The reason why you have 51 shells is because if you have over 10 shells for the Shotgun, picking up the weapon only adds one shell. Same goes for if you had 10 shells to start with, picking up Shotgun would only add one shell. If you had less than 10 shells, then picking up Shotgun would fill it to 10. The same applies for all weapons, but the values are different, and I'm not sure you can change it. The only solution I can think of is just making Ammo = 49.

You can also modify the max ammo fairly easily with Ammo Limit =

Machinegun {
    Cycle = 50;
    Damage = Knockback = 10;
    Regen = 800;
    Ammo = 150;
    Ammo Limit = 900;
    Radial = no;
}

And I have a suggestion, you can also modify the amount of ammo that ammo pickups or weapons give. (or even other items)

modifyitem(0, "AMMO_BULLETS", "", 10, 400);
modifyitem(0, "WEAPON_SHOTGUN", "", 10, 400);

Just an example, but 10 = the seconds it takes to respawn, and 400 is the amount picked up. Same goes for the shotgun, if you pick up the shotgun, it will give you 400 shells (but picking up the weapon again still gives another +1). Modifying the weapon itself though overrides the Ammo = # since its the value you start with on pickup. One piece of advice though, since you have ammo regen then it only regens up to the base ammo value, so I wouldn't set the max ammo of MG to 900 since it only regens up to 150 since that is your base ammo, even though you could modify the ammo from pickup/etc. You should really consider re-balancing all the ammo/regen a little to reasonable values, though.

Hope that helps.