why do i get less than average shots with 24oz co2?

Both of the answers given are dead on. The Stryker isn't known to be very air efficient and without it being regulated it's worse. The Temp. is the main issue here though (if you're talking Fahrenheit), unless you're shooting at 1 ball every few seconds then the liquid CO2 isn't being allowed to warm up as much as it can so it converts back to a gas and expands the best it can. The more rapidly you shoot the more a tank chills and thus the worse the efficiency gets (as you'll be shooting condensed liquid CO2 instead of expanded CO2 gas). Also, the guy that filled the tank may not be chilling the tank like he's supposed to and only giving you a partial fill. Lastly, the condition of your marker and it's accessories play a part. Is your marker always cleaned and freshly lubed or do you play for months without cleaning/lubing it? What barrel does it have? The stock barrel is a large bore barrel, which helps make the marker a bit more of a gas hog. Under boring is more efficient and if decent paint is used it doesn't produce any more/less barrel breaks than over boring does. Also, 14" barrels generally are the most efficient length. Well, any barrel that has a solid 8" of unported barrel. The ported section reduces not only noise, but efficiency. So a heavily ported barrel will not be more efficient than one with fewer ports. I don't expect you to go out and spend a lot of cash upgrading your Stryker (if it's even possible), but a 14" .685 Custom Products Classic barrel ($30) should be more efficient and more accurate as well. I believe your marker uses Spyder threaded barrels. However, I really can't justify investing any amount of cash into such a marker, no offense. You'd be much better off saving up for a better marker such as a Smart Parts Vibe ($150).

The amount of shots you get is determined by a few things. 

Velocity is one, the higher your velocity is the less shots your going to get. 

Weather is another, if it's hot outside you will get better shots out of it than if it was cold. The heat makes the co2 expand more. 

Your marker is also a factor. Some markers just aren't as efficent as others. 

your stryker is most likely the problem because it has no regulator so it takes the maximum psi for every shot if you want to get over 1600 shots i would look in to buying a better marker like a sp1 it is proven to be very air efficient. another possibility could be the person filling it some stores will only fill to a certain amount or are just bad at filling co2 tanks. temperature is also a big factor with co2 in lower temperatures you will get less shots per fill(not sure if you ment 30F or 30C)