... Note the 2nd item on that list is 0.50" ID x 1.155" OD  (0.328" wall), it is up to 8 ft. long, and only $102.00 (per foot)....

Bob

[Scotchmo]

I spent the day with my friend at his 200 acre ranch in the Cambria hills. I brought my 7mm and new .308 builds. First thing on the agenda was to do a short range accuracy comparison between the two rifles. We each shot a five shot group at 100yds. John is probably a better shot than me and he was shooting the 7mm, which has already been proven to be a good long range airgun. He shot an Expert (almost Master) group on a N.U.A.H. target. I shot a Marksman (almost Sharpshooter) group with the .308. After that, we proceeded with the main event. We setup a 25”x23”x0.5” steel plate target at 600yds with a hit indicator light. It took a couple of hours to locate and access (via UTV) a suitable area on the property, and the setup the target. We used my 7mm and 308 builds to shoot. I guess the steel plate was too heavy as we rarely saw a hit indicated with the 105gr 7mm, but occasional hits indicated with the 171gr 308. After lunch, we drove the UTV back down the valley and saw that we had actually been hitting the target more than we thought. 6 hits with the 308 and 8 hits with the 7mm. The larger impacts mostly in the upper half were probably from the 308, and the smaller hits mostly in the lower half were probably from the 7mm. I plan on setting up a 12”x12”x3/8” steel plate next to the larger plate. That lighter weight plate should allow better activation of the hit indicator.

I’m still having issues with the bottle reg (lag and creep) on the .308. So I decided to go ahead with the dual bottle arrangement and also to integrate the Praxair regulator into the rifle. The additional bottle and heavy regulator will add another 9lbs. The gun is already way too heavy, so I don’t even care all that much about the extra weight.

[Scotchmo]

I was using a sub $100 Aliexpress bottle regulator. Although it was a decent fit for this build, it just does not have the consistency that I wanted. I even had it apart, replaced some o-rings, removed a couple of split Belleville washers, re-shimmed and re-lubed it. Still not great.

I came up with a new solution using dual bottles (which I wanted anyway) and the highest quality regulator. With big, quality gauges. I’m still waiting for delivery of some M18x1.5 all-thread so I can finish my dual bottle adapter. Four liters of air on board, at 4500 psi. More shots per fill. Better pressure regulation. A lot more weight.

I was doing some final polishing of the 61” TJ barrel and noticed a loose spot inside, about 12” from the muzzle. It measured 0.004” excess diameter and about 1.5” long. I have no idea how that could happen with only 4000psi air, and I don’t think it was there when I got the barrel. So I cut off 12” and now have a ~49” barrel. That means I’ll need about 50psi more to maintain the desired velocity. But I’m guessing that leaking air past the loose spot was causing a velocity loss anyway. The rifle is now about the same length as my 7mm. Here are some progress pictures.

[sb327]

It’s hard to imagine the air pressure would cause the .004” spot. About the only way I can think would be if you had a squib at some point and ran another behind it. Was the outside of barrel bulged?

You definitely have plenty air on board now 

Dave

[rsterne]

That has to the the biggest regulator anybody has ever mounted on an airgun!.... 

Bob

[Scotchmo]

It’s hard to imagine the air pressure would cause the .004” spot. About the only way I can think would be if you had a squib at some point and ran another behind it. Was the outside of barrel bulged?

You definitely have plenty air on board now 

Dave

In the case of firearms, firing another round with a squib stuck in the barrel can be dangerous. It can easily bulge the barrel or even blow it open. Big pressure difference when gun powder burns in a closed container. But in an airgun?

I’m trying to envision how an airgun could develop the 40,000psi+ that it would take to bulge the barrel. How about this: squib slug stops before exiting. Another shot is fired. Momentum in that new slug compresses the air in front of it faster than the squib can accelerate out of the way? Sounds far fetched, but at least plausible. I’m trying to understand how it happened. I think it’s plausible. Any thoughts from anyone appreciated.

That part of the liner is stuck in the DOM tube part that I cut off. So it’s most likely bulged on the outside of the liner.

