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Starting a .308 ELR build

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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:

--- Quote from: sb327 on June 30, 2024, 09:27:20 AM ---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

--- End quote ---

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

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