Starting a .308 ELR build

[Scotchmo]

I've been making progress on my .308 build. Hopefully, initial range testing starts on Monday.

Yesterday, I pressurized the air tube/valve assembly to 3600psi. Dunked it in water. Bubbles came out of the transfer port, when filling. Almost stops bubbling at 3600psi, but still a few bubbles. I’m not going to worry about it for now. It might seal when I fire it a few times. It’s an acetal (like Delrin) valve head.

Yesterday, I attempted a full assembly of all parts and discovered some issues. Some tolerance stack ups prevented full assembly. Also, there were some difficult access points. I made some design corrections and completed them in the shop today, along with a new slug sizing die. The first .308 die was too short and could not reduce the nose quite enough, so seating of the slug was very difficult.

[Scotchmo]

Everything went together smoothly tonight. New plans are to start range testing tomorrow.

[Scotchmo]

I got off a few shots today. On the 60v setting, I was able to open the valve all the way up to 3600psi which was as high as I got before the plastic receiver let go. The steel housing held any plastic shrapnel in place. The max velocity that I saw was 839fps (238fpe). But shots were accompanied by blow-by out of the breech block as the plastic barrel clamping was not up to the task of holding the barrel and shroud in place axially. Design mods will include a steel or aluminum barrel clamp tied to the steel enclosure rather than the plastic receiver. I’m going for at least 400fpe, so 238fpe is too low. There was significant blow-by, so that could have been one reason, but the dwell also seemed short. I’ll deal with that after I get the barrel secured better.

[Scotchmo]

Finally had a successful day at the range with the .308. Yesterday was cut short. I had mistakenly used a 40v flyback diode and it let the smoke out after I turned the voltage to a little over 60v. I fixed that and went back today. No luck controlling the dwell electronically. This initial balanced valve configuration has just about enough dwell to reach my minimum goal of 400fpe. It was able to get  the 152gr to just over 1080fps. But close to valve lock. I settled on 1060fps. 5 shot groups at 100m were a little over 2moa in light winds. Next trip to the range will be for 300m testing.

[Scotchmo]

I did a few valve mods in an attempt to increase the valve dwell. I got off a few shots before the valve failed in a mode that I had not anticipated. I have a step on the balance piston to keep the hammer from pushing the piston forward through the valve head. But instead, the piston got pushed backward in the valve head by closing pressure. I had also used super-glue to hold it in place and that sorta worked as it sheared off a little bit of Delrin when it shifted. Next iteration will have the balance piston and valve head threaded together to resist axial forces in both directions. I also plan on switching from Delrin to PEEK.

I did reach a milestone as the last 4 shots were above my minimum design goal of 400fpe. The 1122fps shot being at 420fpe.

These few shots are not conclusive but it did give me enough data to start to analyze the "Mach trimming" effect at these velocities.

The highest velocity shot was right at Mach 1. I compared that shot to the next two slightly lower velocity shots. There was almost no Mach trimming between the Mach 0.97 and Mach 0.95 shots. But the Mach 1 shot definitely scrubbed velocity at a higher rate. There was significant trimming between the Mach 1 and Mach 0.97 shots. The velocity spread at the muzzle was 32fps, but was almost half that by 30yds and only 10fps after traveling 100 yards. That reduction in velocity spread will show up as significantly reduced vertical dispersion at farther distances 200yds to 600yds. One series of shots is hardly definitive. I'll be looking into it more later.

I'm going to take a break from the .308 development for a few weeks as I need to prepare for the 2024 RMAC match. The .308 is not at all ready for that match, so I'll be shooting my 7mm again. I took 1st in the RMAC Magnum Slug match last year with the 7mm and some luck. The competition looks just as tough this year.

[Scotchmo]

I’m working on the .308 again. I got about 30 shots off this time before the grate that supports the balance chamber let go. After I had made it, I realized I had selected too large of a drill bit for the holes in the grate. I still thought I might get away with it but it was too weak. There’s a lot of violent action going on during a shot cycle. Picture shows a grate with correct hole size and also shows the broken grate.

Today’s testing was done tethered, while I tried different plenum pressures. I might do more experimenting later at higher pressures, but after today’s tests, I’ve decided to go with a 3200psi regulator set point. That puts me about where I wanted to be for this build.

I tested both the 151gr and 171gr Arsenal BBT slugs. The BC of both slugs is so high that I’m thinking the lighter slug with the higher velocity and more sedate (less recoil) shot cycle will be my choice. The 171gr probably has no appreciable advantage until past 700yds. And I’ve never shot that far.