[sb327]

That was the reason I thought was the momentum of the next slug compressing the air between.

Like I said, it’s hard to imagine how with the relatively low pressures we are dealing with, that was just a guess. I just can’t think of anything else right now.

Dave

[Scotchmo]

That was the reason I thought was the momentum of the next slug compressing the air between.

Like I said, it’s hard to imagine how with the relatively low pressures we are dealing with, that was just a guess. I just can’t think of anything else right now.

Dave

In that scenario, we are compressing a 48” column of 14.7psi air as the newly fired slug converges on the squib.

So how does it create a 1.5” long bulge at 48” down the barrel? 48” of 14.7psi air squeezed down to 1.5” is only about 500psi. Squeezing it down to 40,000psi, that’s only 0.018” long. No where near 1.5” long.

Now looking for another explanation for the bulge.

[cosmic]

Weak steel in a part of the barrel... I had a .457 barrel bulge from a stuck slug...

[sb327]

That was the reason I thought was the momentum of the next slug compressing the air between.

Like I said, it’s hard to imagine how with the relatively low pressures we are dealing with, that was just a guess. I just can’t think of anything else right now.

Dave

In that scenario, we are compressing a 48” column of 14.7psi air as the newly fired slug converges on the squib.

So how does it create a 1.5” long bulge at 48” down the barrel? 48” of 14.7psi air squeezed down to 1.5” is only about 500psi. Squeezing it down to 40,000psi, that’s only 0.018” long. No where near 1.5” long.

Now looking for another explanation for the bulge.

I’m not sure of the full dynamics of how a barrel obstruction causes the bulge but I know it’s quite common. Even with .22 shorts. Keep us updated. It doesn’t sound like you’ll be loosing much performance with the barrel trim, might even help in the harmonic department.

ETA: maybe it’s the lead itself causing the expansion similar to the pressure inside a swage die being transferred to the sides. ?
Dave

[Scotchmo]

That was the reason I thought was the momentum of the next slug compressing the air between.

Like I said, it’s hard to imagine how with the relatively low pressures we are dealing with, that was just a guess. I just can’t think of anything else right now.

Dave

In that scenario, we are compressing a 48” column of 14.7psi air as the newly fired slug converges on the squib.

So how does it create a 1.5” long bulge at 48” down the barrel? 48” of 14.7psi air squeezed down to 1.5” is only about 500psi. Squeezing it down to 40,000psi, that’s only 0.018” long. No where near 1.5” long.

Now looking for another explanation for the bulge.

I’m not sure of the full dynamics of how a barrel obstruction causes the bulge but I know it’s quite common. Even with .22 shorts. Keep us updated. It doesn’t sound like you’ll be loosing much performance with the barrel trim, might even help in the harmonic department.

ETA: maybe it’s the lead itself causing the expansion similar to the pressure inside a swage die being transferred to the sides. ?
Dave

The lead itself would have to expand to 0.004” bigger to make the bulge. I would think that would really make it stuck in the barrel. If there was a squib, it got fired out with the next shot. Nothing was ever stuck.

So the two slugs collide in the barrel and make a 1.5” long bulge. And then get squeezed back down and continue out the muzzle. That’s another plausible theory.

From now on I’ll keep a long rod in the gun case. And push any squibs out the breech with the rod rather than out the muzzle with air. Squibs happen in an airgun when there is a partial valve lock. I just installed a more powerful solenoid, so hopefully less chance of a squib.
 

[rsterne]

I know Marco had a barrel bulge from a squib....

Bob

[Scotchmo]

I know Marck had a barrel bulge from a squib....

Bob

I believe it and have already accepted as fact that it can happen in an airgun. I learned something new.

I had planned to possibly shorten the barrel anyway., so cutting it was not a big problem. Even though the 61” barrel tests ended prematurely, I still have sufficient data for comparison between 61” and 49”. So more to learn.

Since I shortened  the barrel, I now have room in the large gun case to add a moderator which I’m working on right now. I’m still waiting for more hardware for my dual bottle adapter.