I use a 13ci 3000psi tank on my trilogy pump and get about 130 shots from it. I really appreciate the light weight and compact dimensions (that trilogy is a brick). I only go out with ~100 rounds anyway so it serves its purpose for me.
Depends. I'm assuming you are not playing stock class since you are asking about tank size. How long are your arms? When holding your gun in your hand the tank should come to the crook of your elbow. Which ever tank and regulator combo it takes to reach your elbow is your answer. Different companies have different size dimensions for their regulators and their 45/45 68/45 50/45 and 48/45 bottles so you just have to try them and find the right combo that fits you the best.
50/45 stubbys are about 3" shorter than 68/45s. it's all about what you're comforable with. i shoot a CCM S5 (fullblock) and i hate it when my beavertail touches my mask, so i use a 68/45 on it. i can't wait until i get my S6.5, i can use my 50/45 and no mask bite!!!
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