151gr .308 Arsenal BBT
3000psi 1054fps (372fpe)
3200psi 1080fps (391fpe)
3500psi 1106fps (410fpe)

171gr .308 Arsenal BBT
3200psi 1051fps (419fpe)
3250psi 1055fps (423fpe)
3300psi 1064fps (430fpe)
3400psi 1070fps (435fpe)
3500psi 1076fps (440fpe)

[rsterne]

Yep, there is a lot of force on the cage/grate/spider that supports the balance chamber, I shattered one and had to build it stronger.... The total area of the 6 holes in mine is double the throat area, so I don't think the valve is starving for air on the inlet side.... Making it thicker helps as well.... The load is not just a static one, the poppet can hit it like a hammer, either driven by the hammer (a real no-no), or just coasting when opening rapidly.... I use an O-ring to help cushion the blow and decelerate the poppet more slowly instead of allowing it to crash into the spider....

Bob

[sb327]

I float the piston with enough room it doesn’t hit anything. My cage sits far enough forward to give the system room to cycle without crashing.

Looks like the project is progressing nicely Scott.

Dave

[Scotchmo]

I float the piston with enough room it doesn’t hit anything. My cage sits far enough forward to give the system room to cycle without crashing.

Looks like the project is progressing nicely Scott.

Dave

What do you mean by “float the piston”. The piston needs to hit a stop after opening.

I have two grate/cylinder configurations that I have been testing. The first has very little volume remaining when the piston bottoms out. So the “crash” should be a low force impact. It also has a “spherical” connection at the axial grate connection to insure alignment with the piston. A screw head holds the front and a step holds the rear. That’s the configuration that broke the grate. There is over 200lbs of pull as cycling force in the cylinder. It’s possible that’s what broke it rather than the “crashing”.

The alternate grate/cylinder configuration uses a retaining ring to hold the cylinder axially in the grate with a slightly loose connection fit to aid alignment with the piston. It has a larger volume (done to increase dwell) remaining, so the “crashing” force is higher. It was originally bottoming on the Delrin poppit, and deforming the Delrin. I’m going to make a tubular insert to go in the cylinder so that the piston bottoms out on that.

I’m also considering putting an o-ring cushion inside the cylinders.

Bob Sterne did some analysis on the Labradar data and came up with some BC info on the two slugs that I’m using.

151gr
G1=0.35
RA4=0.26
G7=0.15

169gr
G1=0.42
RA4=0.32
G7=0.18

The G1 is what most airgun slug manufacturers use as it gives the biggest number so it’s used for “bragging rights”, however, it is a poor fit for high BC slugs in the transonic  (>900fps) range. I have been using RA4 and it allows for better predictions at high velocity and long distance. The latest slugs are 3 and 3.5 calibers long. That long shape is getting closer to the G7 profile, so I will be checking to see if that would be a more appropriate drag model.

[sb327]

I just noticed that your cylinder/piston are reversed of mine. In yours I would float (maybe not the best terminology) the cylinder. Of course the piston (poppet) will hit the cylinder wherever you design it to, but once open, there is no reason to limit it by having the assembly limited by the cage. The cage, imo, should only serve the purpose of holding the cylinder (piston in my case) and piston apart when valve is closed. My design also allows adjustment of chamber volume without changing parts.

I’m not trying to say it’s the best way to design one but I haven’t had a cage issue and I can tune relatively easy.

Dave

[rsterne]

The piston needs to hit a stop after opening.

I'm not sure that is necessarily the case, Scott.... Valves do not usually open more than about 1/4-1/3 of the throat diameter, so for a 0.300" throat ID, the poppet probably does not open more than 0.075-0.100".... The exception would be if the valve is still open when the slug exits the muzzle (or nearly so).... If it closing anywhere around mid-barrel, I doubt it is opening more than 0.100".... If the stem projects that far behind the back of the valve, then it cannot be driven open more than that.... If you have, say, 0.150" before the piston crashes into the spider, it will only be doing so with it's own momentum, not driven by the hammer.... An O-ring cushion between the two, to prevent it slowing "instantly" on impact with the spider, will greatly reduce the peak impact force....

How are you measuring the dwell?.... Have you ever tried measuring the lift?.... It can often be done with a simple hammer follower with an O-ring sliding on it....

Bob

[Scotchmo]

The piston needs to hit a stop after opening.

I'm not sure that is necessarily the case, Scott.... Valves do not usually open more than about 1/4-1/3 of the throat diameter, so for a 0.300" throat ID, the poppet probably does not open more than 0.075-0.100".... The exception would be if the valve is still open when the slug exits the muzzle (or nearly so).... If it closing anywhere around mid-barrel, I doubt it is opening more than 0.100".... If the stem projects that far behind the back of the valve, then it cannot be driven open more than that.... If you have, say, 0.150" before the piston crashes into the spider, it will only be doing so with it's own momentum, not driven by the hammer.... An O-ring cushion between the two, to prevent it slowing "instantly" on impact with the spider, will greatly reduce the peak impact force....

How are you measuring the dwell?.... Have you ever tried measuring the lift?.... It can often be done with a simple hammer follower with an O-ring sliding on it....

Bob

Dwell is calculated as what’s needed for a given velocity/fpe. Though I now think the actual dwell is considerably more, especially after today’s tests.

