Getting mixed opinions about slug weight and efficiency

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MrP

MrP

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Apr 29, 2020
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New Lebanon, NY, USA
I've heard two things now about the effect of the weight of a slug on how much air a PCP air gun uses, whether a heavier slug will result in more or less air being used per shot. Or whether it will have no effect at all. I'm also hoping to understand why. I recall someone here stating I think that heavier slugs will be more efficient. I read on another form that the opposite is true. It was also suggested that the Air Force gun with its inline design is prone to be efficient with light pellets and less efficient with heavy ones, but that some other designs have the opposite behavior.

Shooting tonight, I noticed that my airforce gun was relatively quiet when shooting 30 grain diablos. I then tried shooting a 60 grain slug. The gun was much louder. It was no surprise there was more recoil since the slug weighed more but I swear way more air came out and the needle dropped farther in that shot then it would in two or three of the shots with the lighter diablos. I've been switched back to Diablo and it was quiet again and using less air. At least I think it was using less air I didn't have a chance to be really quantitative about it.



What's going on here? They heavier slug stays in the barrel longer I assume that has some effect on how much air comes out. LCould it really be opposite for one type of gun compared to another?
 
At either extreme, efficiency is 0. 

A pellet weighing 0gr (no pellet) develops 0fpe.

Likewise, an infinitely heavy pellet also develops 0fpe because it gets stuck in the barrel.

In between, there is a wide range of pellet weights that will generate ever more energy as they increase in mass...until eventually they become so heavy that they stay in the barrel so long that they not only accelerate but then also plateau and begin to decelerate before they leave the barrel. However we almost never reach that point so we arrive at the generalization that heavier pellets generate more energy and therefore higher efficiency. The "almost never" crosses over into the sometimes when shooting heavy slugs, not just because of the weight but because the cylindrical geometry results in vastly higher friction in the bore (compared to a pellet that contacts in just two small bands), which means more of the expanding air's energy is dissipated as friction losses (heat). In turn, the result is lower efficiency.
 
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Regarding your experience with the Air Force rifle, it's not obvious to me why that would be the case. For any PCP system consisting of a conventional knock-open valve design, valve lift and dwell are largely independent of the projectile weight...meaning the amount of air released per shot is basically the same whether there was a light pellet or a heavy slug loaded at the time.


To the small extent they are interdependent, air consumption runs the other direction. That is, the amount of air released per shot is slightly less for the heavier projectile. That's because the amount of air released is constrained by the volume it can fill just before the valve snaps closed. Our projectile acts as a plug of sorts, limiting how much volume the pressurized air can pack during the millisecond or two the valve is actually open. The lighter projectile accelerates quicker and has therefore traveled further down the barrel while the valve is still open, whereby giving a larger volume to be pressurized before the valve snaps closed. Contrast that with the heavier pellet that has moved only a short distance before the valve snaps closed, leaving less room behind it to be pressurized...the result, less air consumed.

I'd be interested in reading the rationale for why the Air Force valve behaves differently if you have a link handy. 
 
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I agree that testing this would be trivial - I was hoping it had been done already in an authoritative way. I know for a fact that I am getting a decent chronograph for Christmas so maybe I can do the testing after I get that.



I have already observed the louder sound and greater pressure drop. I think the effect is pretty obvious but I will double check just in case something else was going on. I can't tell you the difference in rate of pressure drop but like you said this would just involve firing a few shots at the same pressure with different slugs and seeing how far the needle drops. 
 
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Ok - getting some helpful answers from someone with some experience:



"Air force valves close faster with a lighter slug because the pressure differential across the top hat, which holds the valve open, decreases faster as the lighter slug travels down the barrel in less time.... Cothran Powerhouse valves likewise can use less air with a light pellet, depending on how they are tuned.... Fire a Cothran valve without a pellet in the breech, and you often only get a "pop"....
shocked.gif


For the most part, PCPs with conventional valves, where the dwell is proportional to the hammer strike.... and therefore for a constant hammer strike are pretty much of fixed duration (time).... will release less air with a heavier pellet.... This is because the heavier pellet travels a shorter distance from the breech in the fixed dwell time, and therefore less of the barrel volume is available for the air in the valve to expand into.... Less air is therefore released during the shot with a heavier pellet...."



Apparently the effect that I'm noticing (increased airflow with heavier slugs) is so pronounced that with a very heavy slug you can actually get a dump of the entire tank



And in case anyone is wondering why, apparently it has something to do with the top hat design for the Air Force guns. There's an unbalanced back pressure due to the design of the valve and the top hat... this causes valve to stay open when pellet is still in the barrel.
 
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That makes sense to me, if a valves design is greatly effected by pressure differential in the barrel then that explains it, which appears to be the case here. I am not familiar with Air force valves, so this was a refreshing little lesson.

Since barrel pressure helps the valve stay open in the air force valve design, larger slugs gulp more air where as traditional valves that rely primarily on hammer strike to generate dwell don't. I didn't consider how the valves are oriented inline and then struck open in the opposing direction that typical valves are which is in the same direction the projectile is intended. Anyhow, glad you have it figured.
 
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Very grateful that people are willing to venture a guess even if they're not 100% sure, and it's always nice to have someone with more experience on the actual mechanics give the clarifying answer. I find fascinating because there's so much going on that affects performance and if you are in it to tinker and experiment and learn, its is a pretty good place to be.



i actually love the weird behavior of these valves in an unregulated state. I can use that back pressure phenomenon to get two tunes out of the same gun without actually changing anything. If I fill to the point where there is a little bit of valve lock and dial the PW down a bit above where it just knocks the valve open, I can shoot lots of light pellets at decent velocities with small bursts of air before getting down to the valve's sweet spot. This is just as good as putting in a restrictor. When valve locked like this if I dial the hammer spring up to six or higher it easily overcomes the lock. 


The valve I have came with some restrictors that you can screw in but there's no need for them I can just use the valve lock to shoot at lower power.


I just tested it again quiet "pops" with 30 grain, big "foooomp!" sound and a good kick with 60 grain. Same effect as dialing up hammer. Can alternate back-and-forth.


 
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