Usable air in a Disco is about 7 CI, and you are limited to a 2000 psi fill unless you use stronger valve screws, which (with the correct screws) can go to 3000 psi.... However, the pressure drop to maintain a decent ES (velocity spread) is only about 800 psi in either case (52 bar).... That gives you about (52 x 7) = 364 CI of usable air.... Divide that by your FPE to get a shot count estimate, assuming decent efficiency....Bob
A 120 gr. slug at 50 FPE will only be travelling 433 fps.... I don't see that as a practical combination.... Are you looking to build a .30 cal?.... The Disco valve can easily be modified to handle a .25 cal, and I have gone as large as .35 cal, but in reality for a .30 cal or larger you are better to get a Cothran Powerhouse valve.... Be aware that valve cannot be tuned for a bell-curve, the first shot will be the fastest and each subsequent shot will be at lower velocity as the pressure drops.... A regulator will avoid that, but it is almost impossible to get a large enough plenum inside a single Disco tube.... My regulated Disco is a Double, with the upper tube being the plenum and the lower supplying the HPA to the regulator.... The advantage to using 3000 psi instead of 2000, unregulated, is that you will get more power (FPE) with no other changes.... The also applies to a stock, or modified, Disco valve.... Just to give you an idea, I built a .30 cal unregulated Disco Double using a 25" barrel, and with a 3000 psi fill I could get 9 shots at 100 FPE with 46 gr. pellets.... The valve was highly modified, with a 0.250" throat and a 0.219 transfer and angled exhaust port.... The same gun, with a 25" barrel of .35 cal and shooting 117 gr. slugs got 2 shots averaging 170 FPE, the first at 848 fps and the second at 772 fps.... It was VERY loud, and used nearly 800 psi for those 2 shots (and it had the two-tube Disco Double configuration).... A single tube would have only given one shot....Bob
Well, your original post said 120 gr.... Glad you are planning much lighter, that made no sense at 50 FPE.... Yes, properly tuned you should be able to get about 7 shots at 50 FPE from a single tube setup.... with a slightly modified valve.... In fact, I would try just the barrel and bolt change to start and see what you get.... then work from there if you need more power....Bob