I had decided to go with 3200psi, so I adjusted the bottle reg and ran today’s tests from the regulated bottle. The latest valve configuration had the large balance chamber and additional valve lift.

151gr .308 Arsenal BBT
3200psi 1098fps (404fpe)

169gr .308 Arsenal BBT
3200psi 1078fps (436fpe)

I’m likely running at the max FPE for 3200psi. I think that the dwell is too much, as the muzzle blast and recoil was significantly greater. I only got off 12 shots before the valve let go and stuck full open. Velocity was the same when it stuck open, but the muzzle blast and recoil was horrific. Almost got a scope eye.

The valve stem protrudes only 0.098”, so best case, that’s as much lift as the solenoid can provide. However, the balance piston pulls the valve open almost 0.3”. With small lift on a big poppit, with a lot of air flow, there is a strong “suction” attempting to close the valve.  The excess lift is done to reduce the “suction” at the poppit face, and allow the balance piston to hold the valve open a little bit easier/longer.

But as I stated, dwell is probably too long now. I’ll be cutting it back a little until I see 1080fps and 1050fps for the 151gr and 169gr slugs.

The valve failure this time was a circlip coming loose along with the valve stuck full open. The grate was fracturing but had not quite let go.

The plan now it to make a one piece grate/balance chamber out of chromoly steel, and try again. Also time to try a peek poppit.

[rsterne]

Well, you certainly have a very hard crash happening!.... The cylinder appears bent, and the poppet is hitting the end of it!.... Is there somewhere you can install a bumper to slow the deceleration on impact?....

Bob

[Scotchmo]

Well, you certainly have a very hard crash happening!.... The cylinder appears bent, and the poppet is hitting the end of it!.... Is there somewhere you can install a bumper to slow the deceleration on impact?....

Bob

Best I can guess, it’s got to be the inertia of the piston as it strikes the cylinder. I even had a Delrin spacer cylinder inside the brass cylinder, to act as a stop. But the Delrin was smashed farther into the cylinder bottom, squishing past the step. The center of the grate was pushed back like a diaphragm.

Besides the initial plunger strike on the stem, there is also about 200lbs of force accelerating it forward.

There is at least a few fpe of energy in the moving piston as it hits the stop.

Besides switching to chromoly steel, I might reduce the balance chamber volume and incorporate a damper. Or maybe try keeping the chamber volume but limiting the travel to 0.1” to reduce the piston energy.

[sb327]

A crude drawing to help explain what I meant.

Dave

[rsterne]

Yes, Dave, I liked that design and borrowed it for my dual tank benchrest....



I am building 2 versions, one with an all PEEK poppet (2400 psi), and one with PEEK seal and aluminum cylinder....



I had a Spider failure in the first valve in my 6mm....



It was replaced with a much beefier Spider with just 8 holes, and over twice as thick....



The valve at the top of this post is essentially this one with the piston and cylinder positions reversed....

Bob

[rsterne]

Scott, the problem in having the piston slamming into a solid metal stop is that the impact is many times greater than the force accelerating the piston.... I'll try and give a simple example for momentum of piston = 1 FPE....

Maximum force when hitting a stop is therefore the energy divided by the distance travelled during deceleration.... That is an exponential curve, and if the stopping distance is zero, the force becomes infinite.... Distance in feet is inches/12....



You can see that once you are asking the piston to stop in less than 0.010", the force goes through the roof.... It doubles if you ask it to stop in 0.005".... A simple 0.070" CS O-ring as a stop can easily compress 0.010" (or more), and keep the peak force within reasonable limits....

Bob

[Scotchmo]

Scott, the problem in having the piston slamming into a solid metal stop is that the impact is many times greater than the force accelerating the piston....
Bob

I know that. I’ll look into “floating” it.

Thanks David and Bob.

I had the piston floating on a rubber oring on the first version that failed. That might be enough if I reduce the piston travel.

[sb327]

If you do decide to try one Scott, I think you will like it. It just adds some versatility/options by taking the crash limitation out. I’m sure you realize but it can tuned to be more hammer dependent or more blow open dependent. As such, you don’t have to limit your hammer travel (unless you want to for tuning). I think also by taking the crash away, it takes that addition to harmonics out but that’s only speculation.

Good luck,
Dave

[Scotchmo]

I decided to try something with the original valve configuration before I did any major redesign. The poppit front was chewed up a little from the last couple of failures but the Delrin face was still good and still secured to the piston. I made a new grate and assembled everything again with three x-rings (square section o-rings) inside the cylinder to limit travel/lift to about .08” and act as a cushion. I went to the range today and tested for quite awhile, maybe 80 shots with no failures. Velocity was down a little as expected, and the shot cycle was sedate.

151gr at 3200psi
1075fps average

169gr at 3200psi
1020fps average

I took it back apart after I got home and everything looked good. I reassembled it with just two x-rings. That will give about .15” of lift and a little less cushion. I’ll retest tomorrow.