Venturi bolts…

[uv_halo]

I remember the hype of the venturi effect. The OG Spyder had a plastic 'venturi insert' stuck in the bolt. There was a lot of discussion as to what would actually induce a venturi effect. I think it all peaked with the Bob Long Cyclone bolts and given that even those didn't really bring anything tangible to the table and it seems that everyone gives up after this point.

[DocsMachine]

I will readily admit I haven't experimented a great deal with trying the different bolt bores at a wide range of pressures. The only solid example I have off the top of my head is my "junk box" 'Cocker, built off an... '04 Prostock body, I think? Unmodified body, but a Tornado valve, a gas-thru grip and a now-truly-antique Max Flow. Runs about 165 PSI with a good paint/barrel fit, and a custom delrin bolt with a .272" bore from the valve to the bolt face.

If I plunk in a Lightning- the bolt designed to go with the Tornado- I lose about 20 FPS, or have to turn the reg up to about 180. Never tried to see what that did to efficiency over a whole tank.

But, the thing is there's thousands of possible combinations for a 'Cocker. Valve chamber size, valve, hammer weight, spring combination, pressure- and flow from the regulator- bolt style and volume, and heck, paint and barrel. (I've gained 20 FPS going from the same bore of 5" Freak to an 8" Freak, and 20 more by going to a .008" smaller bore.)

I'll tell ya right now the small-bore bolt idea isn't for every possible combination, but it's been my experience that it offers benefits FAR more often than it doesn't.

I've got an old early-gen Spyder around here somewhere... I think it has venturi bolt in it, but I have to say I haven't even looked at that thing in years. Might be interesting to pull that out and try a couple custom bolts. In my spare time...

Doc.

[Fubarius]

The thing with stacked tube poppits, whether a spyder blowback or even an Ego or Timmy ram driven design, the bolt really is quite a bit more than just a hole from the valve to the breech. For one, it's also the hammer. But wait, isn't there a hammer in the lower tube? Yes, and it's connected to the bolt, so they are effectively one piece. A heavier bolt is the same as a heavier hammer. A lighter bolt is also a lighter hammer. And vice versa. I remember some year of the ego they had switched to a titanium ram/hammer, to "reduce recoil", but you needed more LPR pressure to get the lighter ram up to a higher speed to compensate for the reduction in mass. That was ok, middle of the ROF wars and all that, so a faster ram was considered a good thing. A few years later a slower ram speed was the new thing, for a "smoother shot". Problem, the titanium ram was too light, so they put a weight in the bolt. I think a big chunk of brass if I remember correctly. They could have gone back to a stainless ram, but the titanium one was "high end" so they couldn't get rid of it, so a big old chunk of metal in the bolt was the answer.

So not only the weigh of the bolt, but anything that effects how easily it moves. Different coefficient of frictions between different materials. What kind of lube you use. Condition of the o-rings. How much the detents press against the bolt. Even if the bolt pushes the ball into the barrel bore proper and thus bore size starts to effect things mechanically.

Years ago a buddy of mine had an Inferno. A UK made blowback of surprisingly nice quality. However his variant had a kind of weird bolt. It was solid aluminum, un-anodized, and had no o-rings on it. One would think it would be important to keep such a bolt well oiled to help reduce metal on metal friction. However the darn thing would only run correctly if the bolt was bone dry. Practically any amount of lubrication on the bolt would somehow slow it down (may be too tight of fit, no room for the oil between the two surfaces), and then the gun wouldn't strike the valve enough to enable the gun to recock.

[Cunha]

Whatever bolt gives u the most FPS while making no other adjustments is best.

[Tarsun2]

ive boiled a few sypder bolts in my time. lol

my favorite was always the double o-ring delrin bolt. iirc there was one with a little ramp on the top side as well to help push the ball stack up a little as the bolt passed under.
(not stock of course)